28 August 2016

Three areas of interest, two of which could affect U.S. coast

There is a lot going on today... so I'll start in the east and work my way west.  Gaston, which formed back on August 22, is now a Category 2 hurricane, making it the strongest storm of the season so far.  An area of enhanced thunderstorms that was located near Bermuda has gotten much better organized in the past day and is now Tropical Depression 8.  And finally, "Invest 99L" is centered over the Florida Keys and about to enter the Gulf of Mexico.

Hurricane Gaston is centered about 600 miles east of Bermuda in the middle of the Atlantic and is forecast to turn to the north and gradually weaken this week.  But today, conditions favored an impressive intensification, and maximum winds are up to 105 mph.  Gaston is the season's third hurricane, and the first to reach Category 2 status.

Hurricane Gaston
The next system of interest is Tropical Depression 8, which is centered about 300 miles west of Bermuda, or about 430 miles east-southeast of Cape Lookout, NC.  It is experiencing rather strong wind shear, so the surface circulation is nearly exposed with minimal thunderstorm activity displaced to the west.

Tropical Depression 8
TD8 is forecast to intensify further as it heads toward the North Carolina coast, though probably as nothing more than a borderline tropical storm.  The map below shows the official NHC forecast of tropical storm force wind probabilities over the next five days.  There is a slight chance that the NC Outer Banks could experience tropical storm conditions Monday night into Tuesday.  Depending on how soon it reaches tropical storm status, it could be named Hermine or Ian (keep reading for explanation).

Last but not least, Invest 99L.
You've been hearing and reading about 99L for a long time now... in fact, it came off the African coast 12 days ago.  The cryptic name is nothing more than a temporary identifier given to disturbances of interest.  Once it becomes a tropical cyclone, it is given a unique number (08L for example, for the 8th TC of the season).  If it reaches tropical storm intensity, it is also given a unique name (Hermine, for example).  The "invests" are numbered and reused cyclically from 90-99, and the L denotes the Atlantic.  So there will be another 99L this season, but not at the same time as the current 99L.

99L has been a troublesome system to predict.  Model guidance has often forecasted intensification, but has been consistently wrong so far.  It has at times looked quite healthy on satellite images, only to get shredded apart hours later.  It's now just a disorganized mess between Florida and Cuba producing some gusty winds and areas of thunderstorms.

99L is also forecast to organize once it enters the Gulf on Monday.  So here comes the race for the name Hermine.  Either TD8 or 99L could be the next named storm... which is Hermine.  The name after Hermine is Ian, so both could get used in the coming days!  Hermine and Ian are the 8th and 9th names on the list this season... though the average is only 5 named storms by September 1st.

There is pretty good agreement among global and regional models that this system could intensify significantly in the Gulf this week.  Anywhere from LA over to Tampa Bay should be watching this very closely for possible impacts from Wednesday through Friday, particularly the Big Bend area of Florida which is extremely vulnerable to storm surge.

Five-day forecast tracks from several select models (global and regional).

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