Gulf of Mexico System (Invest 99-L) Fueling Heavy Rain; Flood Watches Issued

Jon Erdman
Published: September 29,2015

Even without tropical development, local flash flooding will continue to be a headache along the Gulf Coast Tuesday.
So far, flooding was reported in southern Mississippi and southern Alabama on Sunday, with more than a half foot of rain falling in some locations.
Street flooding was reported in the Pensacola, Florida, area Monday morning from a band of very heavy rainfall. The Pensacola National Air Station had seen more than six inches of rain since late Sunday night as of 8 a.m. CDT Monday. Farther east in Navarre, Florida a private weather station has reported over 12 inches of rain through late afternoon Monday.

On Monday night, flooding of homes and roads was reported in Santa Rosa Beach, Florida where rainfall rates of 2 to 3 inches per hour were observed. Over 10 inches of rain was measured in Destin, Florida through Monday at 9 p.m. CDT.
The National Weather Service has issued flood watches for the continued heavy rain threat along parts of the eastern Gulf Coast.

Development Factors

An area of low pressure in the northeast 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 over the western Gulf of Mexico with weak high pressure aloft centered over the western Caribbean Sea.
This wind flow aloft will pull Invest 99-L ashore Tuesday along the northeast Gulf Coast, likely ending the threat of any tropical development.

Invest 99-L and Jet Stream Level Winds

A Hurricane Hunter reconnaissance mission on Monday found that this system has become elongated but did find tropical storm force winds at the surface. Without a well-defined "closed" circulation, it was not considered a tropical cyclone. Another reconnaissance mission is scheduled for Tuesday has been canceled.

Impacts, Regardless

Current Radar and Flood Alerts

Heavy rain hammered parts of the Gulf Coast on Sunday. 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. This resulted in flooding across the area.
The Mobile, Alabama, regional airport saw 7.5 inches of rain on Sunday, making it the tenth wettest calendar day on record, there. Water rescues were reported in the Mobile area, according to the Mobile Fire-Rescue twitter account.
On Monday, widespread flooding was reported in Santa Rosa Beach, Florida and roads were closed near De Funiak Springs and Rock Hill.
More flooding is possible through Tuesday and the National Weather Service has continued flood watches in parts of the Sunshine State.
Here is a summary of the forecast impacts:

• Locally heavy rain: Bands of locally heavy rain will continue to push ashore along, particularly, into western Florida and the Florida panhandle, pushing into southern Alabama and southwest Georgia. Rain rates of 2 inches per hour, or more, may occur, triggering local flash flooding.
• High surf, rip currents: Persistent onshore winds may whip up high surf and rip currents along Florida's Gulf Coast.

(FORECASTS: Destin | Tampa | Orlando)
Moisture from the Gulf system will eventually ride northward into the Mid-Atlantic and Northeast midweek, fueling a heavy rain threat ahead of a cold front in those regions. Atlantic moisture - partially from Tropical Storm Joaquin in the western Atlantic - may also help to enhance the heavy rain in the Northeast. For more on that story, click here.

Eastern Rainfall Forecast

Check back with us at weather.com and The Weather Channel for updates on this Gulf of Mexico disturbance.

MORE: Retired Atlantic Hurricane Names