Thursday, September 5, 2013

Tropical Storm Gabrielle (2013)

Storm Active: September 4-5, 10-13

On August 26, a tropical wave formed near the coast of Africa and moved westward. Shower activity associated with the system remained minimal due to an unfavorable upper atmosphere for most of the next week. However, by August 31, a broad low pressure center had formed along the wave, and convection increased somewhat, though still remaining disorganized. On September 1, shower and thunderstorm activity associated with the disturbance spread over the islands forming the eastern edge of the Caribbean. Meanwhile, another tropical wave formed a few hundred miles east of the existing system, and the two systems began to interact, forming a widespread area of convective activity stretching from the eastern Caribbean well into the central Atlantic.

Wind shear declined significantly as the first low entered the Caribbean, but the system still struggled with a good deal of dry air aloft for the next few days as it remained nearly stationary in the far eastern Caribbean. On September 3, the low assumed a more northwesterly track, and more concentrated cloud cover appeared in the vicinity of the low pressure center, though the center itself remained poorly defined. However, on September 4, hurricane hunter aircraft investigating the system found a circulation organized enough to merit the low's classification as Tropical Depression Seven.

Partially due to the influence of the tropical wave still churning to its northeast, Seven continued to the northwest toward western Puerto Rico that evening. Small increases in outflow definition and a deepening of convection prompted the upgrade of Seven to Tropical Storm Gabrielle overnight. However, during the morning of September 6, it became clear that the surface circulation of Gabrielle had become decoupled from the mid-level atmospheric center by over 100 miles: the surface circulation was still approaching the channel between Puerto Rico and Hispaniola but the mid-level center and all the associated swirl in satellite imagery was displaced well to the northeast due to upper-level winds. This separation caused the system to quickly weaken into a tropical depression, and since a new surface low did not form in alignment with the structure of the upper-atmosphere, Gabrielle dissipated that evening. Heavy rains continued throughout the northeastern Caribbean due to the remnants of Gabrielle and still because of the lingering disturbance to the northeast over the next day.

The two systems finally combined as the remnants of Gabrielle moved north-northeastward, but the low-pressure center formerly associated with the tropical storm remained intact, and in fact moved into slightly more favorable conditions. On September 9, the low deepened and deep convection appeared near and to the east of its center. By early on September 10, the system had regenerated into Tropical Storm Gabrielle. Meanwhile, an upper-level low situated north of Bermuda altered the bearing of the tropical storm, pushing it in a more northerly direction toward Bermuda.

During that afternoon, Gabrielle became more organized despite experiencing shear out of the west and southwest, and quickly reached a peak intensity of 60 mph winds and a pressure of 1004 mb as it passed within 25 miles of Bermuda, bringing tropical storm force winds and heavy rainfall. However, shortly after Gabrielle reached this intensity, the southwesterly shear displaced the convection associated with the system to the east, exposing the center. This trend caused weakening into September 11. Meanwhile, the upper-level low north of Gabrielle had caused the cyclone to veer left further, and it was now bearing northwest at a slower forward speed. The circulation became almost totally void of convection that evening and Gabrielle weakened to a tropical depression but the cyclone recovered somewhat during the morning of September 12 as shear decreased.

This prompted the upgrade of the system back to a tropical storm that day. Gabrielle also began to accelerate northward and north-northeastward as it entered the flow of an approaching frontal system that evening. By early on September 13, the cyclone had once again weakened to a tropical depression as the associated convection lost its banding features and began to interact with an approaching front. Finally, later that day, the system lost its closed circulation and dissipated. The moisture from Gabrielle contributed to rainfall in Atlantic Canada the next day.

The above image shows Gabrielle shorting after reforming on September 10.

The track of Gabrielle took it very close to Bermuda at its peak intensity, causing some heavy rainfall and gusty winds.

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