Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER  14
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
1100 AM EDT FRI AUG 24 2012

RECONNAISSANCE AIRCRAFT DATA AND SATELLITE IMAGERY INDICATE THAT
ISAAC HAS BECOME BETTER ORGANIZED DURING THE PAST SEVERAL HOURS. 
WHILE THE CIRCULATION CENTER REMAINS ELONGATED EAST-TO-WEST WITH
MULTIPLE EMBEDDED VORTICITY MAXIMA...THE LARGE CENTRAL AREA OF
LIGHT WINDS HAS SHRUNK AS THE STRONGEST WINDS MOVE CLOSER TO THE
CENTER.  AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT REPORTED
66 KT FLIGHT-LEVEL WINDS AT 850 MB NORTHEAST OF THE CENTER...ALONG
WITH SFMR BIAS-CORRECTED WIND ESTIMATES NEAR 50 KT.  BASED ON THESE
DATA...THE INITIAL INTENSITY IS INCREASED TO 50 KT.

THE 12Z POSITION USED A MEAN CENTER...WHICH RESULTED IN A SOUTHWARD
ADJUSTMENT.  SINCE THEN...ISAAC APPEARS TO HAVE ACQUIRED A STRONGER
NORTHWARD COMPONENT OF MOTION...AND THE INITIAL MOTION IS A
SOMEWHAT UNCERTAIN 295/12.  THE TRACK MODEL GUIDANCE IS IN GOOD
AGREEMENT THAT ISAAC SHOULD MOVED NORTHWESTWARD FOR THE NEXT 72
HR...STEERED BY THE SUBTROPICAL RIDGE OVER THE WESTERN ATLANTIC
TOWARD A WEAKNESS IN THE RIDGE OVER THE SOUTHEASTERN UNITED STATES. 
THIS PART OF THE NEW FORECAST TRACK IS SHIFTED A LITTLE TO THE LEFT
OF THE PREVIOUS TRACK BASED ON THE INITIAL POSITION...AND LIES NEAR
THE CONSENSUS MODELS.  AFTER 72 HR...THE SPREAD OF THE TRACK
GUIDANCE INCREASES DUE TO MODEL DISAGREEMENT ON THE EVOLUTION OF A
TROUGH OVER THE EASTERN UNITED STATES.  THE ECMWF KEEPS MORE RIDGE
BETWEEN ISAAC AND THE TROUGH...WHICH RESULTS IN A NORTHWESTWARD
MOTION THROUGH DAY 5.  THE GFS HAS A STRONGER TROUGH...WHICH
RESULTS IN A MORE NORTHWARD MOTION.  THE GUIDANCE ENVELOPE HAS
SHIFTED A LITTLE EASTWARD AFTER 72 HR...AND THUS THE LATTER PORTION
OF THE NEW FORECAST TRACK IS ALSO ADJUSTED EASTWARD.

WHILE THE ORGANIZATION HAS IMPROVED...THE STRUCTURE OF ISAAC STILL
DOES NOT FAVOR RAPID INTENSIFICATION DESPITE A VERY FAVORABLE
UPPER-LEVEL WIND PATTERN.  THE EARLY PART OF THE INTENSITY FORECAST
SHOWS A LITTLE MORE STRENGTHENING THAN EARLIER...AND IT IS POSSIBLE
THAT ISAAC COULD BE NEAR HURRICANE STRENGTH WHEN IT REACHES
HISPANIOLA.  LAND INTERACTION WILL LIMIT THE INTENSITY FROM 24-48
HR...AND THIS PART OF THE FORECAST MAY NEED SOME ADJUSTMENT AFTER
IT IS SEEN WHAT STRUCTURE ISAAC HAS AFTER CROSSING HISPANIOLA. 
ISAAC SHOULD STRENGTHEN OVER THE GULF OF MEXICO...WITH THE FORECAST
SHOWING THE CYCLONE BECOMING A HURRICANE BEFORE REACHING THE
NORTHERN GULF COAST.

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.

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  24/1500Z 16.3N  70.8W   50 KT  60 MPH
 12H  25/0000Z 17.4N  72.6W   55 KT  65 MPH
 24H  25/1200Z 19.2N  75.1W   55 KT  65 MPH
 36H  26/0000Z 20.9N  77.6W   50 KT  60 MPH...INLAND
 48H  26/1200Z 22.3N  79.5W   45 KT  50 MPH...INLAND
 72H  27/1200Z 25.0N  83.0W   55 KT  65 MPH...OVER WATER
 96H  28/1200Z 28.0N  85.5W   65 KT  75 MPH
120H  29/1200Z 31.0N  86.5W   70 KT  80 MPH...INLAND
 
$$
FORECASTER BEVEN
 
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