Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 2:19 PM GMT on September 26, 2007
Tropical Storm Karen is midway between Africa and the Lesser Antilles Islands. Karen is steadily intensifying, as seen in the latest satellite loops. Low level spirals bands continue to organize, and upper-level outflow is becoming established to the north and south. Wind shear of 10-20 knots is keeping intensification slow,and Karen may not be able to attain hurricane strength this week. Hostile wind shear in association with a trough of low pressure is expected to affect Karen over the next few days, and this trough will also turn the storm to the north or northwest. 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.
Figure 1. Microwave image from 6:37 am EDT today showing Karen building heavy thunderstorms on the northeast side. Image credit: Navy/NRL.
Tropical Depression 13
Tropical Depression 13 formed yesterday evening in the far southwestern Gulf of Mexico. Satellite imagery and this morning's QuikSCAT pass show a well-organized surface circulation. Most of the winds seen by QuikSCAT were contaminated by rain, so it is difficult to say what the winds are. Wind shear is about 10 knots over the disturbance, and is expected to fall below 5 knots 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. The next Hurricane Hunter flight is scheduled for 2 pm EDT this afternoon.
Tropical wave 97L bringing heavy rain to Puerto Rico
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 heavy but disorganized thunderstorm activity surrounding Puerto Rico and spreading to the Dominican Republic. These heavy rains will spread over northern Haiti Thursday as the wave tracks west-northwest at 10-15 mph. The wave no longer has a closed circulation, and is under 20-30 knots of wind shear. This shear is expected to remain 20-30 knots through Thursday afternoon, preventing any development. When the wave arrives in the eastern Bahamas Thursday night, wind shear is expected to drop to 10 knots, and stay 10-20 knots through Saturday. This may allow some slow development. The UKMET model is forecasting that 97L will become a tropical depression near South Florida on Saturday or Sunday, but none of the other models go along with this prediction.
Figure 2. Radar estimated rainfall from 97L over Puerto Rico.
Disturbed weather over Florida and the Bahamas
An area of disturbed weather has formed over South Florida and the western Bahama Islands, in association with an upper-level trough of low pressure. NHC has labeled this system "98L" this morning. The region is under about 15-20 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 thunderstorm activity associated with 98L is currently disorganized, as seen on long range radar out of Miami. The disturbance is expected to lift 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 10-20 knots. By Friday, the GFS model predicts 98L should be moving rapidly northeastward past Cape Cod, Massachusetts, and could bring heavy rain and high winds to Nantucket, Martha's Vinyard, and eastern Massachusetts. This would not give 98L much time to organize, and at worst 98L should become a weak tropical storm with 45 mph winds. However, the Canadian model predicts that the trough of low pressure pulling 98L northeast will not be strong enough to finish the job, and the storm will stall off the North Carolina coast. I'll have more on this possibility later.
More action off the coast of Africa
An area of disturbed weather has moved off the coast of Africa, and is located a few hundred miles south of the Cape Verdes Islands. This tropical wave is under about 10 knots of wind shear, and has some potential for development over the next few days. Most of the computer models forecast that a tropical depression will form off the coast of Africa in the next 2-5 days.
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 later today.
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