Monday, August 22, 2016

Hurricane Gaston (2016)

Storm Active: August 22-September 3

During mid-August, a vigorous disturbance developed well inland over the continent of Africa. The potential development of this system once it emerged into the Atlantic was identified by August 17, three days before it encountered water. By the time it had moved over the ocean on August 20, a small area of thunderstorm activity persisted near its circulation center. Organization continued over the next few days and Tropical Depression Seven formed over the eastern Atlantic during the afternoon of August 22. Initially, conditions were favorable for development as the cyclone moved westward, and it quickly strengthened into Tropical Storm Gaston. The one inhibiting factor for rapid intensification over the following day was the Saharan dry air mass located to the north. Some of this air entered the circulation on the 23rd, forming a large eye-like structure near the center of circulation. However, Gaston managed to develop a central dense overcast shortly thereafter and became a strong tropical storm. Meanwhile, the system took a turn toward the northwest. As it was nearing hurricane strength on August 24, wind shear increased, slowing development. Nevertheless, the system overcame increasing upper-level winds out of the southwest to achieve hurricane status early on August 25, becoming the third hurricane of the 2016 Atlantic season.

However, the deteriorating atmospheric conditions soon disrupted Gaston's circulation, weakening it back to a tropical storm later that day. Squeezed between a upper-level low (the source of the shear) and a ridge to its north, the system moved rather quickly toward the northwest for the remainder of that day and gradually weakened. The next day, shear abated and the tropical storm began to reorganize. As a result, strengthening began once again by the morning of August 27. While Gaston was still moving northwestward, its forward speed slowed considerably that day. An eye feature developed overnight, and steady strengthening continued through the morning of the 28th, bringing the cyclone to category 2 hurricane status. That evening, the system slowed to a standstill and became the first major hurricane of the season. Just afterward, it reached an intensity of 120 mph winds and a pressure of 957 mb.

By August 29, Gaston had gained enough latitude to be affected by the mid-latitude westerlies and began a turn toward the northeast. The cyclone came off its peak intensity that day as the eye clouded over temporarily. However, once the eyewall replacement cycle was complete that night, a very large, symmetrical eye appeared on satellite imagery. Therefore, Gaston strengthened once again during the day of August 30 and regained major hurricane status that night, matching its peak winds of 120 mph and beating its former minimum pressure by achieving a mark of 956 mb. However, cooler water and increasing shear finally began to take their toll the next day and a steadier weakening began as Gaston accelerated off to the east-northeast. By the evening of August 31, the winds had decreased to category 1 intensity and the convection had become asymmetrical. This is typical of system beginning to lose tropical characteristics, and the trend continued into September 1, by which time only the northern side remained of the eyewall.

The remaining thunderstorm activity became decoupled from the center that evening as the cyclone took another turn toward due east. On September 2, Gaston weakened to a tropical storm and moved into the Azores. Later that day, however, most of the cloud tops disappeared, leaving a bare circulation. As a result, the system became post-tropical early on September 3. It was absorbed by a large extratropical system about one day later.

Hurricane Gaston achieved its second peak intensity on August 30, at which time its structure included a very large eye (above).

Gaston was the second tropical cyclone of the 2016 season to affect the Azores, after Hurricane Alex in January. For two such storms to affect the Azores in a season is highly unusual.

No comments: