Tropical System Likely to Form and Move Toward the U.S. Gulf Coast this Week

Interests from southeast Louisiana to the west coast of Florida should closely monitor forecasts this week

Author’s note: This is a special Sunday edition of the tropical newsletter to discuss a tropical system expected to form and threaten the Gulf Coast this week.

A broad area of low pressure near the coast of Nicaragua today is expected to slowly organize this week and is likely to become a tropical depression or named storm by the middle of the week as it heads northward toward the U.S. Gulf Coast for Thursday into Friday. How strong it becomes is still an open question, but a hurricane threat is certainly on the table before the system moves inland.

Interests from southeast Louisiana to the west coast of Florida should closely follow the evolving forecasts this week.

For those of us in South Florida, we’ll need to watch for periphery effects for the latter part of the workweek – primarily heavy rain from a surge of tropical air and any outer rainbands – depending on how organized the system gets and how closely it tracks to Florida’s west coast. Forecast models for now do not indicate a direct threat to South Florida from the developing storm system.

Watching for formation

The big early question with the developing system is where exactly a more defined area of low pressure forms within the broader circulation envelope known as the Central American Gyre that we’ve been discussing since early last week.

How quickly it forms early on will influence its future trajectory, with quicker formations favoring a track closer to the west coast of Florida and slower formations favoring a westward path into the central Gulf.

For now, models are leaning toward development around Mexico’s Yucatán Peninsula by Tuesday into Wednesday, so the first step is to see whether this development gets accelerated or delayed in the next day or two.

Northeastern Gulf Coast on alert

Forecast models continue to converge on possible threat zone along the northeastern Gulf Coast for the latter part of the week.

Low pressure tracks from the early morning runs of the European forecast model ensemble system (top map) and the American GFS forecast model ensemble system (bottom map). Both major modeling systems suggest a tropical storm or hurricane threat to the northeastern Gulf Coast by the end of the work week. Credit: Weathermodels.com

Regardless of where the low-pressure center tracks, it’s likely to have a wide reach, which means everyone from southwest Florida to southeast Louisiana will need to keep an eye on the forecast trends in the days ahead.

Rainfall forecast through next Sunday morning (September 29^th) from the National Weather Service. A heavy rain and flood threat will accompany the system from the west coast of Florida into the Florida panhandle and southern Georgia and Alabama beginning on Thursday and Friday. Credit: Pivotal Weather

Since yesterday, forecast models have sped up the timeline slightly, with a potential strike by as early as late Thursday or early Friday along the northeastern Gulf along Florida’s panhandle or Big Bend.

Odds of at least a tropical storm (39+ mph winds) moving within 150-200 miles of a given location from Thursday through Friday, according to the European forecast model ensemble system. Credit: Weathermodels.com

Especially for the storm-surge prone coastal areas along Florida’s Gulf Coast, it’s important to remember that even a disorganized tropical storm – particularly a large tropical system – can bring significant coastal flooding impacts from storm surge.

We won’t know the details of potential storm surge until the system forms, but as always it’s important to not focus on the exact spot where the low-pressure center may come ashore. As Debby illustrated most recently back in August, often the worst flooding and water-related impacts can be experienced well away from a storm’s center.

A high ceiling

Until we have a better-defined low-pressure center and the National Hurricane Center designates the system an Invest, we won’t have access to more reliable intensity guidance. The big forecast models most touted like the European or American GFS models are notoriously poor models for storm intensity, so take the intensity predictions for now with a grain of salt.

The big forecast models do help us to understand the overall environment ahead which shows a largely favorable upper-level pattern for strengthening. The one hiccup for the Gulf system could be increasing wind shear as it nears the coast later in the week courtesy of outflow from a developing storm system in the eastern Pacific.

Regardless, this time of year in the Gulf with near-record sea surface temperatures for the time of year still running well above their average seasonal peak and a deep well of warm water along its projected path, the ceiling is much higher than in June or July, so it deserves our attention this week.

Ocean Heat Content departure from average for the Gulf of Mexico showing in deep red and magenta the areas that have a deeper reservoir of ocean heat than typical for the time of year. These areas of higher-than-average ocean heat content coincide with the possible path of the developing system this week. Credit: Brian McNoldy/University of Miami.

No threats to land elsewhere in the Atlantic

The National Hurricane Center continues to monitor two areas this week over the open Atlantic.

Meanwhile a tropical disturbance moving off Africa today could develop as it moves westward into the eastern Atlantic this week but poses no immediate threat to land.