Caribbean Disturbance 92L Moving Over the Yucatan; Erin Forms Off of Africa

The tropical wave in the Western Caribbean near the Yucatan Peninsula (92L) is growing more organized this morning, after an evening when it lost most of its heavy thunderstorm activity. Satellite loops show a modest-sized area of heavy thunderstorms that are increasing in intensity and areal coverage, but there are a no signs of a surface circulation. Winds at surface stations in the Western Caribbean also do not show a surface circulation. The highest surface wind reports this Thursday morning were at Western Caribbean buoy 42056 about 140 miles east southeast of Cozumel, which had east winds of 25 mph, gusting to 34 mph, at 10 am EDT. Wind shear is a moderate 10 - 20 knots over the the wave, which should allow slow development today until its west-northwest movement at 10 - 15 mph carries it over the Yucatan Peninsula Thursday afternoon. Four hurricane hunter flights were scheduled to fly into 92L today--an Air Force mission tasked to provide a center fix early this afternoon, two NOAA P-3 missions aimed at collecting real-time radar data to feed into the HWRF model, and a flight by the NOAA jet to collect dropsonde data around the periphery of 92L. However, all of these flights were cancelled, given that 92L did not organize as much as much as it could have.


Figure 1. MODIS satellite image of Invest 92L taken at 12:30 pm EDT Thursday August 15, 2013. Image credit: NASA.

Forecast for 92L
92L will trek across the Yucatan Peninsula Thursday evening and arrive in the Southern Gulf of Mexico on Friday, when it will have the opportunity to strengthen. The 06Z Thursday SHIPS model forecast predicts that 92L will remain in an area of low to moderate wind shear through Saturday, and ocean temperatures will be a favorable 29 - 30°C. The topography of the Southern Gulf of Mexico's Bay of Campeche can aid in getting a storm spinning more readily, but 92L may be far enough north that this influence will be negligible. Given all these factors, 92L should be able to become at least a tropical depression by Saturday, A trough of low pressure over the northern Gulf of Mexico will dip down by Saturday evening over the Central Gulf of Mexico, potentially increasing wind shear to a high 20 - 30 knots, stalling any further intensification. This trough may also be able to pull the storm northwards to a landfall between Eastern Louisiana and the Florida Panhandle, as the 00Z Thursday run of the European model is suggesting. This would bring a plenty of tropical moisture into the Southeast U.S., resulting in a large area of 4+" of rain. However, the other models show a more westerly track for 92L, with landfalls possible in Texas or Mexico south of the Texas border, and there is high uncertainty where 92L may go once it enters the Gulf of Mexico. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 92L a 50% of developing by Saturday, and a 60% chance of developing by Tuesday.


Figure 2. MODIS satellite image of Tropical Storm Erin taken at 10:30 am EDT Thursday August 15, 2013. At the time, Erin had maximum sustained winds of 40 mph. Image credit: NASA.

Tropical Storm Erin
The season's fifth named storm of the year, Tropical Storm Erin, has formed over the far Eastern Atlantic off the coast of Africa. Erin is over warm waters of 27°C and is under a moderate 10 - 15 knots of wind shear, which should allow continued development today and Friday. Erin is a small storm, as seen on satellite loops. The 12Z Thursday SHIPS model forecast predicts that wind shear will be low to moderate for the next five days, which favors development. However, the waters beneath Erin will steadily cool to a marginal 26°C by Friday, and the atmosphere will steadily get drier, as the storm encounters the Saharan Air Layer (SAL), discouraging development. Erin's west-northwest motion will cut the storm off on Sunday from a moist source of air to its south--the semi-permanent band of tropical thunderstorms called the ITCZ (Inter-Tropical Convergence Zone.) The storm should weaken beginning on Sunday, which would result in Erin turning more to the west as the east-to-west blowing surface trade winds begin to dominate the steering of the shallower storm. We may see a situation like occurred for Tropical Storm Dorian in late July--intensification to a 60 mph tropical storm, followed by a slow decay and dissipation. The latest run of the GFS model calls for Erin to dissipate well before reaching the Lesser Antilles Islands.

Typhoon Utor dissipates
Typhoon Utor has dissipated after hitting Southeast China about 150 miles southwest of Hong Kong on Wednesday as a Category 1 storm with 90 mph winds. The typhoon is being blamed for 1 death in China, and sank a 21-person cargo ship off the coast. In the Philippines, where Utor hit as a Category 4 storm with 140 mph winds on Monday, 8 deaths are being blamed on the storm, and damage is estimated at $20 million.

Jeff Masters