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 , 1:12 PM GMT on July 17, 2006
A cold front that pushed off the U.S. East Coast this weekend stalled out, and has spawned two areas of low pressure. The first is an extratropical or subtropical low that developed along the north portion of the front. This low is about 300 miles east-southeast of Cape Cod, Massachusetts, and is moving northeast away from the U.S. The storm is over the Gulf Stream where water temperatures are in the upper 70s, which is borderline for a tropical storm. The low has a large area of heavy thunderstorms on the northeast side of the center of circulation, and is undergoing significant wind shear that is exposing the center. To my eye, the system is probably a subtropical storm, and technically should be classified as Subtropical Depression Two. However, is it difficult to tell for sure, and the NHC is conservatively not naming it, since it is headed towards colder water and has little chance of becoming a full tropical storm.
Figure 1. Visible satellite image for 8am EDT July 17 2006, showing the various areas of interest along the 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. There is a narrow area of reduced wind shear of 10-15 knots over this region, and 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 system might go. The models seem to lean towards it heading northeast up the coast, possibly brushing Cape Hatteras and Nova Scotia later in the week. However, the system is currently very broad and disorganized, and any development will be slow to occur.
A very large area of concentrated thunderstorms is a few hundred miles north of Puerto Rico. This is associated with a tropical wave interacting with an upper-level low pressure system. Wind shear is 25-35 knots over this area, and no tropical development is expected. We will have to watch this area later in the week as it approaches the East Coast, as wind shear may fall and allow some slow development.
A solid-looking tropical wave has just emerged off the coast of Africa, and is headed towards the Cape Verde Islands at 15-20 mph. Water temperatures and wind shear are marginal for tropical development in this region, and I'm not expecting this wave to be a threat. It's very common this time of year to see impressive looking waves come off the coast of Africa, only to fizzle after spending a day over the water. By early August, I'll start taking these waves more seriously, when the waters are a bit warmer.
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