Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
1100 AM AST WED AUG 22 2012
 
THE AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT MONITORING ISAAC
DURING THE NIGHT REPORTED THAT THE INNER CORE WIND FIELD BECAME
DISORGANIZED AFTER 0600 UTC.  A COUPLE OF MICROWAVE OVERPASSES
SUGGEST THAT THE CIRCULATION CENTER BECAME ELONGATED DURING THIS
TIME....AND THE CENTER IS NOT READILY APPARENT IN FRENCH RADAR DATA
FROM MARTINIQUE AND GUADELOUPE.  THE INITIAL INTENSITY IS HELD AT
40 KT BASED ON AIRCRAFT-REPORTED WINDS NORTH OF THE CENTER.  WHILE
THE CENTRAL CORE IS DISORGANIZED...THE OVERALL STRUCTURE OF THE
STORM IS IMPROVING...AS SATELLITE IMAGERY SHOWS A LARGE OUTER BAND
FORMING AROUND THE CENTRAL CONVECTIVE FEATURE.
 
THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 280/15...AS IT IS UNCLEAR
HOW MUCH OF THE OBSERVED NORTHWARD NUDGE WAS DUE TO RE-FORMATION OF
THE CENTER. OTHER THAN THAT...THERE IS NO SIGNIFICANT CHANGE TO THE
PREVIOUS FORECAST TRACK OR REASONING DURING THE FIRST 48-72 HR OF
THE TRACK FORECAST. THE EAST-WEST ORIENTED SUBTROPICAL RIDGE TO THE
NORTH OF ISAAC ALONG 30N LATITUDE IS FORECAST BY ALL OF THE GLOBAL
MODELS TO REMAIN INTACT ACROSS FLORIDA AND THE SOUTHEASTERN U.S.
DURING THAT TIME.  THIS EARLY PART OF THE FORECAST TRACK IS AN
UPDATE OF THE PREVIOUS FORECAST.  AFTER THAT TIME...THE RIDGE IS
EXPECTED TO WEAKEN SOMEWHAT AS A MID/UPPER-LEVEL TROUGH MOVES ONTO
THE EASTERN UNITED STATES.  THE MODELS DIVERGE ON THEIR FORECASTS
OF THE RIDGE.  THE ECMWF DOES NOT BREAK THE RIDGE...AND THUS
FORECASTS A MORE WESTWARD MOTION NEAR OR SOUTH OF CUBA.  THE
CANADIAN AND NOGAPS HAVE A LARGE BREAK IN THE RIDGE AND TURN ISAAC
NORTHWARD EAST OF FLORIDA.  THE UKMET HAS A BREAK IN THE RIDGE OVER
THE NORTHERN GULF COAST AND MOVES THE CYCLONE OVER THE FLORIDA
PENINSULA.  THE GFS SHOWS A BREAK IN THE RIDGE EAST OF FLORIDA...
BUT FORECASTS ISAAC TO BYPASS THE BREAK AND MOVE NEAR THE SOUTHWEST
COAST OF FLORIDA.  THIS DECREASED-CONFIDENCE PORTION OF THE
FORECAST TRACK IS SHIFTED A LITTLE WEST OF THE PREVIOUS TRACK TO
LIE NEAR THE CONSENSUS MODELS.
 
WATER VAPOR IMAGERY AND MICROWAVE TOTAL PRECIPITABLE WATER DATA SHOW
DRY AIR TO THE NORTHEAST OF ISAAC...AND A LACK OF CONVECTION IN
THIS AREA SUGGESTS SOME OF THIS AIR IS ENTRAINING INTO THE CYCLONE.
OTHERWISE...ENVIRONMENTAL CONDITIONS APPEAR FAVORABLE FOR
INTENSIFICATION UNTIL THE CENTER REACHES HISPANIOLA IN ABOUT 48 HR.
AFTER THAT...THE INTENSITY WILL BE DETERMINED BY HOW MUCH TIME ISAAC
SPENDS OVER LAND.  THE NEW FORECAST IS SIMILAR TO THE PREVIOUS
FORECAST THROUGH 48 HR...THEN IS REDUCED A LITTLE BASED ON
INCREASED LAND INTERACTION IN THE NEW FORECAST TRACK.
 
A HURRICANE WARNING WILL LIKELY BE REQUIRED FOR PORTIONS OF THE
DOMINICAN REPUBLIC THIS AFTERNOON.  IT SHOULD BE NOTED THAT WHILE
NONE OF THE FORECAST POINTS ARE OVER LAND...THE CYCLONE IS FORECAST
TO MOVE OVER HISPANIOLA AND CUBA FROM 48-96 HR.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  22/1500Z 15.9N  59.3W   40 KT  45 MPH
 12H  23/0000Z 16.3N  61.8W   45 KT  50 MPH
 24H  23/1200Z 16.8N  64.8W   50 KT  60 MPH
 36H  24/0000Z 17.3N  67.8W   60 KT  70 MPH
 48H  24/1200Z 17.8N  70.3W   75 KT  85 MPH
 72H  25/1200Z 19.5N  74.5W   60 KT  70 MPH
 96H  26/1200Z 22.5N  78.5W   60 KT  70 MPH
120H  27/1200Z 25.5N  81.5W   70 KT  80 MPH
 
$$
FORECASTER BEVEN
 
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