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Hurricane Hunters find 50 - 60 mph winds in disturbance 92L north of Puerto Rico

By: Dr. Jeff Masters, 8:54 PM GMT on August 25, 2009

The tropical wave (92L) a few hundred miles north of Puerto Rico is generating a large area of surface winds of 50 - 60 mph, according to the latest information from the Hurricane Hunters. Top winds seen so far at their flight level of 1,000 feet were 69 mph, which would make 92L a strong tropical storm if it had a surface circulation.
However, the aircraft has not found a surface circulation, and the satellite appearance shows virtually no change in the amount, intensity, or organization of the storm's thunderstorm activity. Wind shear has dropped to the moderate range, 15 - 20 knots this afternoon, but the upper low 92L is moving underneath is dumping cold, dry air into the region. Dry air continues to get ingested into 92L's thunderstorms, creating strong downdrafts that are robbing 92L of heat and moisture. These downdrafts are creating surface arc clouds that spread out from where the downdraft hits the ocean surface. NHC continues to give 92L a high (greater than 50% chance) of developing into a tropical depression by Thursday afternoon.

The forecast for 92L
As 92L moves underneath the center of the upper low on Wednesday morning, the upper low is expected to weaken, and wind shear is expected to decline to the low range, 5 - 10 knots. 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. 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 disagree substantially on how close 92L will be to the coast at that time. One camp of models, including the NOGAPS, Canadian, UKMET, and ECMWF models, predict 92L will pass very close to the Outer Banks of North Carolina on Friday night or Saturday morning. The GFS, GFDL, and HWRF models keep 92L several hundred miles out to sea. Both sets of models bring 92L north-northeastwards on Saturday, with a track over Massachusetts or Nova Scotia. The intensity forecast for 92L is problematic, since it's eventual strength depends upon how quickly it manages to become a tropical depression. Given that 92L will find itself in a favorable environment for strengthening for about 36 hours this week, and marginal for the remainder of the week, I give the system these odds:

10% chance of never getting a name.
20% chance of becoming a weak tropical storm (40 - 50 mph winds).
40% chance of becoming a strong tropical storm (55 - 70 mph winds).
30% chance of attaining hurricane strength.

Elsewhere in the Atlantic
The ECMWF and UKMET models predict the development of a tropical wave coming off the coast of Africa late this week. The GFS model no longer shows this.

I'll have an update Wednesday morning.

Jeff Masters


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