Possible Development in the Gulf of Mexico This Week

EDIT: Quick Update. The disturbance I describe in this article has just been designated Invest 91L.

The official start of the Atlantic hurricane season began yesterday and we are already looking at a suspect to become the season’s first tropical cyclone. A large area of disturbed weather  resides across the NW Caribbean Sea, Gulf of Mexico and Bay of Campeche. Part of this disturbance is the remains of former Hurricane Barbara which crossed over Mexico via the East Pacific.

The 8:00pm Tropical Weather Outlook (TWO) had this to say about the disturbance:

CLOUDINESS AND SHOWERS...ASSOCIATED WITH A BROAD TROUGH OF LOW
PRESSURE EXTENDING FROM THE YUCATAN PENINSULA TO THE STRAITS OF
FLORIDA...HAVE BECOME SLIGHTLY BETTER ORGANIZED THIS AFTERNOON. 
ANY DEVELOPMENT OF THIS SYSTEM IS LIKELY TO BE SLOW TO OCCUR DUE TO
MARGINALLY FAVORABLE ENVIRONMENTAL CONDITIONS.  THIS SYSTEM HAS A
LOW CHANCE...20 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS AS IT DRIFTS GENERALLY NORTHWARD.
Visible satellite image of disturbance
Visible satellite image of the disturbance as of 22:45Z

The forecast for this area of disturbed weather is very complex. Most of the reliable computer models show that an area of low pressure will attempt to intensify in the Eastern Gulf of Mexico over the next 72 hours. Complicating this process is a region of high wind shear (about 20-40 knots) from the SW. The disturbance will be sandwiched between an upper-level trough to the North and an upper-level anticyclone (providing lighter and more favorable upper-level winds) to its SE.

Over the past few runs, the computer models have slightly backed off on the magnitude of the wind shear in the Gulf, and thus more ensemble members are forecasting tropical cyclone formation. Further complicating the forecast is the large size of the disturbance.  Over the next few days there will probably be many circulation centers attempting to establish themselves, but which one, if any, become the dominant center remains to be seen. This partially explains why one of the most reliable computer models (the GFS) has been so wishy washy with the details of the development of this system. Tropical cyclone genesis is tricky even in favorable conditions, so it is not surprising the models have been having a hard time getting the details right in this situation. After about 72 hours, the computer models forecast that the low pressure area will begin to eject NE out of the Gulf and towards Florida.

If a tropical cyclone does form, it appears very unlikely it will strengthen too much and will probably remain a tropical depression or a tropical storm at most. The reason for this is the aforementioned wind shear and marginal sea surface temperatures (SST’s). Regardless of development, the main threat this system poses is extreme amounts of rainfall. Here is the latest HPC 5-day total rainfall accumulation forecast:

Latest HPC 5-day total rainfall forecast
Latest HPC 5-day total rainfall forecast. Some parts of South Florida are forecast to receive 5-7 inches of rain with locally higher amounts.

To sum things up, we have a slight/decent chance of seeing Tropical Storm Andrea this week. Even if a tropical cyclone does not form, Florida can expect heavy rainfall over the next five days. The exact hypothetical landfall location is not too important at this time since the system is large and rains will extend far from whatever center establishes itself.

I will try to post updates periodically as conditions warrant.

-Mike

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