Hurricane Hunters find a tropical depression forming; Henriette lashes Pacific coast of Mexico

The Hurricane Hunters have reported back from the tropical wave about 200 miles east of the southern Lesser Antilles Islands, 94L, and found a closed circulation with top winds of 30 mph. This qualifies 94L as a tropical depression, and NHC will likely upgrade it to Tropical Depression Six in their 5pm update. This storm has the potential to become a hurricane in the Caribbean by Sunday or Monday. Visible satellite loops show a considerable improvement in organization occurred today, with low level spiral bands wrapping around the center of circulation, and upper level outflow now visible on both the north and south sides.

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 10 knots over 94L, and is expected to remain below 10 knots for the next four days over the storm's expected path through the Caribbean, according to the 18Z SHIPS model. As seen in the latest microwave satellite image from 12:49pm EDT (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 12:49pm EDT 8/31/07. The heaviest rain (red colors) is just east of the island of Tobago. Image credit: Navy Research Lab.

Both the 12Z GFDL and the 18Z SHIPS intensity models forecast that 94L will intensify into a hurricane by 72 hours from now, as it tracks through the Central Caribbean. The model consensus has a track just north of the ABC islands (Aruba, Bonaire, and Curacao), then west-northwestward through the Caribbean. By Monday night, most of the models have 94L approaching the Honduras/Nicaragua border. The GFDL is further north, taking 94L into Belize on Wednesday as a Category 2 hurricane. 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. However, a strong ridge of high pressure is expected to build in over the region, and I don't see any troughs of low pressure capable of swinging 94L to the north coming.

The next Hurricane Hunter mission will be 8am EDT Saturday. The NOAA jet's first flight will be Sunday morning.

Links to follow today:
Martinique 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 got close to tropical depression status today. However, wind shear from strong upper level westerly winds has almost completely removed all heavy thunderstorm activity from the storm, and 96L has missed its chance to become a tropical depression.

Pacific coast of Mexico at risk from Henriette
Tropical Storm Henriette formed off the Pacific coast of Mexico this morning, and poses a threat to Mexican coast from Acapulco to Puerto Vallarta, as well as the Baja Peninsula. Acapulco radar shows that spiral banding around the center has increased today, and visible satellite loops show a rather disorganized system, with the beginnings of some upper-level outflow to the south. Henriette has brought heavy rains and sustained winds of 25 mph to Acapulco today, but the weather there will improve tonight as the storm tracks west-northwest, parallel to the coast. Wind shear is a rather stiff 20 knots over Henriette today, which should keep any development slow. By tomorrow, wind shear should decrease to 10-15 knots, allowing more rapid strengthening. Residents of Manzanillo should be prepared for tropical storm force winds Saturday evening, and the airport there will probably close for a time late Saturday. Puerto Vallarta is further from Henrietta's projected path and more sheltered, and will probably not get tropical storm force winds. Those planning on being in Baja Monday and Tuesday should keep a close eye on Henriette, as hurricane conditions may arrive late Monday.

Links to watch for Henriette
Acapulco radar
Manzanillo, Mexico observations

I'll edit this blog when the 5pm advisory comes out. Next full update will be Saturday by noon EDT.

Jeff Masters