By: Dr. Jeff Masters, 3:05 PM GMT on August 21, 2014
Heavy rain showers are sweeping through the Lesser Antilles Islands as a strong tropical wave (Invest 96L) heads west-northwestwards at about 15 - 20 mph through the islands. Satellite loops on Thursday morning showed a pretty unimpressive system, with a broad, elongated surface circulation and a modest amount of heavy thunderstorm activity that had changed little in intensity or organization since Wednesday. The storm was poorly organized, with a clumpy appearance and just a few small low-level spiral bands. Radar loops from Barbados and Martinique showed only a slight amount of rotation in the radar echoes. An 8:37 pm EDT Wednesday pass from the ASCAT satellite showed top surface winds near 35 mph on the east side of 96L. Wind shear is moderate, 10 - 20 knots, and water vapor satellite images and the Saharan Air Layer analysis show that 96L has moistened its environment considerably since Wednesday, though some dry pockets persist in the vicinity, particularly to the west. Sea Surface Temperatures (SSTs) are near 28°C, which favors development.
Forecast for 96L
Given 96L's disorganized appearance on satellite imagery, the Thursday afternoon flight of the Hurricane Hunters may get cancelled. The earliest I would expect 96L to become a tropical depression would be Friday morning. The 0Z Thursday runs of our three most reliable models for predicting tropical cyclone genesis, the UKMET, GFS, and European models, had one model, the UKMET, predicting potential development into a tropical depression by Saturday. The storm will pass through the Lesser Antilles Islands Thursday and Friday, bringing heavy rain showers and strong winds, which will also affect the Virgin Islands, Puerto Rico, and Dominican Republic Friday through Saturday. A Flash Flood Watch is in effect from late Thursday night through Saturday morning for the Virgin Islands, Puerto Rico, Culebra, and Vieques. Rainfall amounts of 4 - 6" with locally higher amounts are expected.
The circulation center of 96L has jumped considerably to the northwest over the past day, resulting in northward shifts in the expected track of the system from all of the major models. On Saturday, 96L may pass near or over the islands of Puerto Rico and Hispaniola, whose rugged terrain would likely disrupt the storm. By Sunday, 96L is expected to be near the Southeast Bahamas and Turks and Caicos Islands, and both the GFS and UKMET models predict that 96L will be able to develop into a tropical depression by Sunday or Monday. The 8 am EDT Thursday run of the SHIPS model predicted that wind shear would stay in the moderate range, 10 - 20 knots, though Sunday, then rise on Monday. With dry air expected to be in the region, wind shear would likely be able to drive the dry air into the circulation of 96L, keeping any development slow. A trough of low pressure is expected to be over the U.S. East Coast early next week, and the GFS and European models predict that this trough will be strong enough to turn 96L north and then northeast, keeping the storm away from the Southeast U.S. coast. However, long-range model forecasts of disturbances that haven't formed into a tropical depression yet are unreliable, and we should not be confident that 96L will miss the Mainland U.S. yet. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 50% and 70%, respectively.
Jeff Masters update...
Even if recon can close 96L off right now, which I find unlikely, I doubt the NHC pulls the trigger until convection gets more organized, because right now, its a mess.