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Danny weakens; not likely to become a hurricane

By: Dr. Jeff Masters, 7:39 PM GMT on August 27, 2009

Tropical Storm Danny has weakened. Data from the Hurricane Hunters between 2 - 3 pm EDT showed top winds of just 40 - 45 mph at the surface. Satellite intensity estimates of Danny's strength have also declined since this morning. Danny's satellite appearance is pretty unconventional, and the storm doesn't look much like a tropical cyclone. The low level circulation center is exposed to view, with the heaviest thunderstorms lying several hundred miles east of the center. The center has remained nearly stationary the past four hours, and has grown less distinct. It is possible that a new center will form up to 100 miles to the east, in the region with the heaviest thunderstorm activity. Danny has already undergone several center relocations over the past 12 hours.

Figure 1. Afternoon satellite image at 3:17 pm EDT of Danny, showing the exposed swirl of clouds where Danny's center is, well displaced from the heaviest thunderstorm activity to the east.

The forecast for Danny
There's not much new to report from the latest set of 12Z model runs for Danny. The models have pretty much maintained their previous forecast, calling for Danny to intensify to a Category 1 hurricane by Saturday morning, and pass close to both Cape Hatteras, NC, and Cape Cod, MA. Given that the models did not correctly forecast Danny's current struggles, I believe it is unlikely that Danny will attain hurricane strength. Dry air from the upper-level low that has been keeping Danny disorganized continues to be a problem for the storm, despite the presence of moderate wind shear of 10 - 15 knots. Danny only has about 24 hours of relatively favorable wind shear left, and that is probably not enough time to intensify into a hurricane. By Friday night, a trough of low pressure will approach the U.S. East Coast and bring high wind shear of 20 - 35 knots. Danny will be close enough to this trough that the trough may be able to feed energy to Danny as the trough converts Danny to an extratropical storm. As a result, Danny may not weaken only slowly. A landfall in Cape Cod, eastern Maine, or Nova Scotia as a tropical storm with 45 - 55 mph winds is a reasonable forecast.

Figure 3. Tropical wave 94L off the coast of Africa.

Invest 94L off the coast of Africa
A well-organized tropical wave (94L) lies a few hundred miles southwest of the Cape Verdes Islands, near the coast of Africa. This disturbance continues to look more organized on satellite imagery. Shear is low, about 10 knots, and waters are warm enough to support continued development. The dry Saharan Air Layer is relatively limited in extent and intensity, so dry air may have only a small inhibiting effect on the wave. Expect continued development of this wave as it moves westward over the next few days. NHC is giving this system a medium (30 - 50% chance) of developing into a tropical depression by Saturday. This system is moving rather slowly, 10 - 15 mph. The GFDL and HWRF models both predict 94L will be a Category 1 hurricane five days from now. These models also predict that 94L will take a significant northward jog 3 - 5 days from now, in response to a strong trough of low pressure passing to the north (this is the same trough of low pressure that will be recurving Danny out to sea).

I'll have an update Friday morning.

Jeff Masters


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