Saturday, December 15, 2012

2012 Season Summary

The 2012 Atlantic hurricane season was above average, with

19 cyclones attaining tropical depression status
19 cyclones attaining tropical storm status
10 cyclones attaining hurricane status
1 cyclones attaining major hurricane status

Before the beginning of the season I predicted that there would be

15 cyclones attaining tropical depression status
13 cyclones attaining tropical storm status
5 cyclones attaining hurricane status
3 cyclones attaining major hurricane status

The number of tropical depressions, tropical storms, and hurricanes was well above my predictions prior to the start of the season, though there was only 1 major hurricane relative to the 3 I predicted.

Much of the discrepancy in the forecast can be explained by the status of the El Nino over the course of the season. Early in 2012, the NOAA predicted the development of a moderate El Nino event by midyear, which was to inhibit the development of cyclones in the latter half of the season. However, the development of the El Nino was delayed and inconsistent, allowing for periods of explosive cyclonic formation.

In spite of the very large number of cyclones, there was only a single major hurricane, Michael, during the season. Broader cyclones such as Ernesto, Isaac, and Leslie struggled mightily in strengthening and grossly underperformed in intensity relative to short-term forecasts. Leslie, for example, was predicted to reach strong Category 2 strength, but its peak intensity was only as a minimal Category 1. Smaller cyclones, though, did the opposite, often surviving or intensifying more than expected. Michael, the strongest cyclone by winds during the season, intensified in an only marginal environment, even doing so in relatively close proximity to the much larger Hurricane Leslie. These data are indicative of the prevalence of dry air throughout the basin during the season and also of differences in the direction of upper- and lower-level winds over the season.

Another interesting trend that emerged around early September was the development of unusual blocking patterns and lack of steering currents over the northern part of the Atlantic basin. This was due to an anomalous shift of the jet stream to the north over the Atlantic, as this allowed subtropical ridges to remain in place, and prevented cyclones from being caught in the flow of troughs, which followed a more poleward path. The trend became evident with Leslie and Michael, and was exemplified by Nadine, which survived for a very long time over the north Atlantic, and finally by Hurricane Sandy, which was able to recurve westward due to the unusual shape of the jet stream.

Some notable cyclones and facts about the season include:

  • 2012 was the third Atlantic hurricane season in which two preseason storms (in this case Alberto and Beryl) formed, after 1887 and 1908
  • Beryl was also the strongest Atlantic preseason cyclone since 1972
  • Debby, becoming a named storm on June 23, surpassed the previous record for earliest fourth tropical cyclone formation of July 7 set during the 2005 Atlantic Hurricane Season
  • Though Gordon developed into a tropical storm before Helene, the tropical depression that became Helene formed before that which became Gordon, and thus the names are reversed on the chronological listing
  • Michael was the strongest storm of the season by wind speed, achieving 115 mph winds, or minimal Category 3 intensity, the lowest highest wind speed for an Atlantic hurricane season since 1994
  • Hurricane Nadine spent 21.75 days as a tropical cyclone meandering around the northeast Atlantic, making it fifth on the all-time list of longest lasting tropical cyclones
  • Hurricane Sandy recorded the season's minimum pressure of 940 mb as it transitioned to an extratropical cyclone and made landfall in New Jersey
  • The 2012 season overall was tied for the third most active in terms of number of named storms
Overall, the 2012 Atlantic hurricane season was quite active, and included several hurricanes, but was not very intense. Hurricane landfalls included Ernesto in the Yucatan Peninsula and Isaac in Louisiana, both of which were not particularly strong, and caused relatively little damage. The Caribbean Sea was relatively quiet, as wind shear and dry air mostly kept cyclones in the region to tropical storm intensity, with the exception of Hurricane Sandy, the most notable storm of the season. Sandy, which made landfall in Jamaica, Cuba, and the Bahamas as a hurricane, went on to develop into a hybrid system and slam into the northeast coast, causing widespread damage and unprecedented storm surge along the New Jersey and New York coastlines.

Friday, October 26, 2012

Tropical Storm Tony (2012)

Storm Active: October 22-25

A tropical wave formed in association with an area of disturbed weather accompanying an upper-level low about midway between the western coast of Africa and the Lesser Antilles on October 18. This interaction sporadically produced concentrated thunderstorm activity as it moved to the northwest over the next several days.

Atmospheric conditions improved markedly on October 21, and allowed the system to organize rapidly into Tropical Depression Nineteen on October 22. The cyclone began to curve towards the northeast later that day, as a front approached from the west. By late on October 23, convection had developed close enough to the center of circulation that the cyclone was upgraded to Tropical Storm Tony.

Despite shear from the front to its west, Tony experienced modest strengthening as it accelerated to the east-northeast, and the storm reached its peak intensity of 50 mph winds and a pressure of 1000 mb on October 24. Meanwhile, Tony was beginning to exhibit nontropical characteristics; the banding features became more linear, and the circulation elongated. The transition was very gradual, however, and the system remained a tropical storm through October 25, at which time it lost any remaining tropical characteristics and was downgraded to a remnant low.



Tony as a moderate tropical storm moving rapidly to the northeast over the Central Atlantic.



Track of Tony.

Wednesday, October 24, 2012

Hurricane Sandy (2012)

Storm Active: October 22-29

A low pressure trough embedded in the Intertropical Convergence Zone moved into the Caribbean sea on October 18, and began to increase in shower activity the next day. On October 20, as the area of disturbed weather moved west, the pressures in the area dropped precipitously, and the circulation became much better organized.

Deep convection did not consistently accompany the system on October 21, but conditions continued to be favorable as the disturbance moved southwest, bringing some showers to Jamaica and neighboring areas. By October 22, a swirl was evident amid the clouds, and the low was classified as Tropical Depression Eighteen. The system drifted southward and organized further later that day, and strengthened into Tropical Storm Sandy.

Sandy adopted a slow but accelerating northward motion early on October 23, as a front lifted out of the northwestern Caribbean. Meanwhile, convection steadily increased with the system, and became closer to the center by later that day, causing steady strengthening. In the evening, the cloud tops of Sandy's central dense overcast cooled considerably, and the first hints of an eye feature appeared, indicating that the cyclone was undergoing rapid strengthening. Meanwhile, the outflow had improved, with heavy rain bands sweeping across Jamaica, Hispaniola, and eastern Cuba as Sandy approached. These factors caused the cyclone to be upgraded to a hurricane later that morning.

During the afternoon, the center of Sandy passed directly over eastern Jamaica, but the land interaction did almost nothing to disrupt the circulation and the system continued strengthening, as an eye appeared on infrared as well as visible satellite imagery. Over the next twelve hours, Sandy put on a burst of extremely rapid strengthening, bringing its pressure down to a value of 954 mb. Very early on October 25, the cyclone made landfall in eastern Cuba with its peak winds of 110 mph!

Sandy weakened slightly as it moved over Cuba, but emerged over water still maintaining Category 2 intensity. The cyclone slowed down considerably and turned to the north-northwest that night as it interacted with an upper-level low. Higher shear weakened the system as it lashed the Bahamas, but the structure of the storm also underwent a transformation. Convection became displaced from the center in all but the northwestern quadrant, the windfield broadened, and the outflow became more extratropical in appearance on October 26.

However, shear declined somewhat, and thunderstorm activity more completely covered the center by early on October 27. By this time, Sandy had begun to moved towards the north-northeast, fluctuating in intensity but remaining near minimal hurricane strength.

By later that day, rain bands associated with the combination of a front stalling near the U.S. east coast and the circulation of Sandy swept across numerous states, causing tropical storm force winds in the North Carolina and heavy rain in localized areas up through Virginia. Dry air also invaded the circulation of Sandy, creating a narrow ring devoid of thunderstorm activity between the central convection and outer bands. However, this did not weaken Sandy, as the system was exhibiting some subtropical behavior.

Early on October 28, the central pressure of Sandy dropped again as the cyclone deepened further, plunging to a new low of 951 mb. Meanwhile, the cyclone accelerated to the northeast, and gale force winds expanded even further, stretching from North Carolina all the way to Bermuda, and rain bands moved further up the coast, sweeping across Pennsylvania and New Jersey.

During the night, Sandy began a highly unusual turn towards the northwest, under the influence of an exceptionally strong high pressure ridge over northeastern Canada. This ridge caused an inversion in the normal path of the jet stream, diverting it so that it doubled back on itself. The cyclone began to be drawn in by this feature, and so curved in the opposite direction that tropical cyclones typically turn.

Meanwhile, as Sandy traversed the warm waters of the Gulf Stream, it actually intensified somewhat, despite being at a fairly high latitude. In addition, the pressure continued to drop. Conditions deteriorated rapidly along the Delaware and New Jersey coastlines that afternoon as the central bands of the cyclone came onshore. Hurricane force wind gusts and storm surges in excess of 5 feet were recorded up and down the coast. Sandy accelerated rapidly that afternoon, and was losing tropical characteristics as its central band became frontal in nature. Early that evening, the system recorded its minimum pressure of 940 mb, and winds of 90 mph.

Shortly afterward, around 7:00 pm EDT, Sandy was recognized as an extratropical cyclone, and the remnants of Sandy made landfall in southern New Jersey an hour later. High wind and occasional heavy rain continued as the low crossed into Pennsylvania late that night and weakened to the equivalent of a tropical storm early on October 30. The low continued westward and weakened, still causing rain and snow in the Appalachian areas until it dissipated on October 31. The remnants still caused shower activity for another few days as they moved northeast away from the United States.

Hurricane Sandy set a new record for the largest Atlantic hurricane, with a gale diameter of 945 miles a few hours before landfall in New Jersey, and was one of the costliest in U.S. history. Sandy caused widespread damage in a large swath extending from Jamaica, through Cuba and the Bahamas, and up the east coast from North Carolina to New England.



Sandy near peak intensity near landfall in eastern Cuba.



Track of Sandy.

Saturday, October 13, 2012

Hurricane Rafael (2012)

Storm Active: October 12-17

On October 5, a tropical wave emerged off of Africa, but remained weak for the next several days as it traversed the eastern Atlantic. The system gradually moved over warmer waters and increased in shower activity. By October 9, the system had a well-defined low pressure center associated with it. Shear out of the west still impacted the low, but atmospheric conditions improved over the following few days.

Rain and windy conditions began to affect the Lesser Antilles and surrounding areas during the day of October 12. At this time, the convection was becoming concentrated at the center of circulation, but was still somewhat displaced to the east. Later that day, aircraft indicated that the cyclone had developed a closed center, and was thus classified Tropical Storm Rafael. At the time of its formation, the convection was still distributed linearly along the former trough boundary, and the circulation remained slightly elongated. However, the thunderstorm activity was very vigorous; there were widespread areas of heavy squalls and tropical storm force wind gusts.

Rafael moved generally to the north-northwest over the following day, and very slowly organized, with a more defined region of cold cloud tops appearing near the center during the afternoon of October 13. This initiated a period of strengthening as the center moved closer to the Virgin Islands. The system passed close to the northeastern Caribbean islands late that night with maximum winds of 50 mph as the cyclone continued its trek north.

On October 14, Rafael took a slight turn towards the northwest as the ridge over the north-central Atlantic strengthened, slowing as it did so. By this time, the cyclone had assumed a more symmetric appearance, and on October 15, the circulation finally achieved gale force winds on all sides of the center, and was near hurricane intensity. Later that night, a flare up of very strong convection appeared at the center, prompting the upgrade of Rafael to a Category 1 hurricane.

Early on October 16, the system began to accelerate northward in the flow of a trough emerging off of the U.S. coast, and vertical shear increased. Despite the intense shear, however, outflow remained remarkably healthy in all quadrants, and Rafael strengthened further, reaching its peak intensity of 90 mph winds and a pressure of 969 mb that morning. By the evening, the cyclone was approaching Bermuda as an impressively large cyclone.

The system made its closest approach to the island that night, and continued to accelerate to the north-northeast, moving away from the island at over 25 mph. Early on October 17, Rafael's circulation assumed an extratropical appearance, with a very large area of gale force winds and bands extending many hundreds of miles from the center. However, the hurricane maintained a small amount of deep convection near the center until that afternoon, at which time is was pronounced extratropical, still producing hurricane-force winds as an extratropical low. The low continued northeast before combining with another powerful system over the north Atlantic the next day.



Rafael as a Category 1 hurricane moving north into open waters.



Track of Rafael.

Friday, October 12, 2012

Tropical Storm Patty (2012)

Storm Active: October 11-13

On October 11, a low pressure center formed at the tail end of a frontal boundary extending from the northeast Atlantic down to near Hispaniola. Disturbed weather increased near the low during the following day, as the low became disassociated with the trough to its northeast.

Over the next several days, another frontal boundary began to approach the low, causing a sharp increase in wind shear. However, the low did not get caught in the flow ahead of the front, but instead remained nearly stationary just to the northeast of the Bahamas through October 10. Despite being expected to merge with the front, the system maintained its identity, and in fact became more organized, as thunderstorm activity concentrated near the center.

By the afternoon of October 11, the low had achieved enough deep convection to be considered a tropical cyclone and so was classified Tropical Depression Sixteen. That evening, the convection increased and covered the exposed circulation, and the cyclone was therefore updated to Tropical Storm Patty. Late that night, Patty unexpectedly strengthened further, and reached its peak intensity of 45 mph winds and a pressure of 1005 mb.

On October 12, a combination of strong southwesterly upper-level winds and a northeasterly low-level flow started to pull the circulation apart. Patty weakened to a tropical depression that evening as the center once again became completely exposed. By the morning of October 13, the circulation was no longer closed, and Patty was declared a remnant low. The remnants combined with a trough of the U.S. east coast shortly afterward.



Patty as a disorganized tropical storm struggling to survive just north of the Bahamas.



Track of the short-lived Patty. Most of the positions indicate occurred when the cyclone was non-tropical (triangles), with only the tiny clump of circles accounting for Patty's time as a tropical cyclone, in which it moved little.

Saturday, October 6, 2012

Tropical Storm Oscar (2012)

Storm Active: October 3-5

On September 30, a tropical wave off of the coast of Africa began to show signs of an organized circulation, though convection remained limited. The disturbance moved generally west-northwest over the following two days, and increased markedly in thunderstorm activity. The circulation remained slightly elongated into October 3, but the system was sufficiently organized to be designated Tropical Depression Fifteen.

The cyclone had internal structure issues ab initio; at its formation, convection was displaced to the east and south of the circulation, and the center featured multiple vortices that only gradually consolidated. A trough descending into the central Atlantic had displaced the Bermuda high by this time, and Fifteen began to turn northward.

Overnight, winds increased slightly, and the system was upgraded to Tropical Storm Oscar. Strong upper-level winds kept the center of the cyclone exposed all through October 4, but deep convection moved a little closer to the center, and the winds within the area of shower activity increased. This brought Oscar to its peak intensity of 50 mph winds and a pressure of 997 mb.

By October 5, the trough was encroaching on the circulation of Oscar, and the rapidly deteriorating cyclone was accelerating to the northeast. By late that morning, the cyclone's elongated circulation combined with the trough of low pressure, and the system was announced dissipated.



Oscar as a strongly sheared cyclone over the far east Atlantic.



Track of Oscar.

Tuesday, September 11, 2012

Hurricane Nadine (2012)

Storm Active: September 11-October 4 (21.75 days as a tropical cyclone)

On September 7, a low pressure system emerged off of the western coast of Africa. The system moved over the Cape Verde Islands the next day, bringing some periods of heavy rain and gusty winds to the islands. The low slowly gained organization, with convection becoming closer to the center over the next few days as it moved westward.

Dry air briefly hindered development somewhat on September 10, but the system became slightly better organized on September 11, meriting the designation Tropical Depression Fourteen late that morning. The system turned toward the northwest that afternoon, as a weakness developed in the ridge to its north.

A central overcast developed late that night, and the central pressure dropped, so Fourteen was upgraded to Tropical Storm Nadine. Steady strengthening occurred over the next day as convection deepened, bringing the system to near hurricane strength by September 13. However, strengthening leveled off that day, as Nadine came under the influence of some shear out of the southwest. On September 14, Nadine skated around the periphery of a subtropical ridge, turning to the north and northeast by that evening, still maintaining strong tropical storm intensity.

During that day, the direction of the wind shear affecting Nadine had shifted and upper-level winds began to blow out of the west, allowing the cyclone to become slightly better organized. Therefore, Nadine was upgraded to a hurricane overnight. Another ridge began building to the north of the cyclone the next day, and it was pushed into an eastward motion on September 15. Since it was moving away from the source of the shear, it was able to maintain Category 1 intensity, despite a slight elongation of the circulation, and achieved winds of 80 mph and a pressure of 983 mb.

Some changes occurred within the circulation of Nadine on September 16. The western side of the cyclone eroded considerably, and though the circulation became less tilted, satellite images indicated that the system had weakened slightly as cooler waters began to take their toll. On September 17, Nadine became embedded in the flow of a trough over the Azores, and it turned to the northeast once again, with a decrease in forward speed. The cyclone weakened to a tropical storm, as its inner core became all but void of convection that morning, but a flare up of deep thunderstorms to the north of the center allowed the system to maintain strong tropical storm intensity through the evening.

Nadine continued to persist as tropical on September 18, maintaining an eyewall on the north side of the center and fairly prominent rain bands. Additionally, surf increased in the Azores as the center as the cyclone approached. By this time, Nadine was, in many respects, a hybrid system. The cyclone exhibited some extratropical properties such as resilience to strong shear, but still maintained a tropical-like central structure. This unusual combination allowed Nadine to survive the marginal conditions of the northeast Atlantic.

On September 19, a blocking pattern formed over the northeast Atlantic, a very rare event. This caused Nadine to remain nearly stationary through the afternoon, and eventually to move slowly to the east-southeast, actually increasing in organization on September 20 as its central pressure dropped to 981 mb, a new low for the system.

Overnight, a cold front that had exited the east coast about six days earlier began to interact with the circulation of Nadine, pushing it to the south at a slightly faster speed. The front also caused it to lose some tropical characteristics, and its convection to decrease. However, on September 21, the cyclone developed a prominent banding feature, and though the windfield had expanded, it was not yet non-tropical. Therefore, during the afternoon of that day, Nadine was reclassified a subtropical cyclone. Such an event, a transition from tropical to subtropical, is somewhat rare, and Nadine was only the fifth Atlantic cyclone to do this since subtropical storms were introduced in 1968.

However, as Nadine continued to lose convection overnight, it no longer qualified as even a subtropical cyclone and was downgraded to a remnant low. On September 22, the system regained much of its lost convection as it drifted southward over warmer waters, though most was displaced to the north of the center. By the morning of September 23, satellite classifications indicated that Nadine had regenerated into a tropical cyclone, continuing the lifetime of this unusual storm.

After its regeneration, it lost nearly all the convection it had recovered, but retained just enough to remain tropical into September 24. A trough of low pressure to the northeast of Nadine continued to cause wind shear that day, but upper-level winds gradually lessened as the storm moved generally westward, steered by another ridge to its north.

On September 25, Nadine weakened slightly, but its outflow greatly improved in the favorable upper-atmospheric environment. Also, a prominent central void appeared on infrared imagery that morning, closely resembling an eye, despite the fact that the cyclone was only a weak tropical storm. That evening, the system became more organized as stronger bands developed and the eye contracted, and the system strengthened slightly.

On September 26, the cyclone was steered southward under the influence of the ridge, and convection increased further, causing gradual strengthening over the next day, and bringing Nadine to strong tropical storm intensity by the afternoon of September 27. Overnight, Nadine steered around the edge of the ridge, and began to accelerate to the northwest, and ultimately north-northwest. Meanwhile, Nadine began to interact with an upper-level low to it west, which actually helped to shield it from the worst of the shear. During the morning of September 28, an eye began to sporadically appear on visible imagery, and Nadine was upgraded to a hurricane.

Nadine fluctuated in intensity over the next day, as its interaction with the low altered the contour of the circulation. The system weakened to a tropical storm, and then regained hurricane strength for a third time, as it moved north-northwestward across its former path through the Atlantic and completed a loop, bringing it back to a position it occupied 12 days earlier.

Early on September 30, Nadine increased further in organization, as the eye became better defined and more circular, bringing Nadine to its peak intensity of 90 mph winds and a pressure of 978 mb that afternoon. At the same time, the forward motion of Nadine decreased, and it began a turn to the west as an oncoming trough blocked its progress. The same trough began to erode the northern hemisphere of the circulation that evening, and induced some steady weakening, as Nadine also moved over cooler waters.

The cyclone began a counterclockwise loop overnight, turning to the southwest, and soon weakening to a tropical storm. On October 1, Nadine recovered some of its convection, and maintained strong tropical storm intensity over the next day as it gradually made a turn to the east.

However, shear increased significantly over the next day as Nadine became entrenched in the flow of a trough and began to accelerate eastward. On October 3, the center became separated from the deep convection as the latter was displaced to the southeast, and Nadine weakened to a low-end tropical storm. Overnight, the system accelerated further in the northeast direction, and affected the Azores with thunderstorms and gusty winds as it passed near the islands for the second time. By the morning of October 4, the circulation of Nadine was indistinguishable from the oncoming trough. The long-lived cyclone had finally dissipated.

Nadine's lifetime spanned 23 days, and the cyclone spent 21.75 days as a tropical cyclone, placing it fifth on the list of all-time longest lasting Altantic hurricanes. Its longevity was a product of weak steering over the northeast Atlantic and a resilient structure that allowed the circulation to survive repeated transitions from tropical to extratropical, and even subtropical.



Nadine near peak intensity as a Category 1 hurricane over the north Atlantic. A trough to the north is also visible. This trough would block Nadine and cause it to perform a final loop before dissipation.



Track of Nadine.

Tuesday, September 4, 2012

Hurricane Michael (2012)

Storm Active: September 3-11

On September 1, a trough of low pressure formed in the eastern Atlantic. Over the next day, it began to interact with an upper-level low, the resulting union producing a wide area of isolated showers activity. Later on September 2, a surface low formed at the southern edge of the trough, accompanied by a small area of dense overcast.

Though upper-level winds were initially unfavorable, the circulation organized fairly quickly, and by September 3, the convection had increased enough for the low to be designated Tropical Depression Thirteen.

Up to that point, the system had been moving slowly west to west-southwest as a dip in the jet stream prevented it from accelerating significantly. After formation, Thirteen began to drift northwestward, and eventually northward in the wake of a trough to its north.

Some shear was evident on the west side of the circulation through September 4, but the compact system maintained its integrity and increased slightly in deep convection that day, keeping the center of circulation close to the most intense thunderstorm activity. As a result, the cyclone was upgraded to Tropical Storm Michael.

Overnight, due to the approach of an anticyclone from the west, the tropical storm turned to the northeast. However, its forward speed was still quite slow for a cyclone of its latitude, as it was also hemmed in by the Bermuda high to its east. As the low to Michael's north moved away, shear diminished, and allowed Michael to steadily strengthen through September 5. By the afternoon of that day, an eye feature had made a brief appearance, but it was quickly replaced by a central overcast. These factors supported an intensity near hurricane strength that evening.

During the evening, Michael's center contracted, and the cyclone underwent rapid intensification, bringing the cyclone to major hurricane strength by the morning of September 6, the first of the season. That afternoon, the system had reached its peak intensity of 115 mph winds and a pressure of 964 mb. Michael had expanded somewhat by this time, and had an extremely well-defined eye. However, as the cyclone continued to move northeast, sea surface temperatures began to decline, and Michael began to gradually weaken.

On September 7, Michael gradually made a turn to the northwest, but remained in an area of weak steering, and moved slowly, still maintaining Category 2 intensity for the next day. On September 9, the ridge to Michael's north strengthened considerably, and the cyclone turned toward the west, and even west-southwest that evening. The eye was still prominent, but convection was degrading by this time, especially in the northwest quadrant. Therefore, Michael weakened to a category 1 hurricane.

On September 10, Michael navigated around the western edge of the ridge to its north, and made a sharp turn to the north over the following day. Around the same time, shear increased sharply in the vicinity of Michael, partly due to the outflow of Tropical Storm Leslie. The eye disappeared for the last time, and Michael weakened to a tropical storm. By September 11, all convection had been ripped away from the system by upper-level winds, and the system was declared extratropical that afternoon, and absorbed the next day.



Michael at peak intensity as a low-end Category 3 hurricane.



Track of Michael.

Friday, August 31, 2012

Hurricane Leslie (2012)

Storm Active: August 30-September 11

A tropical wave that emerged off of Africa on August 27 produced scattered showers in the Cape Verde Islands the following day. It continued westward and organized quickly, developing a large area of convection by August 29. The next day, thunderstorms concentrated near the center of circulation, and a spin in the clouds became evident. Therefore, in the afternoon of August 30, the system was designated Tropical Depression Twelve.

Later that day, Twelve's winds increased, and it was upgraded to Tropical Storm Leslie. The center generally became closer to the deep convection overnight, and strengthening began, bringing Leslie to a strong tropical storm by the morning of August 31. The cyclone began to curve towards the north as it exploited a weakness in the Bermuda High, formed by Hurricane Kirk.

Though Leslie was near hurricane strength early on September 1, upper-level conditions deteriorated later that day, and wind shear increased significantly, displacing the convection to the southeast of the center. Meanwhile, the tropical storm made its closest approach to the Windward and Leeward islands of the Caribbean, bringing high surf, especially to the northeasternmost areas.

The cyclone turned northwest overnight, and despite the center being outside the deep convection, the winds associated with the system remained strong and actually increased slightly early on September 2, again bringing Leslie close to hurricane strength.

An upper-level low to Leslie's east, over the Bahamas, continued to induce moderate shear during the day. However, the convection associated with Leslie increased in area and intensity over the following day. By that night, an extremely large blob of thunderstorms, well over 500 miles wide, was churning southeast of the circulation center, featuring extremely cold cloud tops. However, the circulation itself remained disorganized, and Leslie's intensity change little into September 3.

By the afternoon, Leslie had turned farther to the north into the weakness of the aforementioned ridge. Despite this, the forward speed of the cyclone slowed to a crawl that evening, as the ridge restrengthened.

During the night, the system became much more organized as the deep convection assumed a more symmetrical appearance and persisted closer to the center. Soon after, though, dry air invaded the circulation, and the center once again became nearly devoid of thunderstorm activity. By this time, Leslie began to produce rough surf along the U.S. East coast, elevating the rip current threat.

Over the next few days, Leslie drifted northward, and gradually atmospheric conditions improved, allowing the system to periodically develop bursts of central convection. Additionally, on September 4 and 5, the cyclone generally increased in size, with gale force winds extending over 200 miles from the center by the afternoon of September 5. Around this time, an eye briefly appeared on visible satellite imagery, allowing the cyclone to be upgraded to Hurricane Leslie.

Since Leslie was nearly stationary during the day of September 6, strengthening was inhibited by the decrease of water temperature just below the system. Due to this, Leslie could not achieve any further eyewall development, and the inner core actually degraded in organization overnight and into September 7. This caused Leslie to weaken back to a tropical storm later that morning.

The cyclone finally began to move northward at a steadier pace that afternoon, as it became entrenched in the flow of a trough to its northwest. As Leslie moved over warmer waters not affected by upwelling, it began to regain convection, particularly in the northern semicircle.

The outflow of Leslie improved further overnight, and a large curved rain band developed on September 8 that surrounded the inner core. However, the center itself had a large eye feature, too large for a compact eyewall. Thus, the maximum winds of Leslie remained below hurricane strength, and the pressure, formerly near 980 mb, stayed relatively high.

Conditions also deteriorated in Bermuda that evening, as the outer bands swept across the island. By September 9, tropical storm conditions swept across Bermuda, and the center of Leslie made its closest approach late that morning, passing well to the east. The cyclone accelerated further that afternoon, and recovered some deep convection, though winds near the center remained somewhat light.

On September 10, Leslie started to rapidly accelerate north-northeastward, and already showed signs of extratropical transition that afternoon; the already broad center expanded further, and the cyclone became very asymmetric. However, winds recorded in a band north of the center supported raising Leslie's winds to 70 mph that evening. During the morning of September 11, Leslie made landfall in Newfoundland, bringing a wide area of gale force winds to the island and surrounding areas as it became extratropical.

The powerful cyclone continued over the open waters of the north Atlantic, passing just south of island on September 12, and finally combining with another low pressure system over northern Europe on September 13.



Leslie at peak intensity, with a small eye visible.



Track of Leslie.

Tuesday, August 28, 2012

Hurricane Kirk (2012)

Storm Active: August 28-September 2

On August 23, a tropical wave, already featuring a broad circulation, emerged off of Africa. Though the circulation deepened significantly over the next two days, convection remained scant, as the bane of the 2012 season, dry air, prevented further organization. The low moved northwest into less favorable environment the following day and wind shear took its toll on the system.

There was convection associated with the system as it moved over open waters, but it was displaced to the northwest. However, as shear abated slightly on August 27, the convection became nearer to, and even covered the center at times. On August 28, the system became more organized, and took a turn back to the west-northwest. By later that day, convection had persisted to such a degree that the cyclone was classified Tropical Depression Eleven.

Deep convection generally increased with the system overnight, giving it a more symmetrical appearance by the morning of August 29. Therefore, the cyclone was upgraded to Tropical Storm Kirk. Kirk increased in organization rapidly during that day, and, being a small cyclone, was able to quickly develop strong winds in its eyewall. On August 30, the structure of the system improved further as shear decreased, and a small eye formed, meriting the upgrade of Kirk to a hurricane that morning.

The cloud tops of the eyewall continued to cool that day, and the circulation became even more defined later that day, and Kirk rapidly strengthened through the night, bringing the cyclone to its peak intensity of 105 mph winds and a pressure of 970 mb early on August 31. By this time, the small cyclone was beginning to accelerate northward, as it came under the influence of mid-latitude winds.

However, conditions surrounding Kirk started to degrade later that day, as shear increased and the hurricane moved over cooler waters. Dry air finally invaded the system that afternoon, and the eye quickly vanished, as the compact circulation lost organization. Rapid weakening ensued.

By early September 1, Kirk had weakened to a tropical storm, and was accelerating towards the northeast. The system had already begun extratropical transition by later that day, but an area of deep convection near the center allowed Kirk to remain tropical through the morning of September 2, at this time still a moderate tropical storm. However, by the afternoon, the storm had lost any vestige of central convection, and was declared extratropical. It had been absorbed by a frontal boundary by the next day.



Kirk near peak intensity. Though a small system, Kirk featured a very symmetrical eye feature.



Track of Kirk.

Thursday, August 23, 2012

Tropical Storm Joyce (2012)

Storm Active: August 22-24

On August 19, a tropical wave, already associated with a low pressure center, emerged off of the African coast. Over the next few days, the circulation gradually became more organized, as the system assumed a general west-northwest motion on August 21. Convection decreased early on August 22, but the circulation became sufficiently defined for the cyclone to be upgraded to Tropical Depression Ten.

Throughout that day, deep convection slowly increased, but the center remained to the south of the coldest cloud tops, and the cyclone remained a tropical depression into August 23.

Late that morning, Ten gained the necessary organization to be upgraded to Tropical Storm Joyce. However, the cloud cover over the center was very brief, and shear out of the southwest quickly displaced all deep convection to the north. Though isolated areas of deep convection continued to form, they were well north of the center, and the circulation itself was losing definition.

Joyce weakened to a tropical depression later that day, and on August 24 was designated a remnant low. The remnants continued northwest, producing shower activity as they tracked over open waters. However, upper-level winds were unfavorable for regeneration, and the low soon dissipated.

Joyce, being the 10th tropical storm of the 2012 season, was named on August 23, and this is tied for the second-earliest a 10th named storm has ever formed in the Atlantic with 1995, behind only 2005's Jose, which formed on July 22 if that year.



Joyce as a minimal tropical storm on August 23. Shortly afterward, shear tore the system apart.



Track of Joyce.

Tuesday, August 21, 2012

Hurricane Isaac (2012)

Storm Active: August 21-September 1

A tropical wave moved off of the African coast on August 16, quickly developing widespread shower activity. A circulation slowly became defined in association with the system as it moved west over the open Atlantic over the following few days.

The low jogged to the north on August 18, and became exposed to some dry air the next day, limiting the convection near the center. However, the circulation continued to deepen, and early on August 21 the system was organized enough to be classified Tropical Depression Nine.

Nine's center was exposed by shear several times during the day, but deep convection persisted sufficiently for it to be upgraded to Tropical Storm Isaac that evening The general satellite appearance of the cyclone improved on August 22, but dry air completely prevented consolidation of the center and the concentration of deep convection. Meanwhile, Isaac crossed over the Lesser Antilles, bringing heavy rain and tropical storm force gusts.

The system consisted of two hemispheres of convection for much of the evening of August 22 and August 23, one northeast of the circulation, and the other, also the more intense, to the southwest. These areas of heavy rain were quite broad, but the winds near the center remained very light. Early on August 23, Isaac center reformed to the south of its former position.

During the day, Puerto Rico experienced rain and scattered downpours as Isaac passed well to the south. The convection had by this time amalgamated into a single mass, but the center was still slightly displaced from the deep convection. The system's motion shifted toward the west-northwest that night, and modest strengthening began as conditions in Hispaniola steadily deteriorated, with tropical storm force winds sweeping through the island beginning on August 24. That day, Isaac generally increased in organization, as the numerous vortices associated with the low-level center finally combined, and the circulation became more defined.

The cyclone made landfall in Haiti very early on August 25, packing winds just below hurricane force. Though land disrupted the development of deep convection in association with Isaac, its circulation remained largely intact and the storm weakened only slightly that morning as it emerged back over water. However, the center experienced a westward shift a few hours later, bringing it near the coast of eastern Cuba. Despite Isaac's northwestward motion away from Hispaniola, much of the moisture associated with the system remained entangled in the mountains of the island, causing drenching rainfall to continue over most of that day.

The cyclone moved roughly parallel to the northern coast of Cuba overnight at a faster clip than before, and so had difficulty recovering convection near its center. Additionally, Isaac's circulation remained somewhat entangled with a larger low-pressure system over the northwest Caribbean, which had an additional weak center south of Isaac, near Jamaica. Not only did this system cause dry air and moderate shear to invade the southern portion of Isaac's circulation, but the two lows, under the Fujiwhara effect, began to orbit slightly about a common center, causing the tropical storm's motion to shift toward the west early on August 26.

The atmospheric conditions steadily improved during the day, and Isaac experienced some strengthening as it approached the Florida Keys. A distinct eyewall finally formed later that day, and the windfield of Isaac broadened, causing tropical storm conditions to sweep over much of southern Florida and northern Cuba during that day. Overnight, the circulation jogged to the north, and appeared to be closer to the deepest convection than previously, but the maximum winds remained nearly constant at strong tropical storm intensity. Though the winds did not change, the central pressure dropped rapidly overnight and into August 27, falling below 985 mb that afternoon as Isaac moved northwest into the central Gulf of Mexico.

No longer under the influence of the low pressure area to its north or the ridge to its northwest, Isaac slowed considerably later that day, though maintaining a general northwestward motion. The outer circulation of the cyclone was impressive, with prominent outflow and powerful rain bands, one of which swept across the northern Gulf that morning. However, the internal structure of Isaac never fully coped with the dry air invading the circulation. A broad eye occasionally formed, but the eyewall was never closed completely. Despite this, the winds increased just enough during the late morning hours of August 28 for Isaac to be upgraded to a hurricane.

Isaac strengthened slightly even as it encountered land; the system reached its peak intensity of 80 mph winds and a pressure of 968 mb that night. At around 7:45 pm, Isaac made landfall in southeastern Louisiana. However, the cyclone barely moved overnight, and in fact briefly emerged over water early on August 29 after wobbling westward. By the morning, the hurricane had resumed a slow northwest motion, bringing it over Louisiana again, slightly farther westward than before.

Isaac weakened very slowly over the following day, losing hurricane status that afternoon. The cyclone moved slowly northwest, its lack of forward speed causing flooding rains throughout Louisiana, Mississippi, and Alabama, with localized amounts over 20 inches. The system maintained tropical storm strength through the morning of August 30, at which point it was moving through northern Louisiana.

It weakened to a tropical depression that afternoon, and became asymmetric, as is typical of systems tracking inland. Most of the thunderstorm activity remained on the north side of the center, though more scattered rain bands to the south and east still persisted. The center moved over Arkansas later that day, and crossed in Missouri on August 31. It took a turn to the east later that day, and dissipated as it combined with a front on September 1.

The remnants of Isaac and the tropical moisture it carried northward caused shower activity throughout the Ohio River Valley and the northeast during the following few days.

Isaac's legacy throughout the Caribbean and U.S. was heavy rainfall. Particularly near its final landfall in Louisiana, the cyclone moved very slowly, and therefore had the opportunity to dump flooding rains over a huge swath of the southern U.S. However, it also provided needed rain to states farther north.



Isaac as a minimal hurricane. Even at peak strength, it had a poorly defined inner core.



Track of Isaac.

Thursday, August 16, 2012

Hurricane Gordon (2012)

Storm Active: August 15-20

A strong tropical wave and associated low pressure system emerged off of Africa on August 9 a produced an organized area of showers and thunderstorms. However, it tracked west-northwest, and took a path slightly north of the Cape Verde Islands. Soon after, unfavorable conditions caused convection associated with the system to diminish.

However, the low continued to move across the open Atlantic, and emerged into a area of lower wind shear on August 14. Later that day, significant shower activity developed, and the circulation became better organized. On August 15, the satellite presentation was sufficiently impressive to merit the classification of the system as Tropical Depression Eight, despite surface pressures remaining somewhat high in the area.

The cyclone was at that time well east of Bermuda, and it moved north as it exploited a weakness in the Bermuda high ridge. By early on August 16, convection had markedly increased, and the system was upgraded to Tropical Storm Gordon. Deep convection continued to concentrate about the center during that day, and Gordon strengthened as it curved to the east under the influence of a subtropical ridge.

By late that night, Gordon had reached an intensity of 70 mph winds and a pressure of 995 mb. However, under the influence of moderate shear, it weakened slightly that night.

Gordon reasserted itself early on August 18, and a prominent eye feature appeared, resulting in the upgrade of Gordon to a hurricane. The cyclone continued to strengthen throughout the day as it moved west, becoming a Category 2 hurricane that evening as it approached the Azores, and reaching its peak intensity of 110 mph winds and a pressure of 965 mb late that night.

By the morning of August 19, Gordon began to interact with a trough to its northwest, and took a turn to the north, adopting a east-northeast motion. In addition, an increase in shear and a decrease in sea surface temperatures caused Gordon to steadily weaken that day. Conditions in the Azores began to deteriorate during the evening.

Early on August 20, Gordon made landfall in Santa Maria Island of the Azores as a minimal hurricane, where winds gusts in excess of 80 mph were recorded. The convection in the southern portion of the system had degraded significantly by this time, and the cyclone was in the midst of extratropical transition. By later in the morning, Gordon had weakened to a tropical storm and lifted out of the Azores. The system was declared extratropical that afternoon, and dissipated a few days later. Minor damage and no injuries were reported in the Azores.



Gordon at peak intensity moving rapidly east over the central north Atlantic.



Track of Gordon.

Friday, August 10, 2012

Tropical Storm Helene (2012)

Storm Active: August 9-11, 17-18

On August 5, a tropical wave and associated low pressure system moved off of Africa. It exhibited small areas of thunderstorm activity and increased in organization over the following days as it moved west over the tropical Atlantic.

During the afternoon of August 9, convection became concentrated at the center of the system, and it became Tropical Depression Seven. From its genesis, the system interacted with dry air, and this inhibited development of shower activity north of the center later that day and into August 10. The persistent ridge over the north Atlantic caused Seven to track westward rapidly.

By August 11, the system was moving so rapidly that an organized center of circulation no longer existed in conjunction with the system. It was thus downgraded to a tropical wave. The wave continued to move into the Caribbean, producing some heavy rain around the Windward Islands. Convection increased as the wave approached central America, but land prevented redevelopment.

By August 14, the remnants of Seven had moved over land, but the system took a turn to the northwest, ultimately allowing it to emerge into the Bay of Campeche on August 16. Convection increased in association with the system, and a center of circulation became defined on August 17, causing the system to be upgraded to Tropical Storm Helene, already at its peak intensity of 45 mph winds and a minimum pressure of 1004 mb.

The system moved northwest towards the coast of Mexico, and weakened upon interaction with land. Convection remained scant, and the center was hard to identify on satellite imagery during the morning of August 18. The cyclone made landfall during the late morning hours in Mexico, bringing heavy rainfall to some inland areas.

The cyclone weakened to a tropical depression that evening, and dissipated completely late that night, its circulation destroyed by the mountains of Mexico. Overall, the impact of the system was heavy rain, particularly in Trinidad and Tobago (as a tropical wave), and the coast of the Bay of Campeche.



Helene at peak intensity after reforming in the Bay of Campeche.



Track of Helene, including its path through the Caribbean as a non-tropical system (non-tropical points indicated by triangles).

Saturday, August 4, 2012

Tropical Storm Florence (2012)

Storm Active: August 3-6

On August 3, a low pressure center center formed in association with a tropical wave near the Cape Verde Islands. The system was already producing a wide area of thunderstorm activity, and it organized rapidly, perhaps fueled by a local surface water temperature anomaly. In any case, the circulation was already very well-defined by the evening of August 3, and it was therefore classified Tropical Depression Six.

At that time, Six was southwest of the Cape Verde Islands. It adopted a west-northwest motion, and, following a more northerly path, encountered some dry air on August 4. However, the satellite presentation indicated that the system's winds had reached tropical storm force and the depression was named Tropical Storm Florence.

A well-defined center of circulation as well as an eyewall developed that day, and Florence strengthened into August 5, reaching its peak intensity of 60 mph winds and a pressure of 1000 mb. Soon after, however, dry air infiltrated the system, and convection decreased markedly. By that evening, the center was exposed, and the circulation already showed signs of elongation. Florence weakened quickly as a ridge to its north turned the cyclone westward. By the morning of August 6, there was no convection within 50 miles of Florence's center. It was downgraded to a tropical depression and a remnant low shortly after.

The remnants of Florence tracked across the Central Atlantic over the next few days, producing small areas of convection, despite the stable air environment. It degenerated further into an open wave on August 8. By August 12, the system had dissipated completely. Florence, becoming a tropical storm on August 4, was tied second-earliest formation of a sixth named storm in the Atlantic with 1936, behind only the most active season on record, 2005.



Tropical Storm Florence shortly after developing into a tropical storm.



Track of Florence over the Eastern Atlantic.

Thursday, August 2, 2012

Hurricane Ernesto (2012)

Storm Active: August 1-10

On July 29, a weak low pressure center was identified southwest of the Cape Verde Islands. Some isolated showers and thunderstorms were associated with the system, and it was monitored for development. The system began to organize over the following days, though its circulation was inhibited somewhat by its proximity to the equator. The wave remained embedded in the ITCZ until July 31. However, on that date, the center shifted northward, allowing the low to organize further.

On August 1, there was enough convection accompanying the center to declare the system Tropical Depression Five. El Ninó-related shear from the northwest affected the system immediately, pushing dry air into the northern portion of the system. Meanwhile, strong steering currents were pushing Five westward at a fast clip, causing it to approach the Windward Islands on August 2. The circulation became more organized that day, and the confirmation of tropical storm force winds east of the center of circulation by aircraft reconnaissance resulted in the tropical depression being upgraded to Tropical Storm Ernesto.

Ernesto's initial winds were 50 mph, but its convection diminished overnight, causing it to weaken slightly as it crossed the Windward Islands early on August 3. The center reformed farther to the south later that morning, and the system began to reorganize as it entered the Caribbean. Ernesto's intensity fluctuated under marginally favorable ambient conditions, but the outflow and satellite presentation consistently improved overnight and into August 4.

As the storm moved through the central Caribbean that day, it decelerated slightly, though still bearing mostly west. Its outer bands skirted Puerto Rico and the southern Dominican Republic that evening. Over the next day, Ernesto lost some organization, its center became less well-defined, possibly due to dry air invading the cyclone. Ernesto's rapid motion also contributed to its disorganization.

On August 5, the weakened Ernesto lost any vestige of organized central convection, but still maintained low-end tropical storm intensity. It changed direction, now heading due west towards Central America.

That night, however, Ernesto continued its erratic behavior, and rapidly organized. Bursts of convection soon covered the center, and the cyclone's forward motion showed. During the morning of August 6, Ernesto quickly strengthened, and the beginnings of an eye became apparent on satellite imagery. Additionally, the system took a turn to the west-northwest.

That night, Ernesto's outer bands swept over Honduras, causing locally heavy rainfall and tropical storm force wind gusts. Dry air was evident in the interior of the northwest quadrant overnight, but the system's outflow continued to improved and the central pressure decreased into August 7.

By later that day, Ernesto finally became sufficiently organized to be upgraded to a hurricane. It continued to strengthen until that evening, reaching its peak intensity of 85 mph winds and a pressure of 980 mb just as it made landfall in the Yucatan Peninsula just north of the Mexico-Belize border at about 11:15 p.m. EDT, or 10:15 local time.

Over land, Ernesto caused localized flash flooding and heavy winds throughout the Yucatan and the neighboring parts of Central America. The system was downgraded to a tropical storm early on August 8, and continued to weaken through the morning, as it moved west toward the Bay of Campeche.

Late in the afternoon, Ernesto's center emerged over water, and the cyclone immediately began to restrengthen, despite its proximity to the Mexican coast. Its organized circulation allowed moisture to be drawn into the center, and the winds increased rapidly, bringing the system to near hurricane strength early on August 9, as torrential downpours continued over land. Under the influence of a ridge to its north, Ernesto turned west-southwest and made its final landfall in Mexico around noontime that day.

The cyclone maintained its structural integrity quite well over land, and so caused rain and tropical storm force winds through the remainder of the day and into early August 10. It was downgraded to a depression and dissipated later that morning. The remnants of Ernesto moved into the Eastern Pacific basin soon after, where they contributed to the development of Tropical Storm Hector.



Ernesto as a hurricane before landfall in Mexico.



Track of Ernesto.

Sunday, June 24, 2012

Tropical Storm Debby (2012)

Storm Active: June 23-27

Throughout mid-June, a trough of low pressure extending through the central Caribbean Sea caused limited shower activity across the regions of Cuba, Jamaica, and parts of Central America. By June 18, this trough had become associated with a low pressure center near the coast of Nicaragua, and more concentrated convective bursts soon appeared. Shear remained strong over the region, however, and no distinct circulation formed.

The system moved to the northwest over the following days, and the low associated with it dissipated. However, conditions gradually improved for development as it crossed into the Gulf of Mexico, causing widespread flooding in Cuba and southern Florida. On June 21, a well-defined low formed just west of the convection in the central Gulf of Mexico. Organization continued, and by June 23, a huge area of strong thunderstorm activity stretched from the Florida panhandle all the way to western Cuba. Later that day, a closed circulation formed, and the system was classified as a tropical cyclone. Having already attained gale force winds, it was named Tropical Storm Debby, with initial winds of 50 mph.

Shear out of the southwest kept all convection and tropical storm force winds to the east of the center overnight and into June 24. Additionally, the motion was a highly uncertain one, as multiple center and vortices were evident on satellite imagery. However, the general trend was a drift north, and conditions deteriorated in Florida as the outer bands of Debby swept across the coast.

The central pressure of Debby dropped on June 24, and the maximum winds increased, with the circulation becoming more symmetrical. At this time, Debby reached its peak intensity of 60 mph winds and a minimum pressure of 990 mb. However, the trend in shower activity was quite the opposite; the area near the center became completely devoid of convection by late that night, and all significant moisture was displaced towards eastern Florida. This lack of convection eventually caused the system to weaken.

Meanwhile, due to the presence of a powerful anti-cyclonic ridge to its north, Debby was nearly stationary, meandering in small loops, but making little overall progress in any direction over the next couple days. On June 26, a trough over the United States caught Debby and caused it to adopt a slow east motion. The only rain bands of Debby were still only to the northeast of the center, but flooding rains continued over portions of Florida, Georgia, and South Carolina that day.

That evening, the center of Debby made landfall in northwest Florida with 40 mph winds and quickly weakened to a tropical depression. Over land, the circulation of Debby became noticeably elongated from northeast to southwest, and the system adopted an unusual southeast motion. However, the center reformed farther east during the morning of June 27, causing a more rapid turn to the east. The cyclone exited the coast that morning. Its circulation was becoming too elongated to maintain tropical characteristics, however, and it was declared extratropical that afternoon. Debby's remnants initially achieved some organization on June 28, but dry air again permeated the low. It was absorbed a few days later.

Forming on June 23, Debby was the earliest fourth named storm of a season ever to form, surpassing the previous record set by Dennis in 2005, which formed on July 7. This record stood until the formation of Tropical Storm Danielle in 2016. Debby caused flooding throughout parts of the Caribbean and southeast U.S., causing 2 fatalities.



Tropical Storm Debby at peak intensity. Even at its best organization, the cyclone did not have convection circumnavigating the center.



Track of Debby.

Wednesday, June 20, 2012

Hurricane Chris (2012)

Storm Active: June 19-22

On June 17, a circulation began to form from a front situated over the western Atlantic. The low gradually deepened over the following day, and caused scattered shower activity in the area around Bermuda. As the low pressure center became disentangled from the front to its east, its circulation became better organized, through convection decreased.

By June 19, gale force winds were being generated near the low's center and the low assumed some tropical characteristics, though the center remained devoid of cloud cover. Later that day, as the system moved to the northeast, the northwest quadrant developed more significant convection, ultimately allowing it to be classified as Tropical Storm Chris.

Chris tracked generally eastward with no significant change in strength over the next day, but convection became more evenly distributed about the center. The motion of the system increased on June 20, and it underwent strengthening as outflow improved. During the morning of June 21, Chris began its turn to the north as it orbited a upper-level low to its north. By this time, a distinct eyewall and the beginnings of an eye had formed. As a result, the cyclone was upgraded to a hurricane, just above the 40°N latitude line. It soon reached its peak intensity of 75 mph winds and a pressure of 987 mb.

Chris began to move over cooler waters later that day, and weakened as its convection diminished, becoming a tropical storm again. It transitioned into an extratropical cyclone on June 22. Having formed into a tropical storm at 39.3°N, the cyclone was the second most northerly forming Atlantic storm on record. Forming on June 19, it also was among the earliest occurrences of a third named storm of the season in the Atlantic basin.



Hurricane Chris at minimal category 1 intensity. At its peak, Chris attained a distinct eye feature, despite only marginally favorable conditions.



Track of Chris.

Saturday, May 26, 2012

Tropical Storm Beryl (2012)

Storm Active: May 25-30

On May 21, a low pressure system developed near the eastern coast of the Yucatan Peninsula. From there, it slowly moved northeast, and accumulated some more significant cloud cover on May 23. Initially, conditions were very hostile for development, but as the low tracked farther northeast, the upper-level winds slowly abated, and the circulation became better organized. On May 24, the system moved over Cuba, bringing some rainfall to it and neighboring Caribbean islands. Later that day, the western edge of the system passed over the Florida keys.

The center of the low became much better defined late that night and into May 25, and the convection increased markedly, though mainly in the northern and eastern quadrants. The system continued to move northeast, but its motion slowed as it encountered strong ridges of high pressure to its north and east. Late that evening, thunderstorm activity flared up near the center, and the low was upgraded to a tropical cyclone. However, due to its proximity to an upper-level low and its relatively broad circulation, it was designated Subtropical Storm Beryl.

Beryl's motion reversed early on May 26 as it adopted a west-southwest track. Its convection remained limited, as dry air was invading the center from the southeast, but the cyclone became more symmetrical in appearance as it moved toward the U.S. coastline. Over the following day, Beryl became slightly more organized, and experienced modest intensification late that night. The system veered more to the west early on May 27 and accelerated somewhat. More powerful rain bands developed that same morning, but the center remained broad as the cyclone approached the Florida coastline.

Later that day, the windfield contracted and intense thunderstorm activity appeared near the center of circulation, meriting a reclassification of Beryl as a fully tropical cyclone. Despite its proximity to land, Beryl was rapidly strengthening during the afternoon of May 27. It nearly reached hurricane strength, achieving its peak intensity of 70 mph winds and a minimum central pressure of 992 mb late that night before making landfall near Jacksonville Beach, Florida very early on May 28.

The convection quickly deteriorated over land that morning, but the circulation remained intact, and the rainbands near the coastline still caused heavy rain, as they tapped into Atlantic moisture. The winds fell quickly, however, and Beryl was downgraded to a tropical depression by late morning. Over the following day, Tropical Depression Beryl tracked further inland, crossing into Georgia, and then slowed in forward speed as the ridge to the north weakened. Soon, the typical steering patten emerged, and Beryl turned to the northeast.

On May 29, the convection associated with the tropical depression began to be entangled with a front to the north. As the depression crossed over land into South Carolina, all of the remaining rain bands were displaced significantly poleward. Despite this, Beryl's circulation deepened, but this was probably due to the beginning of extratropical transition. During the afternoon of May 30, as Beryl regained tropical storm intensity near the North Carolina coast, it became post-tropical. Soon after, it was absorbed by a front.

Beryl was the strongest preseason cyclone since 1972, and, since it was the second named storm to form before the start of the official hurricane season (begins June 1), 2012 was only the third Atlantic hurricane season in history to have two preseason storms, after 1887 and 1908.


Beryl near peak intensity, just before making landfall in Florida. Beryl was one of the strongest U.S. landfalling off-season storms in history.


Track of Beryl.

Sunday, May 20, 2012

Tropical Storm Alberto (2012)

Storm Active: May 19-22

On May 17, a cold front moved off of the U.S. East Coast. The southern end of the front stalled off of the coast of South Carolina, and a low pressure trough developed. Over the following two days, the area of low pressure remained nearly stationary, and began to acquire tropical characteristics, which, during the afternoon of May 19, were enough to classify it as Tropical Storm Alberto, the first tropical cyclone of the 2012 Atlantic Hurricane Season.

The system drifted slowly to the southwest and west-southwest over the following day, and the devtelopment of modest convection near the center late on May 19 allowed the system to reach its peak intensity of 60 mph winds and a minimum central pressure of 995 mb. Early on May 20, the shear from a low pressure system to its northwest intensified, and the circulation became exposed mid-morning, weakening Alberto. As the day went on the blocking pattern that was in place over the U.S. northeast and the northwest Atlantic Ocean receded, and Alberto quickly shifted its motion from southwest to south to east and then northeast.

Accelerating as it did so, the system weakened further to a minimal tropical storm and then a tropical depression on May 21. By May 22, the convection was strongly displaced from the circulation center, and Alberto became extratropical later that morning, while southeast of Cape Hattaras, North Carolina. The next day, it merged with a front coming off of the U.S. Alberto was the earliest forming cyclone in the Atlantic basin since Ana in 2003, and 2012 was the first year on record such that a preseason cyclone formed in both the Atlantic and East Pacific Basins.


Alberto near peak intensity on May 19.


Track of Alberto.

Professor Quibb's Picks-2012

My personal prediction for the 2012 Atlantic Hurricane Season is (written May 16, 2012):

15 cyclones attaining tropical depression status
13 cyclones attaining tropical storm status
5 cyclones attaining hurricane status
3 cyclones attaining major hurricane status

These predictions are near normal for an Atlantic Hurricane Season, with the number of predicted tropical storms slightly above average and the numbers of predicted hurricanes and major hurricanes near their respective long-term averages.

Despite being near the average, this prediction is low relative to recent years, in which 7 out of the last 10 seasons have had 15 or more named storms, and have included some of the most active on record. This active period reflects a theoretical phenomenon called the Atlantic Multidecadal Oscillation (AMO), a cycle involving sea surface temperatures that has a period of about 60 years. The whole of the 2000's was in the "active" period of the oscillation, which is expected to persist for at least another five years.

Despite this, an El Nino event is expected to develop in the coming months, characterized by a stronger jet stream, and areas of strong wind shear across the Atlantic basin. El Nino tends to inhibit cyclone development, and this is reflected in the forecast. The exact intensity of the season is dependent on how quickly the El Nino develops and its intensity, but, even in the case of a strong El Nino event, there will be portions of the season where wind shear temporarily abates, possibly allowing strong cyclones to form.

Below, my anticipated risk factors for four major regions of the Atlantic basin are listed. The risk index runs from 1 meaning very low potential to 5 being very high potential.

U.S. East Coast: 2
Despite high sea surface temperatures off of the U.S. coast following a mild winter, the risk is low for an East Coast landfall this year. With a strong jet stream and a weak Bermuda, the steering currents will strongly push cyclones out to sea, which for the most part will miss the East Coast. Bermuda might not be so lucky, however. The greatest risk for East Coast states is from a system on the Gulf side tracking over land and then up parts of the east coast. Though this minimizes the risk of strong winds, flooding may still be the result if such a system combines with an existing front or extratropical system (for example, Tropical Storm Lee of 2011).

U.S. Gulf Coast/Northern Mexico: 3
The Gulf coast has not experienced a hurricane landfall since 2008, but this may change in the 2012 season. The Gulf waters will again be very warm, and the blocking pattern across the Gulf will not be as strong as in previous years. Also, as is characteristic with the El Nino, any troughs across the Gulf are likely to be impermanent, and it is likely that at least a few cyclones will enter the Gulf.

Yucatan Peninsula and Central America: 4
Central America has been hit by a number of cyclones over the past few years, and this shows few signs of abating as we enter the 2012 Atlantic Hurricane Season. With mountainous terrain over much of Nicargua, Honduras, and Belize, flash flooding is a major concern, and the warm and moist southwest Caribbean is a prime location for tropical cyclone formation. I expect at least two landfalls, though they may be by weak storms.

Caribbean Islands: 3
The Caribbean Islands are always at risk for hurricane damage, as they lie in the center of many common tropical cyclone paths. However, the risk for damage is not as high as last year, as any cyclones will probably be more fast moving in areas of the Caribbean, and the scenario of a Cape Verde type hurricane is not as likely as in the previous few years.

Overall, 2012 will be a moderate season in terms of formation, but there is still a fair likelihood for a devastating hurricane, which would most likely effect Mexico and the U.S. Gulf coast.

Wednesday, May 16, 2012

Hurricane Names List-2012

For the Atlantic Basin, the hurricane names list for 2012 is as follows:

Alberto (used)
Beryl (used)
Chris (used)
Debby (used)
Ernesto (used)
Florence (used)
Gordon (used)
Helene (used)
Isaac (used)
Joyce (used)
Kirk (used)
Leslie (used)
Michael (used)
Nadine (used)
Oscar (used)
Patty (used)
Rafael (used)
Sandy (used)
Tony (used)
Valerie
William

This list is the same as that of the 2006 Atlantic Hurricane Season, as no cyclone names were retired that year.

Tuesday, May 8, 2012

Spinors and Applications

Spinors are a type of mathematical quantity that expand on the notion of regular vectors and vector rotations. In the case of regular vectors in a space, it is a basic property that rotating a vector about any axis 360º will bring it back to its starting point. In other words, if we apply any 360º rotation r to a vector v, then

rv = v,

which can be simplified to r = 1, expressing the fact that the application of an arbitrary 360º rotation r will leave a vector unchanged. However, the key property of a spinor, or a space of spinors, is that a 360º rotation does not transform a spinor into itself, but rather its negative. Only a rotation of 720º, corresponding to two full rotations, will correspond to leaving a vector unchanged. Therefore, if s is a 360º rotation in a spinor space, then

s = -1, and
s2 = 1, i.e. the application of s twice returns any spinor to its original state.

To clarify, the mathematical objects that are actually spinors are elements, not rotations (strictly speaking), of a certain space. The above spinor rotation s expresses the way through which spinors are transformed into their respective negatives, and is not itself, in this context, a spinor. However, in reality, the set of possible rotations on a space often is itself a space, and the example provided below falls under this category.

One geometrically accessible example of a spinor is a quaternion. As discussed elsewhere, quaternions are an extension of the complex numbers to four dimensions, with four basis elements 1, i, j, and k.

The confusion arising from whether spinors are elements or rotations is related to the dual interpretation of quaternions: either as elements of a four dimensional space, or as rotations of three-dimensional space. However, the full "meaning" of i, j, and k, are not captured by the three-dimensional rotation analogy. As the orientation of the three-dimensional space is shifted through, for example, i2, returning to its former position, the space "remembers" that there has only been one rotation, and the orientation now is the negative of what it was before. In this sense, there is an added complexity to the ambient three-dimensional space-it has a way of "knowing" how many rotations have been performed.

The mathematical properties of spinors find a physical application in the area of quantum physics. It happens that the rotational properties of some particles are identical with those of spinors, in that a rotation of 360° produces not the same quantum state, but a "conjugate"-many properties are the same, but some are opposite to what they originally where. In the same way as spinors, they are restored to their former state through only a 720° rotation.

The family of particles that exhibit the properties of spinors are those with spin 1/2. One example is the electron. The electron, of course being very common, is then important to modeling particle interaction phenomena, and the knowledge of its properties is thus equally important. Thus spinors find an application in the study of subatomic particles.



Sources: http://en.wikipedia.org/wiki/Spinor, The Road to Reality by Roger Penrose, Chapter 11

Monday, April 30, 2012

Quaternions

The history of mathematics began with the natural numbers. These numbers: 1,2,3... have a simple universal interpretation, i.e. counting a number of objects. This was followed by the use of fractions, or rational numbers, most notably by the Ancient Greeks (note that negative numbers, and even zero, were only accepted into use after fractions in most cultures) to accommodate the splitting of objects into portions. Gains and losses in finances contributed to the general acceptance of negative numbers (and zero) while the diagonals of squares forced square roots, and therefore irrational numbers, upon humanity.

The rational and irrational numbers form the reals, which collectively describe any point on the one-dimensional line and the manipulation of these quantities includes all of one-dimensional geometry. Similarly, taking the square root of a negative number brings about the introduction of i and the imaginary numbers (the basic description of which can be found here). Combining these with the real numbers, the complex numbers are formed, and operations on such numbers can be used to describe two-dimensional mappings of the plane. In fact, the general 2-manifold can be treated as having complex coordinates.

It follows naturally that one would next wonder whether a similar system for three dimensions exists. This same question was pondered by the mathematician William Hamilton in during the mid-19th century. He failed to invent a three-dimensional system that satisfied certain necessary constraints, including a way to multiply consistently, (a feat that is now known to be impossible) but created a similar four-dimensional system. Rather than "stumbling upon" a new number system through the application of simple operations to known numbers, (division of integers, square roots of negative, etc.) Hamilton simply defined two quantities, in addition to 1 and i, known as j and k. These new numbers were to be connected through the famous Brougham Bridge Equations,

i2=j2=k2=ijk=-1

The general quaternion is simply an arbitrary addition of multiples of the identity, 1, i, j, and k, namely a number of the form a+bi+cj+dk, where a, b, c, and d are real numbers (note that this is the same form as a complex number (a+bi).

Quaternions, like the complex numbers, follow certain rules that make them a consistent group. For any quaternions a, b, and c, the following axioms are satisfied, and can be verified through simple real number multiplications and additions. Among these axioms are the associativity of addition and multiplication,
a+(b+c) = (a+b)+c,
a(bc) = (ab)c,
the identity formulae of addition and multiplication,
a+0 = 0+a = a,
a1 = 1a = a,
and each quaternion a ≠ 0 has a unique inverse a*, such that
(a)(a*) = (a*)(a) = 1

The above equations qualify quaternions as a group† . In addition, quaternion addition is commutative, i.e. a+b = b+a. However, quaternion multiplication is not commutative, and in general ab ≠ ba. This fact, which is taken as obvious in arithmetic, may seem to make the quaternions unusable in application. Nevertheless, there is a redeeming factor, namely that the multiplication of quaternion basis elements is a special type of non-commutative: anti-commutative, this meaning that ab = -ba. Note that this does NOT generalize to all quaternions, ab is equal to -ba only when a and b are i, j, or k.

†The term group in this context, and in later appearances, is a slightly more casual and abstract use than the strict mathematical group, which requires a more rigorous proof. The above identities outline the framework of a group, together with the proviso that operations on elements of the quaternions (in this case addition and multiplication) produce only other quaternions. Assuming that the coefficients of 1, i, j, and k can vary over all real numbers, this is clearly satisfied.

The multiplication of quaternion basis elements is also cyclic, meaning that the product of any pair among i, j, and k will produce the third (or its negative). The six resulting identities are
ij = k
ji = -k
jk = i
kj = -i
ki = j
and
ik = -j

The reader may be curious as to how these mysterious abstract quantities define four-dimensional geometry. To those familiar with elementary vector algebra, the use of i, j, and k may recall that they seem eerily similar to the unit vectors i, j, and k, which point along the x-, y-, and z-axes, respectively.



An illustration of the unit vectors i, j, and k and their positions in relation to the Cartesian three-dimensional coordinate axes.

This is no coincidence. In fact, the quaternions are most commonly used in relation to rotations in three-dimensional space, rather than coordinates in four. There is a connection here to the complex numbers, in which multiplication by i represents a 90º counterclockwise rotation. In quaternion geometry, each of the basis elements stands for a rotation in three-dimensional space, keeping the axis on which the element is oriented still. For example, the quaternion j represents a rotation keeping the y-axis constant. However, for i, j, and k, the degree of rotation is 180º, rather than 90º as in complex numbers.

An easy way to visualize such rotations is to visually manipulate a three-dimensional object, specifically something in the shape of a rectangular prism. A commonly used example is a closed book. Start with the cover facing up, and the text oriented right side-up. The x-axis of the three-dimensional cartesian coordinate plane passes through the center of the book from increasing from left to right. Similarly, the y-axis passes through the center, increasing as it moves away from you. Finally, the z-axis pierces the back cover of the book, travels through the center, and then continues upward out of the center of the front cover. If you are having trouble visualizing this, simply try to superimpose the three-dimensional coordinate plane above on the book.

The rotation i is equivalent to rotating the book 180º about the x-axis and ending with the back cover facing upward, but the text, upside-down, in front of you. Similarly, j flips the book onto the back cover, though with text facing right side-up, and the k simply rotates the book through 180º without flipping it, resulting in the front cover, except with the text upside-down. Further experimentation confirms the identity ij=k and others. However, the difference between ji and ij (the first yielding -k, rather than k) is unclear in 3-space. To understand why this is, a type of quantities known as spinors must be considered, see here.

Sources: Road to Reality by Roger Penrose Chapter 11,
http://programmedlessons.org/VectorLessons/vch12/vch12_10.html.