Invest 99-L Soaks Hispaniola; May Become Hermine This Weekend; Threat to Florida, Gulf Coast

August 25,2016
Invest 99-L, a tropical disturbance near the Bahamas, may develop into a tropical depression or storm, and will move near South Florida and into the Gulf of Mexico this weekend into next week.
(MORE: What is an Invest?)
Right now, Invest 99-L remains a broad area of low pressure pushing through the southeast Bahamas.

Invest 99-L Infrared Satellite Image

Multiple Hurricane Hunter reconnaissance missions have flown through 99-L the past few days, but have been unable to find a tight, consolidated surface low pressure center that would have prompted the initiation of advisories for Tropical Depression Eight.
(MORE: Hurricane Central)

Caribbean Impact

One factor that appears to be taking its toll on an attempt of 99-L to consolidate convection near a surface low-pressure center is wind shear, namely, the change in wind speed and/or direction with height, north of 99-L.

Current Satellite, Wind Shear

Wind shear tends to inhibit the development of tropical cyclones by blowing the convection away from any area of low pressure trying to form. This is indeed the case right now.
Regardless of development, this tropical wave will have one primary impact in its brush with the Caribbean basin, locally heavy rainfall. The heavy rain could cause flash flooding and mudslides in Hispañiola and eastern Cuba into Friday.
(MORE: Why Tropical Waves Are Important During the Hurricane Season)

Rainfall Outlook Through Monday

The National Hurricane Center (NHC) says this system has a low to medium chance of development into a tropical depression or tropical storm over the next 2 to 5 days.
The chance of development in the next two days has decreased compared to earlier projections. However, upper-level winds may become a bit more favorable for development this weekend into early next week when the disturbance is near the Florida Keys or in the southeast Gulf of Mexico, the NHC said.

A consensus of our guidance suggests the disturbance should continue 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

South Florida: Rain, Gusty Winds Possible

This system, in whatever form it's in, should be near or west of the central Bahamas by Friday, then move more slowly toward the Florida Keys by this weekend.
Once it reaches this area, the environment may feature low-enough wind shear and sufficient moisture to allow a tropical depression or storm to finally form.
By this time, upper-level high pressure should be 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.

This steering flow is expected to push the tropical system toward South Florida and the Florida Keys later in the Sunday or Monday timeframe.
(MORE: Where Every U.S. Hurricane Has Hit Since 1985)
For now, the intensity is quite uncertain for when the system nears South Florida, but ranges from an undeveloped tropical disturbance to perhaps a tropical depression or tropical storm. At a minimum, impacts will include locally heavy rain and some gusty winds in southern Florida no matter how well developed this system becomes by that point.
(MORE: South Florida's Increasing Vulnerability to Flooding)

Gulf Coast Threat?

With the upper-level high in place, this system is then expected to be pushed into the Gulf of Mexico, with the potential for a second landfall somewhere along the Gulf Coast next week, possibly as a tropical storm. This is all dependent on if the system develops.
(MORE: Why Long-Range Model Forecasts in the Tropics Can't Often Be Trusted)
For now, the Gulf Coast landfall appears to be sometime in the Tuesday or Wednesday timeframe, with the majority of guidance suggesting a landfall somewhere from the Florida Peninsula to Louisiana.
(MORE: Florida's Record-Setting Hurricane Drought)
One complicated uncertainty regards how close 99-L tracks to the west coast of Florida. This could bring gusty winds and heavy rain to the corridor from Naples to Tampa/St. Pete in the Monday/Tuesday timeframe.
(FORECAST: Naples | Fort Myers | Tampa)
Beyond that, the system may bring a period of heavy rainfall in parts of the Deep South to the Carolinas next week.
Alternate scenarios for "it" to move over the Florida peninsula (over land), or stay offshore moving north, before bending northeast offshore, cannot be counted out, but appear to be of lower probability.
(MORE: Most Intense U.S. Landfalls Have Happened in a 17-Day Period)
It is still too early to make a definitive call for the Gulf coast, especially since the tropical depression hasn't even formed yet.
In fact, if this system isn't able to intensify much for whatever reason, it's possible rainfall flooding may be the biggest impact near and along its track.
For now, if you have interests anywhere along the Southeast and Gulf coast, from Louisiana to Florida to the Carolinas, check back with us at weather.com for any important forecast changes in the days ahead.
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.