Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 AM AST THU AUG 23 2012
 
AN AIR FORCE RECONNAISSANCE AIRCRAFT INVESTIGATING ISAAC THIS
MORNING HAS FOUND A POORLY DEFINED INNER CORE WITH A LARGE AREA OF
LIGHT WINDS AROUND A CENTER...WHICH IS SIMILAR TO WHAT AN EARLIER
NOAA RESEARCH MISSION INDICATED. RADAR DATA FROM GUADELOUPE AND SAN
JUAN ALSO INDICATE A POORLY DEFINED INNER CORE CONVECTIVE PATTERN.
RATHER THAN INITIALIZE THE CENTER OF ISAAC WELL TO THE SOUTH OF THE
PREVIOUS TRACK...I HAVE OPTED TO USE A BLEND OF THE RECON FIXES...
SATELLITE IMAGERY...DATA FROM NEARBY NOAA BUOY 42060...AND A 06Z
CONSENSUS FORECAST POSITION FROM THE GFS...ECMWF...AND UKMET
MODELS. THE INITIAL INTENSITY WAS ALSO DECREASED TO 35 KT BASED ON
DATA FROM THE RECON AIRCRAFT AND NOAA BUOY 42060.

THE INITIAL MOTION ESTIMATE IS AN UNCERTAIN 270/12 KT. THE 00Z
GFS...ECMWF...AND UKMET MODELS DID AN OUTSTANDING JOB PREDICTING
THE RECENT SOUTHWESTWARD JOG OR REFORMATION OF THE CENTER OF ISAAC.
THOSE SAME MODELS ARE ALSO FORECASTING ISAAC TO MAKE A SHARP JOG TO
THE NORTHWEST OVER THE NEXT 12-24 HOURS...AND THEN STEADY OFF ON A
GENERAL WEST-NORTHWESTWARD MOTION THROUGH 48 HOURS. AFTER THAT...
THE GLOBAL AND REGIONAL MODELS HAVE COME INTO BETTER AGREEMENT ON
ISAAC MOVING NORTHWESTWARD TOWARD A BREAK IN THE SUBTROPICAL RIDGE
THAT IS FORECAST TO DEVELOP AFTER 72 HOURS ACROSS FLORIDA AND THE
EASTERN GULF OF MEXICO.  THE ECMWF MODEL HAS CONTINUED ITS EASTWARD
SHIFT OVER THE PAST 36 HOURS...AND EVEN THE LATEST NOGAPS AND
CANADIAN MODEL RUNS HAVE SHIFTED WELL TO THE WEST ACROSS THE
FLORIDA PENINSULA ARE NOW NO LONGER CONSIDERED TO BE OUTLIERS. DUE
TO LESS SPREAD IN THE LATEST NHC MODEL GUIDANCE...AND THE FACT THAT
ECMWF HAS SHIFTED CLOSER TO THE PREVIOUS FORECAST AT DAYS 4 AND
5...THE OFFICIAL FORECAST TRACK IS JUST AN UPDATE OF THE PREVIOUS
ADVISORY TRACK...AND IS CLOSE TO THE DYNAMICAL CONSENSUS MODELS
TVCA AND TV15.

UPPER-AIR DATA FROM ST. MAARTEN AT 00Z CONFIRMS THE DRY AIR IN THE
MID-LEVELS OF THE ATMOSPHERE BETWEEN 600 AND 300 MB AS ALLUDED TO
IN PREVIOUS DISCUSSIONS. THIS LAYER OF VERY DRY AIR HAS BEEN
HINDERING CONVECTIVE DEVELOPMENT IN THE NORTHEASTERN QUADRANT FOR
THE PAST 3-4 DAYS. HOWEVER...THE GLOBAL MODELS ARE IN GOOD
AGREEMENT ON THE DRY AIR MIXING OUT OVER THE NEXT 36 HOURS AND THE
INNER CORE BECOMING QUITE MOIST...WHICH SHOULD ALLOW FOR SOME
STEADY INTENSIFICATION TO OCCUR BEFORE ISAAC INTERACTS WITH THE
MOUNTAINOUS TERRAIN OF HISPANIOLA. GIVEN THE IMPRESSIVE UPPER-LEVEL
OUTFLOW PATTERN THAT THE CYCLONE HAS DEVELOPED...ISAAC IS STILL
EXPECTED TO BECOME A HURRICANE BY 36 HOURS. AFTER THAT...HOWEVER...
THE THE PRIMARY IMPEDIMENT TO STRENGTHENING WILL BE THE INTERACTION
WITH THE LAND MASSES OF HISPANIOLA AND EASTERN CUBA.  HOW MUCH THE
INNER CORE OF ISAAC IS DISRUPTED WHILE OVER LAND WILL DETERMINE
JUST HOW MUCH RE-STRENGTHENING WILL OCCUR ONCE THE CENTER MOVES
BACK OVER WATER AFTER 72 HOURS. THE OFFICIAL INTENSITY FORECAST IS
SIMILAR TO THE PREVIOUS FORECAST AND IS IN REASONABLE AGREEMENT
WITH THE DECAY-SHIPS AND LGEM INTENSITY MODELS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  23/0900Z 15.3N  64.0W   35 KT  40 MPH
 12H  23/1800Z 16.2N  65.9W   40 KT  45 MPH
 24H  24/0600Z 16.9N  68.4W   50 KT  60 MPH
 36H  24/1800Z 17.8N  71.1W   65 KT  75 MPH
 48H  25/0600Z 19.1N  73.4W   55 KT  65 MPH
 72H  26/0600Z 21.8N  77.4W   60 KT  70 MPH...INLAND
 96H  27/0600Z 24.4N  80.9W   65 KT  75 MPH...OVER WATER
120H  28/0600Z 27.4N  83.7W   70 KT  80 MPH
 
$$
FORECASTER STEWART
 
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