Gulf of Mexico System (Invest 99-L) Impacting U.S. and Mexico; Low Chance of Tropical Development

Jon Erdman
Published: September 27,2015

El Niño has certainly exerted its influence on the 2015 Atlantic hurricane season.
(MORE: An Atlantic First In Over 100 Years | Early End to the Season?)
Record high vertical wind shear, which tends to rip apart developing or mature tropical cyclones, has been in place over the Caribbean Sea, according to Colorado State University's Dr. Philip Klotzbach (Wunderblog).
Despite that, it is still late September, and homegrown tropical cyclones spinning up closer to the U.S. can occur. With that in mind, we have an eye on the Gulf of Mexico over the next several days.
Even without tropical development, impacts will be seen along the Gulf Coast. In fact, flooding has already been reported in southern Mississippi and Alabama and a flash flood emergency was issued in eastern Jackson county Mississippi Sunday evening.

Development Factors

An area of low pressure in the southern Gulf of Mexico has been designated as Invest 99-L by the National Hurricane Center (NHC). This means this is a feature the NHC is monitoring for further tropical development.
(MORE: What is an Invest?)
The upper-level wind pattern features a southward dip of the southern-branch jet stream centered over Texas and northeast Mexico, with weak high pressure aloft centered near the northwest Bahamas.
This wind flow aloft will pull Invest 99-L north through the Gulf of Mexico.

Invest 99-L and Jet Stream Level Winds

That may sound ominous if you're concerned about tropical storms or hurricanes. Let's go through a checklist of ingredients to see how likely a tropical system is to form.

Favorable Factor	In the Gulf?	Comments
Warm water	Yes	Near or above average, and plenty warm enough for tropical development
Moist air	Yes	Deep moisture pumped from the tropical eastern Pacific and Central America will be well above late September average over at least the central and eastern Gulf of Mexico.
Unstable for thunderstorms	Yes	Gulf water warming and adding moisture to the air above it + jet stream dip to the west mentioned above = plenty of instability.
Low/no wind shear	Likely Not	That same jet-stream dip, however, will create hostile wind shear over a large swath of the Gulf of Mexico.

That one negating factor, wind shear, is why any threat of tropical cyclone development in the Gulf of Mexico remains low at this time.
Despite that low chance, the NHC has scheduled a Hurricane Hunter reconnaissance mission on Monday to investigate the disturbance, if needed.

Impacts, Regardless

Whether or not an area of low pressure does acquire enough organization to be designated a tropical depression or storm, there will still be impacts along the Gulf Coast through early this week.
In fact, the upper-level weather pattern mentioned above has already tapped moisture in Gulf of Mexico and northwest Caribbean. This has resulted in locally heavy rain along parts of the Gulf Coast. A flash flood emergency was issued Sunday evening for eastern Jackson county in southern Mississippi, including Moss Point, Pascagoula and Excatawpa where 5 to 8 inches of rain fell in less than 2 hours, with the rain contining. This has resulted in flooding across the area.

Gulf Coast Potential Impacts

• Locally heavy rain: Bands of locally heavy rain will continue to push ashore along, particularly, the central and eastern Gulf Coast. We can't rule out a few heavy rainbands, as well, along the western Gulf Coast. Rain rates of 2 inches per hour, or more, may occur, triggering local flash flooding.
• High surf, rip currents: Persistent east to southeast winds may whip up high surf and rip currents from the Upper Texas coast to the Florida panhandle. Some elevated surf may also propagate to parts of the Lower Texas Gulf Coast. 
• Coastal flooding: This persistent onshore flow may lead to areas of at least minor coastal flooding, particularly around high tide, particularly from southeast Louisiana to the western Florida panhandle. The longevity and strength of onshore flow over the western Gulf (Texas), and thus, the coastal flood threat, there, remains uncertain at this time.

(FORECASTS: S. Padre Island | Biloxi | Destin)
We don't recommend canceling any Gulf Coast beach or fall festival plans right now.
However, we do recommend visitors with little experience at the beach to familiarize yourself with the dangers of rip currents.
Look for surf hazard warning flags and heed the advice of lifeguards. Even experienced swimmers should stay out of the water when rip currents are a threat.
Over 30,000 rip current rescues are performed each year in the U.S., often occurring in waves of just 2-3 feet. Rip currents claim about 100 lives each year in the U.S.
Check back with us at weather.com and The Weather Channel for updates on this Gulf of Mexico disturbance.

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