Sunday, September 23, 2018

Hurricane Leslie (2018)

Storm Active: September 23-25, September 28-October 13

On September 18, an extratropical system associated with the remnants of Hurricane Florence moved away from the U.S. east coast over the tropical Atlantic. A new low formed along the frontal boundary around September 22 in the central subtropical Atlantic. Over the next day, the low developed spiral banding and lots its frontal nature. By the morning of September 23, it had transitioned into Subtropical Storm Leslie. This was the sixth subtropical storm of the 2018 season, setting a new record.

At the time of formation, Leslie was drifting westward, but steering currents were quite weak and it turned southward and ultimately eastward over the next day. The system had never had much in the way of deep convection, but what was there diminished further by September 25. Meanwhile, a new front was approaching from the west and interacting with Leslie, elongating its circulation. By late that morning, the system had become extratropical. Upon transition, it underwent a rapid burst of the strengthening and was producing hurricane force winds by the 26th. Since it was non-tropical, however, it was not designated a hurricane.

At the same time it continued to turn toward the north and then back west. Conditions were still fairly favorable for tropical cyclone development so it began to transition back the next day. On September 28, enough deep convection had reappeared near the center for Leslie to again be classified as subtropical. However, its maximum winds had subsided back to around 50 mph, so this was the initial intensity. The system moved slowly southwest over the next few days and gradually developed more banding features south and east of the circulation center. Leslie transitioned to a fully tropical storm for the first time on September 29. Sea surface temperatures increased and wind shear decreased along the storm's path, leading to some slow strengthening over the next few days as thunderstorms finally wrapped entirely around the center.

By October 2, Leslie was approaching hurricane strength and had dipped in latitude to below 30° N due to its unusual southwestward motion. A ragged eye formed that evening and the system was upgraded to a hurricane for the first time. Overnight, the cyclone became stationary around 500 miles east-southeast of Bermuda. It also peaked in intensity at maximum sustained winds of 80 mph and a central pressure of 975 mb. Due to the influence of an upper-level low pressure system to the north, Leslie began to move northward on October 3. This motion took it over cooler water, and convection waned again, with a shallow ring of convection around the center separated from the outer bands by a "moat" of dryer air. Leslie began to weaken as a result and soon was a tropical storm again.

Although the system was still quite distant from any landmasses, the large size of the circulation generated significant ocean swells that led to rough surf in Bermuda and even the east coast of North America. Leslie stalled again about 450 miles northeast of Bermuda on October 5, and began to feel the influence of the mid-latitude westerlies. The cyclone turned sharply eastward that day. Meanwhile, the structure of the storm had changed quite a bit; a central area of strong thunderstorms had replaced the large eye, and a large area of convection persisted to the north of the center. Leslie began to separate from a trough to its north and turned south of east on October 7. The storm accelerated southeastward over the next day, bringing it over warmer waters, and it began to restrengthen.

The cyclone developed a central dense overcast on October 8 and approached hurricane strength on the 9th, achieving category 1 status that evening a week after doing so the first time. Leslie turned due south for a little while on the 10th, reaching a southernmost latitude of 27.8 ° N. However, another trough moving to its north turned the system east-northeast and began to accelerate it toward the far eastern Atlantic. The inner core structure fluctuated a great deal in organization during the following day, but overall it became a bit better defined and Leslie strengthened somewhat. Late on October 11, Leslie reached its peak intensity as a top-end category 1 hurricane with 90 mph winds and a pressure of 969 mb.

The system picked up even more speed the next day and colder waters weakened the storm's convection. A tropical storm warning was issued for the island of Madeira, located southwest of Portugal. This was the first ever warning issued for the island and Leslie was the first known tropical cyclone ever to affect it in modern history. The center passed north of Madeira later on the 12th. Finally, on October 13, Leslie transitioned to an extratropical low just before making landfall in northern Portugal. This transition did not prevent the cyclone from bringing hurricane force winds gusts and heavy rain to the Iberian Peninsula. The low finally dissipated inland a few days later.

This image shows Leslie during its second and final stint as a hurricane, moving east-northeastward toward Europe.

Leslie's convoluted track included some highly unusual southward dips over the central Atlantic. Just after becoming extratropical, it moved over the Iberian Peninsula, though this is not shown above.

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