Invest 99-L Still May Develop in the Gulf of Mexico Next Week

August 26,2016
Invest 99-L, a tropical disturbance we've been following since last week, still has a chance of developing into a tropical depression or storm next week in the Gulf of Mexico.
(MORE: What is an Invest?)
Wind shear, the change in wind speed and/or direction with height, hammered any attempt of 99-L to consolidate convection near a surface low-pressure center the last few days.
Wind shear tends to inhibit the development of tropical cyclones by blowing the convection away from any area of low pressure trying to form.
However, Invest 99-L is now moving into an environment of rapidly diminishing wind shear.
(MORE: Hurricane Central)

Current Satellite, Wind Shear

Late Friday morning, more thunderstorms began to sprout generally to the east and southeast of the location of the broad area of low pressure, or tropical wave.
(MORE: Why Tropical Waves Are Important During the Hurricane Season)

Invest 99-L Infrared Satellite Image

South Florida/Bahamas: Rain, Gusty Winds Possible

A consensus of our guidance suggests the disturbance should still move in a general west or west-northwest trajectory the next several days.
(MORE: Three Things to Know About Spaghetti Model Forecasts)

Forecast Model Tracks: Invest 99-L

The National Hurricane Center (NHC) says this system has a low chance of development into a tropical depression or tropical storm over the next two days.
In essence, wind shear needs to be low enough to allow sufficiently persistent convection to flare up over the actual low pressure area. If that doesn't happen, no tropical cyclone forms.

Regardless, the main threat to South Florida and the Bahamas through the weekend will be pockets of locally heavy rain and flash flooding.
Some areas will likely pick up over 3 inches of rain, possibly in a short period of time, triggering flooding, particularly in urban areas.
(MORE: South Florida's Increasing Vulnerability to Flooding)

Rainfall Outlook Through Wednesday

These pockets of locally heavy rain may continue through at least Sunday, if not into Monday, and may eventually spread into parts of central Florida, as well.
(FORECAST: Miami | Key West | Tampa)

Gulf Coast Threat?

Upper-level high pressure is now established over the southern Appalachians and Mid-Atlantic states.

Forecast steering flow in the upper-atmosphere this weekend. An upper-level high pressure system is expected to steer the tropical system toward Florida once it's near the Bahamas.

With the upper-level high in place, this system will likely be pushed into the Gulf of Mexico.
Any area of low pressure in the Gulf of Mexico in the heart of the hurricane season bears watching closely.
For now, there is a moderate chance, according to the National Hurricane Center, for development of a tropical cyclone next week in the Gulf of Mexico.
That upper-level high, however, is expected to shift east, then weaken. This, plus the uncertain intensity (anything from an undeveloped low to a tropical storm, even low-end hurricane), makes for a highly uncertain outlook for the Gulf of Mexico.
(MORE: Where Every U.S. Hurricane Has Hit Since 1985 | Record Gulf Hurricane Drought)
Regardless of the system's ultimate intensity, rainfall flooding looms as a significant threat along or near its track.
(MORE: Rainfall Flood Concern for Saturated Gulf Coast)
In fact, there already is a plume of moisture and clusters of slow-moving thundershowers over parts of Louisiana and Texas not associated with 99-L. Since Louisiana is so flood prone right now, the situation will have to be closely monitored.
A rather expansive swath of the Gulf Coast region, not simply Louisiana, but also parts of the Florida Panhandle and Texas, have been soaked in August.

August 2016 rainfall, through August 23.

The threat of heavy rainfall is not a function of tropical cyclone intensity, but rather the system's slow movement and availability of deep, tropical moisture, as we saw with an unnamed system earlier this month triggering Louisiana's epic flooding.
For now, if you have interests anywhere along the Gulf Coast, from Texas to Florida, check back with us at weather.com for any important forecast changes in the days ahead.
(MORE: Most Intense U.S. Landfalls Have Happened in a 17-Day Period)
Regardless of Invest 99-L's future, now is a good time to make sure you have a plan before a hurricane hits.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.