Friday, September 2, 2011

Tropical Storm Lee (2011)

Storm Active: September 1-4

During the day of August 30, shower activity increased with a tropical wave moving west-northwestward through the western Caribbean Sea. Meanwhile, a stationary low pressure trough was situated over the Gulf of Mexico. During the next few days, the tropical moisture of these two systems combined, forming a very large area of strong thunderstorms over the eastern Gulf, stretching from the Yucatan Peninsula to Florida. Shear from the west affected the system, but began to slowly abate during the day of September 1. During that afternoon a circulation appeared on the western edge of the clouds, becoming closed by the evening. Tropical Depression Thirteen had formed.

During the night, the depression's center reformed farther south and lost nearly all of its initial motion to the northwest, Due to weak steering currents, the system remained nearly stationary through the morning of September 2. Also, an area of deep convection developed near the center, despite the center itself remaining rather elongated. Due to the system's large size, rain and gusty winds already were moving into southeastern Louisiana. The circulation of the system became more well-defined that afternoon, and Thirteen became Tropical Storm Lee.

Lee had an unusually large windfield even at the time of its formation, and it continued to expand later that day. The cyclone itself also slowly strengthened, moving erratically, but generally northward. Lee still exhibited some characteristics of a non-tropical cyclone even into September 3, with the southwest quadrant still devoid of convection. During that morning, heavy rain continued across Louisiana and Mississippi, and tornadoes were reported within the heaviest bands, located in southern Louisiana.

Lee strengthened further as it slowly moved toward the Gulf coast, reaching its peak winds of 60 mph during the day. However, a second low-level center developed within the system early that afternoon, the original over land, and the second still off the coast. The new formed center took over the circulation while the other dissipated, and Lee therefore stalled off of the coast of Louisiana, with tropical storm conditions still extending far inland all across the central Gulf states. During the night, convection decreased, with the only rain band near the center extending to the southwest. Gradual weakening occurred, but Lee still did not assume any definite motion, and was still hovering over the coast, continuing the already severe flooding of the surrounding areas. Despite the winds having fallen, Lee's central pressure decreased to 987 mb early on September 4. The system finally made landfall in Louisiana later that morning.

The circulation actually became better defined for a brief period over land that afternoon, but the cyclone quickly degenerated, weakening further that evening, and began extratropical transition late that night. The system was fully extratropical by the next day, but the rain was by no means over. During the day of September 5, the remnants of Lee combined with a powerful cold front. With the addition of tropical moisture, a region of heavy rain formed from the tail end of Lee, in Louisiana, through the end of the front, in Canada! Lee's remnants lost their well-defined center on September 8, but rainfall continued for two more days, finally ending on September 10. The several days that Lee spent moving up the coast saw unprecedented flooding in the the mid-Atlantic states, New England, and even in some areas as far north as Canada. Rainfall totals from the combined storm system exceeding 6 inches in widespread areas, with local amounts significantly greater. 21 fatalities resulted from Lee, along with over $250 million in damages.

Lee strengthening over the northern Gulf of Mexico.

Track of Lee.

Note: In post-season analysis, Lee was confirmed to have been subtropical from September 3 up to the points of extratropical transition. This means that the cyclone transitioned from tropical to subtropical, a very rare event.

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