African Wave 94L Has Potential to Develop; 92L Dead; Erin Dying

It's time to turn our attention to the coast of Africa, where a new tropical wave (94L) has just emerged over the Tropical Atlantic. Wind shear is a moderate 10 - 20 knots over 94L, and water temperatures are warm enough for development, 27.5°C. Satellite loops show that 94L has a modest amount of spin, but the storm's heavy thunderstorms are not very intense, and are poorly organized. The 12Z Sunday run of the SHIPS model predicted that during the next five days, wind shear for 94L will be mostly in the moderate range, and ocean temperatures will slowly cool to 26.5°C. These conditions should allow for some slow development. As usual, dry air from the Saharan Air Layer will likely be an impediment to development, as the 11 am EDT Sunday SAL analysis showed a large pulse of dry air and dust exiting from the Sahara just to the north of 94L. The Sunday 06Z run of the GFS model and 00Z run of the ECMWF model did not calling for 94L to develop. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 94L a 10% of developing by Tuesday, and a 30% chance of developing by Friday.


Figure 1. Saharan Air Layer (SAL) analysis from 11 am EDT Sunday August 18, 2013. A large pulse of dust and dry air was exiting the coast of Africa just north of tropical wave 94L. Note the swirl of dry air marking the center of Tropical Depression Erin, near 20°N 40°W. Image credit: University of Wisconsin/NOAA-HRD.

Gulf of Mexico disturbance 92L dies
The tropical disturbance (92L) that was over the Gulf of Mexico the past few days has now degenerated into a trough of low pressure with little heavy thunderstorm activity, and is no longer a threat to develop. However, a flow of moist tropical air will take some of the remnants of 92L northwards over the Southeast U.S. over the next few days, bringing a swath of 3+ inches of rain to the Gulf Coast from Louisiana to the Florida Panhandle.


Figure 2. MODIS satellite image of Tropical Depression Erin taken at 10:30 am EDT Sunday August 18, 2013. At the time, Erin had maximum sustained winds of 35 mph. Image credit: NASA.

Tropical Storm Erin dying
Tropical Storm Erin over the Eastern Atlantic is small and weak and has lost nearly all of its heavy thunderstorms, as seen on satellite loops. Dry air, moderate wind shear, and marginal water temperatures will likely destroy the storm by Monday, as predicted by all of the reliable global computer models and the NHC.

Have a great weekend, everyone!

Jeff Masters