Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:15 PM GMT on April 21, 2011
A tropical disturbance (91L) near 24N, 63W, midway between the Virgin Islands and Bermuda, is moving north-northwest at about 8 mph. The system's heavy thunderstorm activity has increased since yesterday, but 91L has an elongated and poorly-organized circulation, thanks to a hefty 80 knots of wind shear. The storm is over waters of 25°C, and these waters will cool to 24°C by Friday as the storm continues to the north-northwest. Before 91L reaches Bermuda, steering currents will reverse and force 91L to the south-southwest on Saturday, into a region of higher wind shear. The latest SHIPS model forecast predicts wind shear will drop to 50 knots over 91L by Friday, then increase again to 70 knots by Sunday. The high shear and relatively cool water temperatures will make it difficult for 91L to organize into a subtropical depression. I give 91L a 10% of becoming a subtropical depression. Climatology argues against 91L becoming the first named storm of the year; there has only been once named April storm in the Atlantic since 1851, Tropical Storm Ana of 2003. The formation of a tropical disturbance at this location this time of year is unusual, but is not necessarily a harbinger of a active season ahead.
Figure 1. Morning satellite image of the Atlantic tropical disturbance 91L.
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