Friday, August 31, 2018

Hurricane Florence (2018)

Storm Active: August 31-September 17

On August 30, a vigorous tropical wave entered the Atlantic from the west African coastline. While the system was initially large and disorganized, its center became much better defined over the next couple of days. The low passed just south of the Cape Verde Islands on the 31st, bringing some heavy rain and winds. Around the same time, it was classified Tropical Depression Six. Ocean temperatures were only marginal for development, but wind shear was very low and the air surrounding the system quite humid (any Saharan dry air was well to the north). Because of this, the cyclone strengthened as it moved west-northwestward away from the islands, becoming Tropical Storm Florence early on September 1.

Some more intensification occurred that evening, but wind shear out of the southwest suddenly exposed the center of circulation on September 2 and the system weakened slightly. As Florence continued on its west-northwest heading over the open central Atlantic, sea surface temperatures began to warm beneath it. Overnight, the system developed an inner core and strengthened into a strong tropical storm. However, even as temperatures warmed, wind shear increased also and the satellite presentation became a bit ragged. Nevertheless, Florence held its own on the 3rd and approached hurricane strength when a central dense overcast appeared. In defiance of almost all models and forecasts, the system continued to intensify on September 4 and became a hurricane. It turned a bit toward the northwest that evening and an eye became apparent on satellite imagery. The next morning, the eye became better defined and cleared out further. Florence was then upgraded to a category 3 hurricane, the first major hurricane of the 2018 season.

That afternoon, the cyclone peaked as a category 4 hurricane with winds of 130 mph and a minimum pressure of 953 mb before wind shear out of the southwest finally began to take its toll. The southern eyewall thinned and then broke under the shear, the inner core became disrupted, and Florence lost major hurricane status on September 6. Rapid weakening continued throughout the day, and the center nearly became exposed from time to time overnight as the system weakened back to a tropical storm. Meanwhile, mid-level ridging to the north of Florence turned the shallower system back toward the west on September 7. The weakening trend halted that afternoon as shear began to abate and convection gradually worked its way back over the center. But while waters were warm and upper-level winds were more favorable, the circulation had ingested some dry air that it struggled to mix out through the day of September 8. Nevertheless, it approached hurricane strength once again by that evening.

The next day, deeper convection blossomed near the center and hints of an eye reappeared. Florence regained category 1 status that morning and began a process of rapid intensification. This process really accelerated when a symmetrical eye appearing late on September 9. By the afternoon on the 10th, the system was a dangerous category 4, surpassing its earlier peak intensity to reach a new peak of 140 mph winds and a central pressure of 939 mb. Meanwhile, a very strong subtropical ridge began building to its north and Florence turned west-northwest with a greater forward speed. Overnight, the core underwent an eyewall replacement as the small inner eyewall collapsed in favor of a broader outer one. This took some time to consolidate, leaving Florence with a lopsided appearance on infrared imagery. However, the new eyewall eventually closed off and the system maintained category 4 strength. Moreover, eyewall replacement cycles usually are accompanied by a broadening of the wind field; this held true for Florence. Its tropical storm force and hurricane force wind radii increased markedly by September 12.

By September 13, the maximum winds had decreased to category 2 strength, but the system had grown considerably. Its outer bands began to sweep across the coast of southeastern North Carolina that morning. Florence's speed slowed during the day and it stalled off the coastline. That evening, it turned toward the west and ultimately made landfall overnight as a category 1 hurricane. Even as the storm weakened, torrential rains continued to fall over much of coastal North Carolina and pushed into South Carolina on September 14. Florence became a tropical storm that day and the center crossed the border into South Carolina over land. As the circulation spun down, the core lost much of its strength, but the portion of the storm still over water to the northeast of the center was still dragging immense amounts of tropical moisture over coastal North Carolina on September 15. Along with the flooding rains came intermittent tornadoes reported in the northeast part of the circulation.

Overnight, Florence weakened to a tropical depression and finally began to pick up some speed inland toward the west and then north through Appalachia. By September 17, the system turned northeast under the influence of the mid-latitude westerlies and rains spread eastward into the mid-Atlantic, though nowhere near as severe as they had been in the Carolinas. The low pressure center of Florence became elongated later that day and the system finally transitioned to post-tropical near West Virginia. Soon, the remnants moved eastward over the Atlantic. The extratropical successor to Florence eventually spawned a new low in the subtropical Atlantic that would become Leslie. Even well after the storm had passed, river flooding continued for the Carolinas. Florence's devastating rainfall, totaling over 30 inches in parts of coastal North Carolina, made it among the costliest tropical cyclones ever recorded.

The above image shows Florence near peak intensity as a category 4 hurricane.

Florence's track was highly unusual. In a vast majority of instances, cyclones that traversed the Atlantic at the latitude that Florence did would recurve out to sea before hitting land. However, very strong subtropical ridges steered the storm into the Carolinas.

Sunday, August 19, 2018

Tropical Storm Ernesto (2018)

Storm Active: August 15-18

On August 12, a low pressure area formed over the subtropical Atlantic well to the southeast off the coast of Nova Scotia. Over the following few days it moved generally toward the east and then southeast. In the meantime, very warm waters in the region allowed it to organize as atmospheric conditions improved. By August 15, the low's center had become well-defined, though the surface center was still situated under an upper-level low and the radius of maximum winds was rather broad. In light of these features, the system was classified Subtropical Depression Five that day.

The depression turned toward the north that day and strengthened into Subtropical Storm Ernesto. Marginal sea surface temperatures allowed a slight bit of intensification the next day and a transition to a fully tropical storm with convection closer to the center. At the same time, the storm began to feel the influence of the mid-latitude westerlies and accelerated northeast and then east-northeast. Ocean temperatures under the storm plummeted on August 17, but humid and unstable air in the region allowed Ernesto to maintain tropical cyclone status for somewhat longer than originally expected. Early on August 18, the system transitioned into a post-tropical storm west of Ireland. Nevertheless, it brought areas of heavy rain and gusty winds to northern Ireland and the United Kingdom as it merged with a front later that day. In both origin and track, Ernesto was very similar to its predecessor, Debby. In addition, it was the fourth system to become a subtropical storm in the 2018 season. This was the first such occurrence since 1969.

The above image shows Ernesto as a subtropical storm shortly after formation.

While Ernesto did not affect land as a subtropical or tropical cyclone, its remnants did impact the United Kingdom and Ireland.

Wednesday, August 8, 2018

Tropical Storm Debby (2018)

Storm Active: August 7-9

During the first few days of August, a non-tropical low meandered over the north central Atlantic. By the 4th, it was producing gale force winds, but had very little thunderstorm activity associated with it. Over the next couple days, it drifted southeastward, moving over water that was a tad warmer. The system again became stationary and then started to move back to the north, but by this time it acquired more significant convection. On August 7, the system was classified as Subtropical Storm Debby due to the spread of tropical storm force winds and outer banding from the center.

The storm slowed a bit overnight and turned toward the north-northeast on August 8. At the same time, the maximum winds increased somewhat and thunderstorms became more concentrated close to the center of circulation. As a result, Debby was reclassified as a tropical storm that morning. The cyclone also reached its peak intensity of 45 mph winds and a pressure of 1003 mb. Soon, however, the system began to weaken over the cold north Atlantic. Debby transitioned to a post-tropical storm during the afternoon of August 9 as it accelerated to the northeast, far from any land.

Debby was a small and short-lived cyclone that did not have any land impacts.

The above image shows Debby's track over the north Atlantic.