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.

No comments: