Friday, August 31, 2012

Hurricane Leslie (2012)

Storm Active: August 30-September 11

A tropical wave that emerged off of Africa on August 27 produced scattered showers in the Cape Verde Islands the following day. It continued westward and organized quickly, developing a large area of convection by August 29. The next day, thunderstorms concentrated near the center of circulation, and a spin in the clouds became evident. Therefore, in the afternoon of August 30, the system was designated Tropical Depression Twelve.

Later that day, Twelve's winds increased, and it was upgraded to Tropical Storm Leslie. The center generally became closer to the deep convection overnight, and strengthening began, bringing Leslie to a strong tropical storm by the morning of August 31. The cyclone began to curve towards the north as it exploited a weakness in the Bermuda High, formed by Hurricane Kirk.

Though Leslie was near hurricane strength early on September 1, upper-level conditions deteriorated later that day, and wind shear increased significantly, displacing the convection to the southeast of the center. Meanwhile, the tropical storm made its closest approach to the Windward and Leeward islands of the Caribbean, bringing high surf, especially to the northeasternmost areas.

The cyclone turned northwest overnight, and despite the center being outside the deep convection, the winds associated with the system remained strong and actually increased slightly early on September 2, again bringing Leslie close to hurricane strength.

An upper-level low to Leslie's east, over the Bahamas, continued to induce moderate shear during the day. However, the convection associated with Leslie increased in area and intensity over the following day. By that night, an extremely large blob of thunderstorms, well over 500 miles wide, was churning southeast of the circulation center, featuring extremely cold cloud tops. However, the circulation itself remained disorganized, and Leslie's intensity change little into September 3.

By the afternoon, Leslie had turned farther to the north into the weakness of the aforementioned ridge. Despite this, the forward speed of the cyclone slowed to a crawl that evening, as the ridge restrengthened.

During the night, the system became much more organized as the deep convection assumed a more symmetrical appearance and persisted closer to the center. Soon after, though, dry air invaded the circulation, and the center once again became nearly devoid of thunderstorm activity. By this time, Leslie began to produce rough surf along the U.S. East coast, elevating the rip current threat.

Over the next few days, Leslie drifted northward, and gradually atmospheric conditions improved, allowing the system to periodically develop bursts of central convection. Additionally, on September 4 and 5, the cyclone generally increased in size, with gale force winds extending over 200 miles from the center by the afternoon of September 5. Around this time, an eye briefly appeared on visible satellite imagery, allowing the cyclone to be upgraded to Hurricane Leslie.

Since Leslie was nearly stationary during the day of September 6, strengthening was inhibited by the decrease of water temperature just below the system. Due to this, Leslie could not achieve any further eyewall development, and the inner core actually degraded in organization overnight and into September 7. This caused Leslie to weaken back to a tropical storm later that morning.

The cyclone finally began to move northward at a steadier pace that afternoon, as it became entrenched in the flow of a trough to its northwest. As Leslie moved over warmer waters not affected by upwelling, it began to regain convection, particularly in the northern semicircle.

The outflow of Leslie improved further overnight, and a large curved rain band developed on September 8 that surrounded the inner core. However, the center itself had a large eye feature, too large for a compact eyewall. Thus, the maximum winds of Leslie remained below hurricane strength, and the pressure, formerly near 980 mb, stayed relatively high.

Conditions also deteriorated in Bermuda that evening, as the outer bands swept across the island. By September 9, tropical storm conditions swept across Bermuda, and the center of Leslie made its closest approach late that morning, passing well to the east. The cyclone accelerated further that afternoon, and recovered some deep convection, though winds near the center remained somewhat light.

On September 10, Leslie started to rapidly accelerate north-northeastward, and already showed signs of extratropical transition that afternoon; the already broad center expanded further, and the cyclone became very asymmetric. However, winds recorded in a band north of the center supported raising Leslie's winds to 70 mph that evening. During the morning of September 11, Leslie made landfall in Newfoundland, bringing a wide area of gale force winds to the island and surrounding areas as it became extratropical.

The powerful cyclone continued over the open waters of the north Atlantic, passing just south of island on September 12, and finally combining with another low pressure system over northern Europe on September 13.

Leslie at peak intensity, with a small eye visible.

Track of Leslie.

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