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 , 10:53 PM GMT on July 16, 2006
A cold front that pushed off the U.S. East Coast this weekend and stalled out has created an interesting situation to watch. An extratropical or subtropical low has developed along the north portion of the front, about 300 miles southeast of Cape Cod, Massachusetts. Water temperatures are right at the borderline for a tropical storm--about 80 Fahrenheit--so this low is unlikely to develop into a tropical storm as it heads northeast out to sea towards colder water. The low has a large area of heavy thunderstorms on the southeast side of the center of circulation, well removed from the center. This is characteristic of a subtropical storm experiencing significant wind shear. This is the expected development I was referring to this morning in my blog.
Figure 1. Current satellite image of the U.S. East Coast.
The departing low has left behind the tail end of the old cold front over the waters off the South Carolina/North Carolina coast. This old front could serve as the focus for some tropical development over the next few days. Already, some tropical-looking thunderstorms have built over the front, in a narrow area of reduced wind shear of 5-10 knots that has developed. The GFS computer model is indicating that the area of reduced shear will remain. Steering currents are weak, so it is difficult to tell where this tropical blob might go. The models seem to lean towards this system heading northeast up the coast, possibly brushing Cape Hatteras and Nova Scotia later in the week.
Wind shear visualized
I've linked a photo of an odd fog formation a wunderphotographer in Alaska took--I've never seen anything like this photo! It shows very graphically what strong wind shear can do to contort a cloud into strange shapes. The wind speed and direction are different at the bottom of this fog bank than at the top, creating a twisting, shearing effect on the cloud that bends it into strange shapes. Now imagine what strong wind shear can do to a developing tropical depression--ouch!
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