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 , 1:09 PM GMT on July 18, 2006
The National Hurricane Center issued this special advisory at 8:20am EDT this morning:
Satellite and surface observations this morning indicate the low pressure area located about 250 miles southeast of North Carolina coast has become better organized this morning... and a tropical depression may be forming. An Air Force Reserve reconnaissance aircraft is scheduled to investigate the system this afternoon. Interests along the North Carolina and Virginia coasts should closely monitor the progress of this system.
Indeed, the first visible satellite images from this morning show a clear surface circulation developing near 32N 74W, about 250 miles south-southeast of Cape Hatteras, North Carolina. There is an impressive area of thunderstorms to the southeast side of the center of circulation. Upper level winds of 10-15 knots from the northwest are keeping any thunderstorms from building on the northwest side of the disturbance.
Water temperatures are 26 - 28C in the storm's vicinity, which is above the 26C threshold needed for tropical storm formation. The axis of the warm Gulf Stream current lies just 100 miles to the storm's northwest, so any motion towards the North Carolina coast will bring the system over very deep warm waters of 28 - 29C that should aid in intensification. The GFS computer model is indicating that wind shear will remain in the 5 - 20 knot range the next few days, which is low enough to allow some modest intensification. The exact magnitude of this shear will be critical in determining how strong this storm gets, and is difficult to predict at this time.
As we can see from the historical plot of the 15 tropical cyclones to form in July and August off the Carolina coast in July and August (Figure 3), all these systems moved north or northeast out to sea. Only one hit land, and only two got as strong as a Category Two hurricane. This storm will follow the historical trend, as a strong trough of low pressure is expected to push off the East Coast by Thursday, turning the disturbance northwards and then recurving it out to sea. North Carolina, Virgina, Maryland, and Nova Scotia appear to be the only land areas at risk from this storm.
Figure 1. Latest satellite image of the Southeast Coast disturbance.
Figure 2. Preliminary forecast model tracks for the Southeast Coast disturbance.
Figure 3. Historical tracks of tropical cyclones that formed off the Carolina coast in July and August.
Elsewhere in the tropics
A large area of thunderstorms is a few hundred miles northeast of Puerto Rico. This is associated with a tropical wave interacting with an upper-level low pressure system. Wind shear has dropped to 10 - 25 knots over this area, which is still too high to allow tropical development today, as there is a large amount of dry air in the vicinity. The system is expected to track northwest towards Bermuda and recurve out to sea.
A somewhat ragged tropical wave is near Africa, just south of the Cape Verde Islands. Water temperatures are marginal for tropical development in this region, and wind shear is high, 10-30 knots. The wind shear forecast shows the possibility of more favorable conditions later in the week if the wave can hold together as it moves westward across the Atlantic at 15-20 mph. I'm not expecting this to happen, however.
The Hurricane Center has scheduled a reconnaissance aircraft to check the Carolinas system out at 2pm EDT today. I'll be back with an update when the plane has had a chance to check the storm out.
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