Invest 99L Organizing Over Gulf of Mexico

By Dr. Jeff Masters
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Published: 2:22 PM GMT on September 01, 2014

The center of a broad area of low pressure associated with tropical wave 99L is now over the warm waters of the Gulf of Mexico's Bay of Campeche, and the disturbance is growing more organized as it heads west-northwest at about 10 mph. Heavy rains are falling over Mexico's Yucatan Peninsula and southeastern Bay of Campeche coast, and radar loops out of Sabancuy, Mexico show a pronounced rotation to the echoes. Satellite loops on Monday morning showed the storm's heavy thunderstorm activity was increasing in intensity and organization, with a number of low-level spiral bands beginning to form. Sea Surface Temperatures (SSTs) were very warm, near 30°C (86°F), the atmosphere was moist, and wind shear was moderate, 15 - 20 knots. These conditions are favorable for development. The 8 am Monday run of the SHIPS model predicted that conditions will remain favorable for development over Bay of Campeche through Thursday, with moderate wind shear, a moist atmosphere, and warm SSTs of 30°C (86°F.) None of our three reliable computer models for predicting tropical storm formation showed 99L developing into Tropical Storm Dolly in their Monday morning runs. However, I expect that 99L will be at least a tropical depression by Tuesday morning. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 99L 2-day development odds into a tropical cyclone of 60% (a tropical cyclone is a generic term for all tropical depressions, tropical storms, and hurricanes.) The storm should continue to track to the west-northwest or northwest, with landfall occurring on the Mexican coast several hundred miles south of the Texas border on Tuesday or Wednesday morning. This likely will not give the storm time to intensify into a hurricane, though landfall as a strong tropical storm would not be a surprise. An Air Force hurricane hunter aircraft has been tasked to investigate 99L on Monday afternoon.


Figure 1. Latest satellite image of Invest 99L in the Western Caribbean.

New African tropical wave this weekend may develop
Our three reliable computer models for predicting tropical storm formation all show development by Saturday of a tropical wave expected to come off the coast of Africa on Friday. This wave will be capable of bringing heavy rain and strong winds to the Cape Verde Islands on Friday and Saturday.

Eastern Pacific's 93E a potential threat to Baja
In the Eastern Pacific, tropical disturbance Invest 93E is gathering strength a few hundred miles south of the southwestern coast of Mexico. In their Monday morning runs, both the GFS and European models predicted that 93E would develop into a tropical storm late this week, and pass very close to the tip of Mexico's Baja Peninsula on Friday. Satellite loops show that 93E is poorly organized today, and I expect that the earliest the disturbance would become a tropical depression is Wednesday. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 93E 2-day and 5-day odd of development of 20% and 70%, respectively.

Jeff Masters

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About The Author
Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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