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 , 8:04 PM GMT on July 18, 2006
A hurricane hunter airplane is currently in Tropical Depression Two at 600 - 900 feet in altitude. As one can gather from the decoded recco reports we now make available, the aircraft has made one pass through the storm from SW to NE, and measured strongest surface winds of 28 mph in the northeast quadrant. The lowest central pressure was 1008 mb, which is 3 mb lower than the 2pm advisory from NHC. The airplane put out a vortex report at 3:15 pm EDT, which said that they found strongest winds of 35 mph at the surface. So, this is still a tropical depression--we need winds of at least 39 mph at the surface to have a tropical storm.
Upper level winds from the northwest are keeping thunderstorm activity limited on the storm's west side. However, wind shear has decreased from 10-15 knots this morning to 5-10 knots this afternoon. This decrease in wind shear has allowed some thunderstorms to wrap around to the north side of the storm, and the storm is now attempting to wrap deep convection (large thunderstorms) all the way around its center. Given the improving satellite presentation and the low central pressure found by the hurricane hunters, I expect that this will be Tropical Storm Beryl by midnight.
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 NHC's forecast of a more northwesterly 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, as well. The exact magnitude of this shear will be critical in determining how strong this storm gets, and is difficult to predict at this time. A Category 1 hurricane is certainly a good possibility by Thursday morning, as the storm should stay over the warm waters of the Gulf Stream until then. Once the storm gets north of Cape Hatteras North Carolina, the chances for continued intensification lessen, since water temperatures are in the 70s close to the coast.
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 2), only one hit land, and only two got as strong as a Category Two hurricane. If this storm does hit land, it will definitely buck the historical trend. North Carolina, Virgina, Maryland, and Nova Scotia appear to be the only land areas at risk from this storm.
Figure 1. Latest satellite of Tropical Depression Two.
Figure 2. Historical tracks of tropical cyclones that formed off the Carolina coast in July and August.
Elsewhere in the tropics
An area of thunderstorms a few hundred miles northeast of Puerto Rico is associated with a tropical wave interacting with an upper-level low pressure system. Although wind shear has dropped to 10 knots, there is a lot of dry air around, and the system is very disorganized. No tropical development of this system should occur through Wednesday as it tracks northwest at 15 mph towards Bermuda.
A large tropical wave is 300 miles southwest 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.
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