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 , 9:02 PM GMT on September 26, 2007
Tropical Storm Karen is now suffering the effects of 15-20 knots of winds shear, and has stopped intensifying. The latest satellite loops show a large and expanding cloud pattern with good outflow to the north, but no eye. The wind shear affecting Karen is expected to stay at 15-20 knots the next four days, and Karen will probably not change much in strength during that period. The long range outlook for Karen remains unchanged. The models are all fairly unified in taking Karen to a latitude north of the Lesser Antilles Islands. However, the trough of low pressure expected to steer Karen to the northwest is unlikely to be strong enough to recurve Karen out to sea. A new ridge of high pressure may build in, forcing Karen westwards towards the U.S., just north of the Lesser Antilles Islands. This scenario, favored by the UKMET model, would put Karen in a position to threaten the U.S. East Coast late next week. The GFS model does not build in such a strong ridge, and instead forecasts that Karen will stall for 2-4 days a few hundred miles north of the Lesser Antilles Islands, and eventually recurve out to sea when the next strong trough of low pressure comes along. This scenario would put Bermuda at risk from Karen. It is too early to speculate which of these scenarios is more likely, and how much of a risk Karen may present to Bermuda or the U.S. East Coast.
Tropical Depression 13
Tropical Depression 13 remains nearly stationary in the far southwestern Gulf of Mexico. Satellite imagery show a well-organized system that should be a tropical storm, but this is not supported by Hurricane Hunter reports. The top winds found this afternoon at the surface by the airplane were less than 30 mph. Wind shear is about 10 knots over the disturbance, and is expected to remain 10 knots or below for the next three days. This should allow Tropical Depression 13 to develop into a strong tropical storm--possibly a Category 1 hurricane--before it makes landfall along the Mexican coast 2-3 days from now. Steering currents are weak in the southwest Gulf of Mexico, and the storm will move slowly and erratically. This storm is primarily a threat to Mexico, due to a very strong ridge of high pressure that will prevent the storm from turning northwards.
Tropical wave 97L grows disorganized
A tropical wave (97L) has brought heavy rains of up to 2-4" to Puerto Rico today. Satellite imagery and Puerto Rico long range radar show that this wave has degenerated. High wind shear of 20-30 knots has seriously disrupted the wave, and this will be my last mention of it.
Tropical disturbance 98L near South Florida and the Bahamas
A surface low pressure area (98L) formed near Key West this morning, and has moved northeast to a position off the SW coast of Florida south of Naples. The rotation of the low is clearly evident on long range radar out of Miami. However, there are no organized spiral bands, and winds measured throughout South Florida this afternoon have been 12 mph or less. The system is under about 10-20 knots of wind shear, which should keep any development slow today. This disturbance has brought rains of up to four inches to portions of the Florida Keys and the western Bahamas as seen on Miami radar.
The disturbance is lifting northeastward in response to a strong trough of low pressure swinging off the U.S. East Coast. On Thursday, when 98L will be over the warm waters of the Gulf Stream approaching North Carolina, the system has the potential to organize into a tropical depression, as wind shear is expected to fall to 10 knots. The GFDL develops 98L very rapidly, and predicts it will approach Category 1 hurricane status as it passes about 200 miles east of Cape Hatteras on Friday night. This is too aggressive a forecast, and I prefer the HWRF forecast, which forecasts top winds of 35-40 mph by Friday night. A very strong band of wind shear associated with the jet stream will affecting 98L by Friday night, and could rapidly destroy the storm. 98L or its remnants may then move rapidly northeastward past Cape Cod, Massachusetts, or may stall off the coast of North Carolina and wander erratically for several days. The models are split on this matter.
Coast of Africa wave
A tropical wave a few hundred miles south-southeast of the Cape Verdes Islands is under about 10 knots of wind shear, and has some potential for development over the next few days.
Figure 1. Tropical Storm Karen and the latest tropical wave coming off the coast of Africa at 8:45 am EDT Sep 26, 2007. Image credit: NOAA Visualization Program.
Wind shear tutorial
For those interested, I've posted a wind shear tutorial. This page is permanently linked on our tropical page.
I'll have an update Thursday morning.
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