93L in Middle Atlantic Close to Tropical Depression Status

An area of disturbed weather located near 10°N, 39°W at 8 am EDT Tuesday, about 1600 miles east of the Lesser Antilles Islands, (93L), has the potential to develop into a tropical depression by Wednesday, but is struggling with high wind shear today. Visible satellite loops on Tuesday morning showed improved organization to 93L with more spin and some low-level spiral bands beginning to form. A 7:30 am EDT July 29 pass from the ASCAT satellite showed plenty of west winds on the south side of 93L's center of circulation, so the storm is close to having a well-defined closed surface circulation. However, infrared satellite images showed that the system's heavy thunderstorm activity had diminished somewhat since Monday, and the storm is now fighting high wind shear of about 20 knots. Water vapor satellite loops and the Saharan Air Layer analysis showed that 93L had more dry air to contend with than on Monday, with some tendrils of the dry Saharan Air Layer to the north encroaching into the circulation. Ocean temperatures have cooled since Monday, and are now marginal for development, about 27°C.


Figure 1. Analysis of the Saharan Air Layer (SAL) made at 8 am EDT Tuesday July 29, 2014 using data from the Meteosat-9 satellite. Dry, dusty air was present from the coast of Africa westwards across the tropical Atlantic, and was beginning to encroach from the north into tropical disturbance 93L. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Forecast for 93L
Given the high levels of wind shear affecting 93L today, it is more likely that NHC will classify it as a tropical depression on Wednesday than today. The 12 UTC Tuesday forecast from the SHIPS model predicted that the current high wind shear affecting 93L will relax to the moderate level, 5 - 15 knots, Wednesday afternoon through Friday, aiding development. Sea Surface Temperatures (SSTs) will slowly warm as the system approaches the Caribbean, reaching 28°C by Friday and Saturday. Two of our three reliable models for predicting tropical cyclone genesis, the GFS and UKMET models, predicted in their 00Z Tuesday runs that the disturbance would develop into a tropical depression by Thursday. The fact that two out of three of the reliable genesis models predict development bolsters the odds that development will actually occur. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC put the 2-day and 5-day odds of development at 70% and 80%, respectively.

All of the models predict that the disturbance will continue west or west-northwest at 10 - 15 mph for the next four days. The UKMET and the European models offer the fastest solution, predicting that the disturbance will arrive in the northeast Lesser Antilles Islands on Friday evening, move over Puerto Rico on Saturday evening, and approach the Southeast Bahamas and Turks and Caicos Islands on Sunday evening. The GFS is slower and more to the northeast, predicting a Saturday morning arrival in the northern Lesser Antilles, with passage a few hundred miles northeast of Puerto Rico occurring on Sunday morning. Dry air to the north of 93L will likely interfere with development throughout the week, and we will have to see if the moderate levels of wind shear forecast to occur will be capable of driving this dry air into the core of the system, disrupting formation. The disturbance may also have trouble disentangling itself from the Intertropical Convergence Zone (ITCZ), the band of heavy thunderstorms that circles the globe in the tropics, which lies just to the south of the disturbance. Clusters of thunderstorms in the ITCZ may compete for moisture and energy, slowing development of the disturbance. The Tuesday morning runs of our two most reliable models for predicting intensity, the LGEM and DSHIPS models, forecast that once 93L became a tropical depression, it would intensify into a hurricane within 72 hours. However, the dynamical GFDL and HWRF models, which made good intensity forecasts for Hurricane Arthur, were much less bullish. The Tuesday morning runs of these models predicted that 93L would never reach hurricane strength. I give a 10% chance that 93L will be a hurricane on Saturday when it makes its closest approach to the Lesser Antilles Islands. If 93L hits the Northern Lesser Antilles as a wet Tropical Storm Bertha (not a hurricane), the storm could be more boon than bane for the islands. The Northeast Caribbean suffered its driest June in recorded history last month, according to NOAA, and many of the islands have significant drought problems.

The GFS and European models have come into better agreement on the long-range fate of 93L. The great majority of the 20 members of the 00Z and 06Z Tuesday runs of the European and GFS ensemble models (which run at low resolution 20 times with slightly different initial conditions to show a range of possible outcomes) showed 93L taking more of a northwesterly track early next week, passing to the north of Hispaniola and not making an extended track through the Caribbean Sea. This raises the odds that the strong trough of low pressure over the Eastern U.S. will be able to recurve 93L out to sea without the storm hitting the mainland U.S. coast.


Figure 2. Image taken at 9:54 am EDT July 29, 2014, from the Guam radar, showing heavy rains from an outer spiral band of Tropical Storm Halong affecting Guam. The eyewall of the intensifying storm is visible to the east.

Tropical Storm Halong approaching Guam
In the Western Pacific, Tropical Storm Halong is close to becoming a Category 1 typhoon, and is expected to pass within 50 miles of Guam near 12 UTC (8 am EDT) on Wednesday. Guam radar is showing that heavy rains from an outer spiral band of Halong have already arrived. Halong may be a long-range threat to Japan.

Jeff Masters