Wednesday, October 5, 2016

Hurricane Nicole (2016)

Storm Active: October 4-18

During late September, a tropical wave was traversing the central Atlantic. Strong upper-level winds made it difficult for the system to organize, but by October 1, a low pressure center developed and the disturbance was producing strong winds. It moved northwest over the following few days and slowly organized as conditions became somewhat more favorable. By October 3, thunderstorm activity had become more concentrated. The next day, the system was producing winds to tropical storm force, and the circulation became better defined. Therefore, the system was designated Tropical Storm Nicole late in the morning on October 4.

It exhibited organized banding features and a well-defined circulation through the next day despite moderate shear. Meanwhile, the small system moved generally west-northwest into October 5. The cyclone continued to defy somewhat unfavorable atmospheric conditions and rapidly intensified over the following day, achieving hurricane status during the afternoon of October 6. An eye appeared on satellite imagery at the same time, and Nicole sped to an intensity of 105 mph winds and a pressure of 968 mb. The cyclone then stopped in its tracks and reversed course toward the south by October 7. Wind shear increased drastically and weakened Nicole as quickly as it had strengthened in addition to periodically exposing the center. By October 8, the system was again a fairly weak tropical storm, though vigorous convection bursts appeared intermittently throughout the day.

Nicole once again slowed to a standstill overnight and upper-level winds slowly relaxed. This allowed the system to recover some organization and initiate a slow strengthening trend. Later on October 9, Nicole finally assumed a more typical northward motion after meandering over the open Atlantic for several days. The next day, the system was again a strong tropical storm. However, the system still struggled with some dry air aloft even as it passed over very warm waters. It managed to develop a ragged eye feature during the morning of October 11, and intensification restarted. In fact, the system rapidly intensified into a hurricane that evening, and a category 2 storm by early on October 12. By this point, the cyclone was moving north-northwestward toward Bermuda. Later that day, the eye became quite large and symmetric and the system had outstanding outflow in all quadrants. These new increases in organization merited an upgrade to a major hurricane that evening and Nicole reached category 4 status briefly that night. Its peak intensity of 130 mph winds and a minimum pressure of 950 mb occurred as the system accelerated toward the north and tropical storm force winds began to engulf Bermuda.

Just after achieving peak intensity, the cyclone experienced a huge increase in wind shear, causing significant weakening to begin early on October 13. Nicole's center passed within 10 miles of Bermuda that morning with the cyclone still at category 3 strength, bringing hurricane force sustained winds to the island. By this time, the system was turning toward the northeast and gaining forward speed. Meanwhile, shear and decreasing ocean temperatures quickly weakened Nicole back to category 1 strength. The system acquired some extratropical characteristics the next day, but rather than transitioning fully, it became a sort of hybrid cyclone: the windfield and size of the system were typical of an extratropical system, but the inner core remained that of a tropical cyclone.

Late on October 14, Nicole briefly weakened to a tropical storm, but baroclinic processes reintensified the system the next day to a very powerful north Atlantic hurricane. It also turned toward the east on October 15 and slowed in forward speed. By the next morning, it had maximum winds of 85 mph and a pressure of 958 mb and was generating large ocean swells throughout the north Atlantic. Some weakening ensued the following day, but Nicole remained a hurricane through the morning of October 17. Diminishing ocean temperatures finally caught up to the system that night, purging the remaining tropical characteristics, and Nicole became post-tropical early on October 18. The cyclone remained extremely powerful for the next few days as it interacted with a frontal low and brought unusually strong winds to the southern coast of Greenland before being absorbed.

The above image shows hurricane Nicole at peak intensity shortly before impacting Bermuda. Since Matthew and Nicole both were category 4 hurricanes in October, the 2016 season was the first in recorded history to have two storms of at least category 4 strength in this month.

Nicole's meandering track brought it southward before doubling back toward Bermuda, stalling over the far northern Atlantic, and finally having impacts as far north as Greenland as an extratropical system.

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