Monday, August 29, 2016

Hurricane Hermine (2016)

Storm Active: August 28-September 3

On August 18, a tropical wave just southwest of the Cape Verde Islands began to display scattered shower activity. The system remained rather broad and disorganized during its trek westward across the Atlantic for most of the following week. Throughout much of this period, the wave and associated low were little more than an large swirl of sparse clouds with limited convection farther south. This was due to the influence of the Saharan air mass to the north. Organization increased some around the 22nd and 23rd as the low passed through the Lesser Antilles, but a well-defined circulation did not yet develop. In fact, organization decreased markedly over the following several days, as the convection became scattered across many of the islands of the Caribbean away from the center. It was not until the system passed just south of the Florida Keys on August 28 that it finally acquired the status of Tropical Depression Nine.

The location of the center of circulation remained difficult to spot on satellite imagery through the next day, but convection increased overall, particularly to the south of the center. This caused heavy rains for much of western Cuba. Organization increased markedly on August 30 and the subsequent overnight period: a large area of very deep convection developed over the southeastern Gulf of Mexico surrounding the center and the central pressure dropped appreciably. This did not immediately correspond to an increase in maximum winds, but aircraft data collected the afternoon of August 31 indicated that the system had finally strengthened into Tropical Storm Hermine.

By this time, the cyclone had assumed a track toward the north-northeast due to the influence of a trough over the southeastern United States. At the same time, atmospheric conditions improved, allowing Hermine to take advantage of the very warm Gulf waters and intensify quickly. That evening, the storm developed more organized banding features, with a huge band extended from northwest to southeast of the center. Shortly afterward, this band began to affect the Florida panhandle. By the morning of September 1, heavy rain was moving across western Florida as far south as Tampa. An eyewall appeared later that morning and completed its circumnavigation of the center that afternoon. Hermine became a category 1 hurricane soon after. A few hours later, the cyclone reached its peak intensity of 80 mph winds and a central pressure of 984 mb and made landfall in the Florida panhandle, causing storm surges of several feet, strong winds, and over 5 inches of rain for many areas.

Hermine weakened to a tropical storm overnight and move inland over Georgia by the morning of September 2. The cyclone traversed South Carolina that afternoon and entered North Carolina that evening. All the while, it weakened and gradually lost tropical characteristics. This did not prevent it from causing flooding rains from the Carolinas north to the Delmarva peninsula by the morning of September 3. Around this time, Hermine became post-tropical and moved east-northeast back over water. There it reintensified some and developed winds near hurricane force. While storm surge remained a threat to the coast, the system moved farther away from land over the next day, lessening direct impacts. Early on September 5, Hermine changed course, moving first slowly northward, and then toward the west-northwest by the afternoon. Though it remained post-tropical, the system generated some shallow convection to the north and west of the center, bringing continued shower activity to coastal New England. The circulation began to spin down on the 6th, and while it moved a little closer to the coast, impacts diminished. By September 7, the weakening cyclone had switched direction and was moving generally east-northeast. It dissipated shortly after.

The above image shows Hermine shortly after it became a hurricane.

Hermine continued to affect the U.S. east coast even after transitioning to a post-tropical cyclone (post-tropical track points are indicated by triangles).

Sunday, August 28, 2016

Tropical Depression Eight (2016)

Storm Active: August 28-31

On August 26, an area of showers and thunderstorms developed in association with a low pressure system just south of Bermuda that had originated from a stalled frontal boundary. While the atmosphere was fairly dry, organization increased over the next day as the low moved toward the west-northwest. By the evening of August 27, the system had developed a roughly circular area of convection with the center of circulation on the eastern edge. The next morning, it was well-defined enough to be classified Tropical Depression Eight. Over the next day, shower activity blossomed only intermittently, leaving the circulation bare a majority of the time. Thus the system remained a tropical depression into August 29 as it turned toward the northwest. Its forward speed also decreased as it moved toward a break in the subtropical ridge to its north. As a result, it stalled off the coast, missing the Outer Banks of North Carolina by less than 100 miles. As a result, scattered showers and gusty winds affected portions of eastern North Carolina on August 30.

Later that day, Eight began to move slowly northeast away from the U.S. east coast. At the same time, the depression lost organization as it began to interact with a frontal boundary just off of the U.S. east coast. Convection became well displaced from the remaining circulation, and the system opened up into a trough early on August 31.

The above image shows Tropical Depression Eight near the coast of the Carolinas.

Tropical Depression Eight faced marginal conditions at best for development throughout its short lifetime.

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.

Wednesday, August 17, 2016

Tropical Storm Fiona (2016)

Storm Active: August 16-23

On August 14, a tropical wave formed near the western coast of Africa and began to produce a large region of disorganized thunderstorm activity. Over the next couple days, the system moved generally west-northwestward and developed a small but vigorous circulation. Late on the 16th, the system was designated Tropical Depression Six. Water temperatures were warm in the region and wind shear was abating. However, as with many tropical cyclones in the central Atlantic, the depression had to contend with low humidity values in the surrounding air that constantly threatened to overwhelm the small system.

On August 17, a small concentrated burst of convection replaced the larger but less powerful banding features, resulting in the cyclone's upgrade to Tropical Storm Fiona. Thunderstorm activity waxed and waned over the next day following the usual cycle by which convection preferentially forms at certain times of day due to solar heating of the ocean and atmosphere. Overall, Fiona's intensity remained virtually unchanged on August 18. Meanwhile, the cyclone moved generally northwest in towards a gap between high pressure systems centered in the east Atlantic and near Bermuda. Wind shear increased somewhat on August 19 but water temperatures continued to climb as Fiona traversed anomalously hot waters in the central Atlantic. As a result, bursts of convection continued to form. Fiona fluctuated in intensity a bit but this state of affairs remained mostly unchanged through the day on August 20. Overnight, the circulation center became exposed again, and the cyclone weakened to a tropical depression. Fiona struggled more to develop a convective canopy on the 21st, but it held its own enough to remain a depression through the day.

Over the following two days, the system continued to wane, with the circulation becoming elongated by August 23. This, coupled with a lack of persistent convection, prompted its downgrade to a remnant low that morning. The low continued west-northwestward for another few days before it dissipated over the western Atlantic.

Tropical Storm Fiona struggled with dry air for most of its lifetime.

Fiona did not approach land at any time during its trek through the central Atlantic.

Tuesday, August 2, 2016

Hurricane Earl (2016)

Storm Active: August 1-6

On July 25, a tropical wave formed just off of the African coast. It moved at an unusually fast rate (around 30 mph) across the eastern and central Atlantic, being among the first of the season to exhibit vigorous shower activity this far east. However, conditions were not very favorable and it is in any case more difficult for fast-moving systems to develop organized circulations. During the final two days of July, convection increased with the wave and it brought rainfall to the Lesser Antilles as it passed westward into the Caribbean Sea. Only toward the end of this period, however, did surface pressures begin to fall in the region. Organization increased significantly by early on August 1 as a large, roughly circular area of thunderstorm activity developed, bringing some rain and high winds to Puerto Rico as it passed to the south. However, it was not until August 2 that a closed circulation was identified. Since the system already had tropical storm force winds, it was designated Tropical Storm Earl as it passed quickly to the south of Jamaica.

Earl was quite disorganized at first, but shear lessened and the storm's forward motion slowed significantly later that day. Combined with the quite high sea temperatures of the western Caribbean, these factors allowed the cyclone to strengthen overnight as its southern bands swept across Honduras. The circulation of Earl had become quite broad by the afternoon of August 3, resulting in heavy rainfall for larger portions of Honduras even as the center remained offshore. Hints of a mid-level eye appeared intermittently throughout the day and by the late afternoon Earl had become a category 1 hurricane. It did not have much time to strengthen further, however. At around 2:00 am EDT, August 4, Hurricane Earl made landfall in central Belize at its peak intensity of 80 mph winds and a minimum central pressure of 979 mb. Hurricane-force winds affected an area near Belize City, and total rainfall amounts of 8-12 inches were common in the central peninsula.

Earl rapidly weakened as it moved inland. By the time it crossed into Guatemala, it was again a tropical storm. That evening, the center took a trajectory just north of west, bringing a the circulation toward the southernmost waters of the Bay of Campeche. As a result, weakening had halted by August 5, with Earl still clinging to minimal tropical storm strength as it continued generally westward. Significant convective bands redeveloped in association with the system as the center traveled over water that morning and through most of the afternoon. The tropical storm strengthened unexpectedly during this period, enhancing the rainfall totals for regions of Mexico. That evening, Earl made landfall in Mexico with maximum winds of 60 mph. Rapid weakening commenced over land, and the system dissipated over the mountainous terrain the next day. Having crossed Mexico, the remnants of Earl contributed to the development of Tropical Storm Javier in the Eastern Pacific basin on August 7.

Earl is shown above near peak intensity on August 3, about 5 hours prior to landfall in Belize.

The high ocean temperatures of the western Caribbean Sea were favorable for Earl's intensification, but its rapid movement and interaction with land prevented it from more than a minimal hurricane.