Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER  15
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 PM EDT FRI AUG 24 2012
 
RECONNAISSANCE AIRCRAFT DATA INDICATE THAT THE CIRCULATION OF ISAAC
HAS BECOME BETTER ORGANIZED THIS AFTERNOON...WITH A WELL-DEFINED
AND MORE CIRCULAR INNER CIRCULATION.  THE MAXIMUM 850 MB
FLIGHT-LEVEL WINDS HAVE BEEN 70 KT...THE MAXIMUM RELIABLE SFMR
SURFACE WIND ESTIMATE HAS BEEN 55 KT...AND THE MINIMUM REPORTED
PRESSURE HAS BEEN 994 MB.  BASED ON THESE DATA...THE INITIAL
INTENSITY IS INCREASED TO 55 KT.  ONE SIGN OF ONGOING ORGANIZATION
ISSUES...HOWEVER...IS THE LACK OF PERSISTENCE OF THE INNER CORE
CONVECTION...WHICH HAS WANED OVER THE PAST FEW HOURS.

ISAAC HAS MADE ITS EXPECTED TURN TOWARD THE NORTHWEST AND THE
INITIAL MOTION IS NOW 315/14.  DURING THE FIRST 48-60 HR...THE
TRACK GUIDANCE MADE AN EASTWARD SHIFT SINCE THE LAST ADVISORY. 
THIS WAS LED BY THE GFS...WHICH CHANGED TO A TRACK INTO THE
SOUTHEASTERN GULF OF MEXICO TO LANDFALL IN SOUTHEASTERN FLORIDA. 
THE ECMWF AND UKMET ARE NOW ON THE LEFT EDGE OF THE GUIDANCE...
SHOWING A TRACK ALONG THE NORTH COAST OF CUBA.  THIS PART OF
THE FORECAST TRACK IS SHIFTED EASTWARD BUT STILL LIES A LITTLE TO
THE LEFT OF THE CENTER OF THE GUIDANCE ENVELOPE AND THE CONSENSUS
MODELS.  THERE REMAINS A SPREAD IN THE MODEL GUIDANCE AFTER 48
HR.  THE GFS-BASED GUIDANCE SHOWS A NORTHEASTWARD MOTION AFTER
LANDFALL ON THE NORTHERN GULF COAST...WHILE THE UKMET AND ECMWF
SHOW A NORTHWARD MOTION.  THE LATTER PART OF THE FORECAST TRACK IS
SHIFTED EASTWARD CLOSE TO THE CONSENSUS MODELS.
 
THE MAIN CONVECTIVE BAND AND STRONGEST WINDS NORTHEAST OF THE CENTER
ARE MOVING INLAND AT THIS TIME.  BASED ON THIS AND THE CURRENT LACK
OF CONVECTION NEAR THE CENTER...LITTLE CHANGE IN STRENGTH IS
EXPECTED BEFORE LANDFALL.  THE INTENSITY FROM 12-48 HR WILL BE
DETERMINED BY THE AMOUNT OF LAND INTERACTION.  THE CURRENT FORECAST
IS BASED ON THE PREMISE THAT ISAAC WILL REMAIN OVER LAND LONG
ENOUGH TO ALLOW GRADUAL WEAKENING.  HOWEVER...ANY CHANGE IN THE
TRACK THAT ALLOWS IT TO REMAIN OVER WATER FOR A LONGER TIME COULD
ALLOW IT TO BE STRONGER THAN FORECAST.  AFTER 48 HR...THE
ENVIRONMENTAL CONDITIONS APPEAR GENERALLY FAVORABLE FOR ISAAC TO
STRENGTHEN INTO A HURRICANE OVER THE GULF OF MEXICO...AND THE
INTENSITY FORECAST REFLECTS THIS.  CONVERSELY...A LONGER TRACK OVER
LAND MAY DISRUPT ISAAC ENOUGH TO BE UNABLE TO TAKE ADVANTAGE OF
THESE FAVORABLE CONDITIONS.
 
IT IS IMPORTANT NOT TO FOCUS ON THE EXACT TRACK DUE TO THE
UNCERTAINTIES IN THE FORECAST AND THE FACT THAT ISAAC HAS A LARGE
AREA OF TROPICAL STORM FORCE WINDS ASSOCIATED WITH IT.  THE TROPICAL
STORM WATCH FOR SOUTH FLORIDA AND THE FLORIDA KEYS MAY HAVE TO BE
UPGRADED TO A HURRICANE WATCH IF THE INTENSITY FORECAST IS REVISED
UPWARD.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  24/2100Z 17.2N  71.9W   55 KT  65 MPH
 12H  25/0600Z 18.7N  73.6W   50 KT  60 MPH
 24H  25/1800Z 20.7N  76.2W   50 KT  60 MPH...INLAND
 36H  26/0600Z 22.4N  78.6W   45 KT  50 MPH...NEAR COAST OF CUBA
 48H  26/1800Z 23.9N  80.7W   55 KT  65 MPH...OVER WATER
 72H  27/1800Z 26.5N  84.0W   65 KT  75 MPH
 96H  28/1800Z 29.5N  86.0W   75 KT  85 MPH
120H  29/1800Z 32.0N  86.0W   45 KT  50 MPH...INLAND
 
$$
FORECASTER BEVEN
 
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