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Disturbance 95L over South Florida bringing heavy rains

By: Dr. Jeff Masters, 1:27 PM GMT on June 29, 2007

Clouds and showers associated with a weak low pressure system over South Florida have increased markedly this morning. NHC has designated this system "95L". Long range radar from Melbourne, Florida shows an expanding area of echoes, but there are no signs of any organization or spin. With wind shear remaining high at 20-30 knots, I'm not expecting any development of this disturbance today. Some of the computer models are forecasting that wind shear will slowly drop the next three days, so we'll have to keep a closer eye on 95L over the weekend. The system could bring heavy rains to South Florida and the northwestern Bahama Islands the next two or three days. A Hurricane Hunter airplane is on call to investigate the system Sunday at 2pm, if necessary.

A trough of low pressure is scheduled to push off the U.S. Southeast Coast on Monday, and will probably sweep 95L out to sea in front of it. There is a chance something could develop along the remains of a cold front the trough leaves behind over the Gulf Stream. However, none of the reliable computer models are forecasting any tropical storm formation over the coming week.

Figure 1. Preliminary model tracks for the disturbance near South Florida.

One other area to watch is the region just north of Panama in the Southwest Caribbean. Wind shear values are forecast to drop below 5 knots there by Sunday.

Wind shear decline expected
The jet stream usually divides itself into two branches this time of year--a strong jet whose average position is near the U.S.-Canadian border (the polar jet), and a weaker branch whose average position is over the Gulf of Mexico (the subtropical jet). Both of these branches of the jet stream bring high upper level winds (and thus high wind shear) over the Atlantic Ocean. All of the computer models are forecasting that the subtropical jet will weaken substantially over the next ten days, bringing much lower than average wind shear to the tropical Atlantic. It is normal to see the subtropical jet weaken in the summer, but it usually happens a month later than this--in August. The expected early weakening of the subtropical jet should give us an above-average risk of a July tropical storm. I'll have a full analysis of the possibilities on Monday, when I post my bi-monthly 2-week outlook.

Jeff Masters

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