Wednesday, November 9, 2011

Tropical Storm Sean (2011)

Storm Active: November 8-11

On November 4, a frontal boundary moved off of the U.S. east coast. A low pressure system along the front deepened as it moved off the coast of North Carolina later that day. The section of the front to the north of the low had most of the cloud cover associated with it, but the convection moved closer to the circulation of November 6, as the system drifted southeast. After temporarily losing definition, the low strengthened again on November 7. By this time, gale force winds occupied a region around the low, extending hundreds of miles in each direction.

Over the following day, the southern extension of the frontal boundary degenerated, devolving into a banding feature expanding clockwise from the low. Meanwhile, the remainder of the front had moved away to the east, and the leftover moisture became entrenched in the circulation of the cyclone. Early on November 8, convection had circumnavigated the center, and the system was upgraded to Subtropical Storm Sean.

The cyclone continued to increase in organization that afternoon, the eye contracting, and the surface circulation becoming better defined. The movement of the circulation into the lower levels of the atmosphere merited a reclassification of Sean into a tropical storm. Throughout the day, Sean remained nearly stationary, initially revolving around a broader cyclonic center, and later adopting a slow westward motion. Convection developed in earnest during the morning of November 9, and the system intensified into a strong tropical storm, also forming an eye feature.

By this time, Sean had entered the steering currents of the west Atlantic and began to accelerate northward. On November 10, the system curved to the northeast, also reaching its peak intensity of 65 mph winds and a pressure of 983 mb. Late that night, the windfield of Sean enveloped Bermuda, causing tropical storm force winds on the island, along with periods of heavy rain.

As it moved away from Bermuda on November 11 winds shear drastically increase and extratropcal transition began as Sean cam into close proximity with a front. By that evening, the center had elongated, and convective bands associated with the circulation ere stripped away. As a result, the system became extratropical that night. Sean caused minor damage and one fatality in Bermuda.

Tropical Storm Sean near peak intensity on November 10. The Outer Banks of North Carolina are visible on the upper left.

Erratic track of Sean through the Western Atlantic.