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Tropical Storm Danny likely to be named today

By: Dr. Jeff Masters, 1:25 PM GMT on August 26, 2009

First-light satellite images of the tropical wave (92L) a few hundred miles northeast of the Bahama Islands show that the system has developed an organized surface circulation, and 92L will likely be named Tropical Storm Danny later today. This morning's QuikSCAT pass confirms the presence of a surface circulation, and the satellite saw top winds of 50 mph in a cluster of thunderstorms well east of the center. Wind shear has dropped to the moderate range, 10 - 15 knots. The upper-level low 92L is moving underneath has plenty of dry air in it, and the upper low is injecting this dry air into 92L's west side, keeping and heavy thunderstorm activity from developing on that side. The Hurricane Hunters are in 92L, and have found surface winds up to 57 mph so far this morning. NHC continues to give 92L a high (greater than 50% chance) of developing into a tropical depression by Thursday afternoon.

Figure 1. Water vapor satellite image from 8:15 am EDT 8/26/09 showing dry air associated with an upper-level low pressure system to the west of 92L's center of circulation. Image credit: NOAA/SSD.

The forecast for 92L
As 92L continues to plow through the upper low, the low will weaken, and wind shear is expected to decline to the low range, 5 - 10 knots, by tonight. However, the upper-level low will continue to dump dry, cold air into 92L through Thursday afternoon, slowing down development. By Thursday night, when 92L should be several hundred miles off the coast of northern Florida, the upper-level low may be weak enough and far enough away that 92L will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development. Most of the intensity models, including the GFDL, HWRF, and SHIPS model, forecast that 92L will become a hurricane by Friday. However, this favorable environment will not last long, since a strong trough of low pressure will be approaching the U.S. East Coast on Friday. This trough will bring high wind shear of 20 - 30 knots by Friday night. This trough should be strong enough to turn 92L to the north. The models have come into better agreement keeping 92L offshore as it passes North Carolina, though the storm is certainly capable of giving the Outer Banks a direct hit. As 92L passes North Carolina, it should start heading north-northeast, with a second landfall likely Saturday night or Sunday morning somewhere between Massachusetts and Nova Scotia. At that time, 92L is likely to be a strong tropical storm or weak Category 1 hurricane, with winds in the 55 - 80 mph range. It currently appears that 92L will not bring tropical storm-force winds to the Bahamas or to Florida.

Elsewhere in the Atlantic
The models have been inconsistently predicting formation of several tropical waves coming off the coast of Africa over the next week, and I am going to stop mentioning these forecasts until we get two models predicting the same thing, several model runs in a row.

I'll have an update later today.

Jeff Masters


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