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 , 1:57 PM GMT on September 27, 2007
Tropical Storm Karen is being ripped apart by 20-40 knots of wind shear, thanks to strong southwesterly winds aloft (Figure 1). Satellite loops show that these strong winds have exposed the center of circulation, now visible as a swirl of low clouds, and pushed the remaining heavy thunderstorm activity to the storm's northeast side.
Karen was probably a hurricane yesterday morning, since a NOAA hurricane hunter aircraft that arrived at the storm during the afternoon found winds near hurricane force. These winds were much stronger than the storm's satellite presentation suggested. This flight occurred after Karen had already peaked in intensity, so it is likely Karen was a hurricane for a few hours. The storm may be upgraded to a hurricane in post-storm analysis. It looks right now that Karen does not have much chance to regain hurricane status over the next five days, and may not survive at all. Wind shear over the next five days is predicted to range between 20-30 knots, and there is at least a 30% chance Karen will be destroyed by this. None of the computer models forecast total destruction, though.
Figure 1. This morning's visible satellite image, with yellow contour lines showing the amount of wind shear (in knots) superimposed. Strong upper winds winds blowing from the southwest (shown by the large white arrow) were creating 20-40 knots of wind shear over Karen. These strong winds pushed Karen's heavy thunderstorm activity to the downwind (northeast) side of Karen, exposing the low-level center of circulation to view. Image credit: University of Wisconsin CIMSS.
A severely weakened Karen will not turn to the north as much as the models have been predicting, since the storm will not extend as high in the atmosphere. Shallow storms respond to the wind field closer to the surface. These surface winds are blowing more east-to-west, and Karen is expected to follow a more westerly track to a point a few hundred miles north of the Lesser Antilles Islands five days from now. Steering currents will weaken then, and Karen will move slowly for a few days. It appears likely now that a ridge of high pressure will then build in and force Karen (or its remnants) to the west towards the U.S. East Coast late next week.
Tropical Depression 13
Tropical Depression 13 remains virtually unchanged from a day ago. Satellite imagery shows a small, compact storm that is affecting a very limited portion of the Mexican coast. Wind shear is a low 5 knots, and TD 13 could still strengthen to a 50-55 mph tropical storm before its expected landfall in Mexico on Saturday, between Poza Rica and Tampico. TD 13 will not affect Texas, due to the storm's small size. The next Hurricane Hunter mission is scheduled to arrive at 2 pm EDT today.
Tropical disturbance 98L near the east coast of Florida
A surface low pressure area (98L) moved over South Florida last night, and now appears to be reforming about 100 miles southeast of Cape Canaveral. The system is bringing heavy rain to the waters offshore Florida and the northern Bahama Islands, as seen on long range radar out of Miami. However, there are no organized spiral bands, and winds measured throughout Florida this morning have been 8 mph or less. The system is under about 10-15 knots of wind shear, which may allow some development today. This disturbance has brought rains of up to four inches to portions of South Florida and the western Bahamas, as seen on Miami radar. The center is forming far enough off the coast that 98L will probably not be a big rainmaker for Florida.
The disturbance is lifting northeastward in response to a strong trough of low pressure swinging off the U.S. East Coast. The system has the potential to organize into a tropical depression today or tomorrow, and 98L will pass several hundred miles east of Cape Hatteras, North Carolina Saturday. The system is expected to accelerate to the northeast and could bring heavy rain and high wind gusts to the Canadian Maritime provinces early next week. It is unlikely 98L will have time to become any stronger than a 45 mph tropical storm, and any effects on the U.S. will be minimal.
Coast of Africa wave
A tropical wave about 150 miles south of the Cape Verdes Islands is under about 10-20 knots of wind shear, and has some potential for development over the next few days. This morning's QuikSCAT pass showed an elongated circulation, with top winds of 35 mph to the storm's northeast side.
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 Friday morning at the latest.
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