Sunday, August 30, 2015

Hurricane Fred (2015)

Storm Active: August 30-September 6

On August 27, the National Hurricane Center began to monitor a disturbance that was still well inland over the west African coast for tropical development. By the time it entered the Atlantic early on August 29, the system already had a well-defined circulation and convection increased immediately. Though initially very far south, the disturbance was able to gain latitude after moving over water due to a weakness in the subtropical ridge north of the Cape Verde Islands. Early on August 30, the circulation became well-defined, indicating the formation of Tropical Depression Six. Surrounded by warm water and low wind shear, the depression intensified shortly after its classification, becoming Tropical Storm Fred just a few hours later. Fred was named at longitude 18.9°W, only the fourth Atlantic system to be named east of 19°W.

The system continued to strengthen quickly during the day, exhibiting healthy outflow and a dense area of central convection. Meanwhile, its outer bands began to affect the Cape Verde Islands, bringing wind and rain as well as a storm surge to the islands. Overnight, Fred began to develop an eye, and was upgraded to a hurricane. The system began to pass through the Cape Verde Islands as a category 1 hurricane during the morning of August 31, becoming the strongest known to affect this area for over a century. While near the islands, Fred reached its peak intensity of 85 mph winds and a central pressure of 986 mb. Immediately after passing the northern islands that afternoon, however, the system encountered less favorable conditions including higher shear and lower atmospheric instability. Thunderstorm activity diminished rapidly overnight, and Fred weakened to a tropical storm.

The system held its own during the day of September 1 even as sea surface temperatures declined. Meanwhile, the ridge north of Fred began to rebuild, causing a more west-northwesterly motion. During the evening, however, Fred moved into an area of powerful wind shear out of the west that tore the entire convective canopy away from the surface circulation. Only a swirl of clouds was left of the system during the morning of September 2, with any significant thunderstorm activity now displaced over 100 miles to the east. Despite hostile conditions, a little bit of activity managed to redevelop near the center of Fred late that afternoon and it was able to maintain tropical cyclone status. In fact, it even strengthened slightly early on September 3. A cycle began that day in which Fred would lose its thunderstorm activity in the face of strong upper-level winds, recover with a new convective burst, and lose it again in a few hours. In this way, the system clung to life as a tropical cyclone for two full days as it tracked slowly west-northwestward towards warmer waters. Later on September 6, the system began to move northward into a weakness in the subtropical ridge and slowed its forward motion. Later that day, Fred weakened to a tropical depression. On September 7, though shear relaxed slightly, the circulation became elongated and soon lost its definition. The system dissipated late that afternoon.

This image shows Hurricane Fred at peak intensity before it entered a region of more stable air and weakened.

Fred formed almost immediately after entering the Atlantic and passed directly over the Cape Verde Islands as a category 1 hurricane, an extremely rare event.

Tuesday, August 25, 2015

Tropical Storm Erika (2015)

Storm Active: August 24-29

Around August 20, a tropical wave moved off of the African coastline. It passed just south of the Cape Verde islands shortly afterward, and moved swiftly into the central Atlantic. A low pressure center soon developed along the wave, and it begin to organize in earnest by August 23. The next day, the system acquired enough definition to be considered a tropical cyclone. Since an Atlantic buoy measured gale force winds near the center, it was classified Tropical Storm Erika.

Erika initially struggled with moderate wind shear and dry air aloft. On August 25, the center became exposed as convection retreated into the southeast quadrant. However, thunderstorm activity made a comeback overnight as the system approached the Leeward Islands from the east. Conditions deteriorated in the islands that night, for while Erika still possessed no visible banding features and was poorly organized, it packed a large area of heavy rainfall and thunderstorm activity. The center reformed to the south during the morning of August 27 as Erika entered the Caribbean.

Continuing to struggle with strong shear, the system remained badly organized that day, and though it passed fairly close to Puerto Rico from the south, most of the strongest winds remained in the southeastern quadrant over open water. By the morning of August 28, Erika had turned west-northwestward toward the island of Hispaniola. Later that day, the cyclone moved ashore on the south coast near the border of the Dominican Republic and Haiti, bringing scattered areas of heavy rain and tropical storm force winds. Land interaction weakened and disorganized the system further, making the center tremendously difficult to locate overnight. Though the circulation emerged over water, the system could not recover and Erika dissipated during the morning of August 29 near Cuba. The remnants of Erika continued toward the northwest, bringing rain to Cuba and eventually Florida. Moisture associated with Erika also contributed to thunderstorm activity over the Carolinas during the final days of August.

Erika failed to achieve significant organization due to constant vertical wind shear. The above image shows Erika over the Lesser Antilles.

Erika dissipated on August 29; the last several points of its track indicate the positions of its remnants over the following day.

Tuesday, August 18, 2015

Hurricane Danny (2015)

Storm Active: August 18-24

At the beginning of the third week of August, a the most vigorous tropical wave of the season thus far moved off of the African coastline. The system developed a surface low pressure center almost immediately. The low moved westward, tracking well south of the Cape Verde Islands early on August 16. Within two days, the low had acquired a closed circulation, and was classified Tropical Depression Four. Low shear conditions dominated the central Atlantic at the depression's low latitude (near 10°N) and the system strengthened to Tropical Storm Danny within hours.

The system experienced modest strengthening through the morning of August 19, but the center became exposed during the day as Saharan dry air infiltrated the northern half of the circulation. Meanwhile, a weakness in the ridge north of Danny allowed it to slow its forward motion and turn toward the west-northwest. Overnight, the cyclone's convective structure recovered, and a small area of concentrated thunderstorm activity appeared near Danny's center by the morning of August 20. Later that morning, an eye formed at the center of the system and Danny rapidly intensified into a category 1 hurricane, the first of the season. After its reformation, the structure of the system was very compact: tropical storm force winds extended only about 50 miles from the center, and hurricane force winds only 10 miles.

Surrounded by a low-shear environment and recovered from its encounter with dry air, Danny continued to strengthen over the next day. A period of rapid deepening brought the hurricane to its peak intensity as a category 3 major hurricane during the afternoon of August 21, with sustained winds of 115 mph and a central pressure of 974 mb. After attaining this peak intensity, however, wind shear increased and the system's cloud pattern decayed. Danny had become a minimal hurricane by the afternoon of August 22 and weakened to a tropical storm overnight. Meanwhile, the ridge north of the cyclone reformed, turning its motion back toward the west and increasing its forward speed. On August 23, Danny's center became exposed, and it became a minimal tropical storm that night. The system finally reached the Caribbean the morning of August 24, passing near Guadeloupe now as a tropical depression. By late that morning, Danny had lost its closed circulation and degenerated into a trough of low pressure in the far eastern Caribbean. Its remnants continued to spread areas of heavy rain westward over Puerto Rico on August 25 before fading away.

The above image shows Danny on August 21 when it briefly attained major hurricane intensity.

Danny dissipated on August 24, just as it entered the Caribbean Sea.