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Gulf of Mexico Disturbance 92L Less Organized; Erin No Threat to Land

By: Dr. Jeff Masters, 3:30 PM GMT on August 17, 2013

Tropical disturbance 92L over the South-Central Gulf of Mexico is looking less organized today due to dry air from an upper-level low pressure system over the Gulf. Satellite loops show that 92L has lost the well-developed surface circulation it had Friday, and its heavy thunderstorms are elongated and not well-organized, even though wind shear is a moderate 10 - 20 knots over the the wave, and ocean temperatures are a very warm 29 - 30°C. The hurricane hunter flight scheduled for today has been cancelled and re-scheduled for Sunday afternoon.

Figure 1. Visible satellite image of 92L taken at 10:30 am EDT Saturday August 17, 2013. Image credit: NASA/GSFC.

Forecast for 92L
The 12Z Saturday SHIPS model forecast predicts that 92L will remain in an area of low to moderate wind shear through Monday, and ocean temperatures will be a favorable 29 - 30°C. However, the dry air due to the upper-level low over the Gulf will continue to keep any development slow. A trough of low pressure is over the northern Gulf of Mexico and is bringing high wind shear values of 20 - 30 knots just to the north of 92L. This trough may also be able to pull the storm northwestwards to a landfall in Louisiana, Texas, or Northern Mexico on Monday of Tuesday, as suggested by all of our top models. The farther north 92L penetrates, the more difficult intensification becomes, due to higher wind shear values to the north. Regardless of 92L's track, a flow of moist tropical air along the storm's eastern flank will form an atmospheric river of moisture that will bring a swath of 5+ inches of rain to the Gulf Coast from Louisiana to the Florida Panhandle over the next few days. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 92L a 40% of developing by Monday, and a 50% chance of developing by Thursday.

Figure 2. Predicted precipitation for the 7-day period ending on Saturday, August 24, 2013. Tropical moisture flowing north and northeastwards from 92L over the Southeast U.S. is expected to create a swath of 5+ inches of rain along the coast. Image credit: NOAA/HPC.

Tropical Storm Erin
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. This is due, in part, to the 25.5 - 26°C waters the storm is traversing, which is a marginal water temperature for tropical cyclones. Erin is also having trouble with dry air from the Saharan Air Layer (SAL), and the storm's northwest motion has begun to cut Erin off from a moist source of air to its south--the semi-permanent band of tropical thunderstorms called the ITCZ (Inter-Tropical Convergence Zone.) Several of the major global computer models call for Erin to dissipate over the next few days, and it is unlikely that the storm will ever threaten any land areas.

Figure 3. MODIS satellite image of Tropical Storm Erin taken at 10:30 am EDT Saturday August 17, 2013. At the time, Erin had maximum sustained winds of 40 mph. Image credit: NASA.

The tropical wave that spawned Erin began over East Africa on August 9, dumping 0.83" of rain on Khartoum, Sudan, worsening a devastating flooding situation there. Periodic torrential rains in Sudan that began on August 2 have triggered flash flooding that has killed 53 people. More rain is on the way for the waterlogged nation, as a series of strong tropical waves associated with an unusually active African monsoon parade across the continent. Several of these tropical waves have the potential to grow into tropical storms once they exit the coast, but the latest 06Z run of the GFS model and 00Z run of the ECMWF model are not calling for any new tropical waves to develop off of the coast of Africa over the next seven days.

Wunderblogger Lee Grenci just posted this excellent analysis of Typhoon Utor's eyewall replacement cycle before the storm made landfall in the Philippines last week.

Video 1. Debris flies as Typhoon Utor hits Zhapo, China on 14th August 2013. Video taken by storm chaser James Reynolds, and brought to my attention by wunderground member Robert Speta.

Have a great weekend, everyone!

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.