Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER  29
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
400 AM CDT TUE AUG 28 2012
 
REPORTS FROM THE AIR FORCE HURRICANE HUNTER AIRCRAFT INDICATE THAT
THE CENTRAL PRESSURE HAS DROPPED A LITTLE MORE...BUT SO FAR...THE
MAXIMUM FLIGHT-LEVEL AND SFMR-OBSERVED SURFACE WINDS STILL DO NOT
QUITE SUPPORT HURRICANE INTENSITY.  THE CURRENT INTENSITY IS HELD
AT 60 KT PENDING ADDITIONAL OBSERVATIONS FROM THE HURRICANE
HUNTERS.  ALTHOUGH THE WIND FIELD REMAINS RATHER BROAD AND
RELATIVELY FLAT...RECENT AIRCRAFT DATA SUGGEST THAT THE MAXIMUM
WINDS ARE NOW OCCURRING SOMEWHAT CLOSER TO THE CENTER OVER THE
NORTHERN SEMICIRCLE OF THE CYCLONE.  THIS WOULD SUGGEST THAT THE
INNER CORE CIRCULATION IS BECOMING BETTER DEFINED...AND
STRENGTHENING MIGHT BE IMMINENT.  UPPER-LEVEL OUTFLOW IS
WELL-ESTABLISHED OVER THE SOUTHERN SEMICIRCLE OF THE SYSTEM...BUT
CENTRAL CONVECTION CONTINUES TO FLUCTUATE.  THE LATTER IS
PRESUMABLY DUE TO A CONTINUED INTRUSION OF DRY AIR INTO THE CORE
REGION.  THE STATISTICAL-DYNAMICAL GUIDANCE IS A LITTLE LOWER THAN
IN PREVIOUS RUNS...AND THE STORM HAS ONLY ABOUT 12-24 HOURS BEFORE
MOVING INLAND.  THE OFFICIAL INTENSITY FORECAST IS JUST SLIGHTLY
LOWER THAN THE PREVIOUS ONE.
 
CENTER FIXES FROM THE AIRCRAFT INDICATE THAT THE STORM HAS WOBBLED
ON A GENERALLY WESTWARD COURSE OVER THE PAST FEW HOURS.  THIS IS
BELIEVED TO BE TEMPORARY...HOWEVER...AND THE LONGER-TERM MOTION
ESTIMATE IS ABOUT 305/10.  THE CURRENT AND FORECAST STEERING REGIME
ARE BASICALLY THE SAME AS IN THE PREVIOUS PACKAGE.  ISAAC IS
EXPECTED TO MOVE INTO A WEAKNESS IN THE SUBTROPICAL RIDGE NEAR THE
NORTH-CENTRAL GULF COAST...WITH SOME SLOWING OF FORWARD SPEED
DURING THE NEXT COUPLE OF DAYS AS A RIDGE BUILDS A LITTLE TO THE
NORTHWEST OF THE CYCLONE.  LATER ON...THE SYSTEM SHOULD TURN
NORTHWARD AND NORTHEASTWARD INTO THE OHIO VALLEY REGION AS IT MOVES
AROUND THE WESTERN PERIPHERY OF A MID-LEVEL ANTICYCLONE.
 
ISAAC IS A LARGE TROPICAL CYCLONE.  A DANGEROUS STORM SURGE...HEAVY
RAINFALL...AND STRONG WINDS EXTEND WELL AWAY FROM THE CENTER AND
ARE EXPECTED TO AFFECT A LARGE PORTION OF THE NORTHERN GULF COAST. 
FOR THIS REASON...IT IS IMPORTANT NOT TO FOCUS ON THE EXACT CENTER
LOCATION.  THE THREAT OF HEAVY RAINFALL AND FLOODING IS ALSO
EXPECTED TO SPREAD INLAND OVER THE LOWER MISSISSIPPI VALLEY REGION
DURING THE NEXT FEW DAYS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  28/0900Z 27.5N  88.1W   60 KT  70 MPH
 12H  28/1800Z 28.5N  89.1W   70 KT  80 MPH
 24H  29/0600Z 29.5N  90.1W   75 KT  85 MPH...NEAR THE COAST
 36H  29/1800Z 30.3N  90.9W   55 KT  65 MPH...INLAND
 48H  30/0600Z 31.4N  91.5W   45 KT  50 MPH...INLAND
 72H  31/0600Z 34.5N  92.5W   25 KT  30 MPH...INLAND
 96H  01/0600Z 38.0N  91.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  02/0600Z 40.5N  87.0W   15 KT  15 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER PASCH
 
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