Two Atlantic storms are near tropical depression strength

The tropical wave about 200 miles east of the southern Lesser Antilles Islands, 94L, continues to become better organized, and will probably become a tropical depression today. This system has the potential to become a hurricane in the Caribbean Sea early next week. Visible satellite loops show a steadily organizing system, with low level spiral bands trying to form, and upper level outflow to the north, visible as cirrus clouds wafting from west to east. QuikSCAT winds from 5:36am EDT this morning showed that 94L's circulation was not well-defined yet. Sustained winds of 25-35 knots were observed, and some wind gusts to 55 mph are probably occurring on the south side of 94L's circulation. Water vapor satellite loops show the presence of a large amount of dry air to the north of 94L, but the storm has generated enough thunderstorm activity to moisten the surrounding environment, which should speed development. Wind shear is a favorable 5-10 knots over 94L, and may decrease some by Saturday, allowing further development. As seen in the latest microwave satellite image of 94L (Figure 1), the heaviest thunderstorm activity is on the south side of the system. Trinidad and Tobago will get the heaviest rains from 94L, although the northern coast of South America plus the island of Grenada could also see some heavy rains.


Figure 1. Microwave satellite image of 94L taken at 6:07am EDT 8/31/07. Image credit: N avy NRL Research Lab.

None of the reliable computer models forecast that 94L will develop into a tropical depression, but it look like the models will be wrong on this. The usually unreliable Canadian model appears to have the best idea on what 94L will do. This model gradually strengthens 94L as it passes along the northern coast of South America. By Monday night, the Canadian model has 94L approaching the Honduras/Nicaragua border. Yesterday afternoon's run of the GFDL model had a similar solution, and predicted 94L would be a Category 2 hurricane at that time. The future strength of 94L depends critically upon how close it passes to the South American coast over the next two days. The Southeastern Caribbean just north of the South American coast is a climatologically unfavorable region for tropical cyclones, as they tend to pull dry continental air off of South America into their circulations. Many tropical cyclones passing through this region of the Caribbean die or become severely weakened. If 94L does survive and pass into the Central Caribbean, a more northwesterly turn to threaten Haiti, Jamaica, and Cuba is not out of the question.

The Hurricane Hunters will be in the storm this afternoon. The NOAA jet's first flight will be Sunday morning.

Links to follow today:
Guadaloupe radar
Barbados weather
Trinidad Crown Point weather
Piarco, Trinidad weather
Grenada weather

96L
An area of low pressure ("96L") a few hundred miles south of Nantucket, Massachusetts is close to tropical depression status. The storm has a well-defined surface circulation, but strong upper-level winds from the southwest have kept heavy thunderstorm activity confined to 96L's northeast side. Low-level spiral bands on its northeast side are impacting Cape Cod and Nantucket today, as seen on long-range radar out of Boston. QuikSCAT saw winds as high as 30 knots (35 mph) in this morning's pass. The combined wind and pressure plot from buoy 44004, 200 nm east of Cape May, NJ, shows the passage of 96L nicely.

96L has about a day before increasing wind shear and cooler sea surface temperatures convert the storm into an extratropical system. The eastern tip of Nova Scotia and the south coast of Newfoundland could experience tropical storm-force winds from this extratropical system over the weekend.

Coast of Africa
None of the computer models are forecasting the development of a tropical depression during the coming week off the coast of Africa.

I'll post an update later today.
Jeff Masters