Tropical Storm ISAAC

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TROPICAL STORM ISAAC DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
1100 PM EDT THU SEP 28 2006

THE OVERALL CLOUD PATTERN OF ISAAC HAS CHANGED VERY LITTLE THIS
EVENING.  A CONVECTIVE BAND HAS WRAPPED AROUND THE WEST AND
SOUTHWEST SIDE OF THE CIRCULATION...WHICH HAS MADE IT A LITTLE 
MORE DIFFICULT TO DETERMINE THE CENTER LOCATION WITH INFRARED
IMAGERY.  SINCE THE CLOUD TOPS ARE NOT VERY COLD AND THE CONVECTIVE
BAND IS RATHER SMALL...DVORAK DATA T-NUMBERS HAVE DECREASED DURING
THE PAST 6-12 HOURS.  THIS IS A CASE IN WHICH THE DVORAK TECHNIQUE
MAY BE UNDERESTIMATING THE INTENSITY...SINCE THE CONVECTION IS NOT
VERY DEEP AND THE CYCLONE HAS SOME SUBTROPICAL CHARACTERISTICS.  A
SHIP WELL SOUTHEAST OF THE CENTER AT 0000 UTC REPORTED 33 KT
WINDS...AND STRONGER WINDS COULD BE OCCURRING CLOSER TO THE CENTER. 
MAKING THAT ASSUMPTION...THE ADVISORY INTENSITY REMAINS 40 KT.  THE
WIND RADII WERE ALSO EXPANDED SLIGHTLY TO ACCOUNT FOR THE SHIP
REPORT.

ISAAC HAS BEEN MOVING ALONG THE EASTERN SIDE OF THE MODEL GUIDANCE
DURING THE PAST DAY OR SO.  MOST OF THE DYNAMICAL MODELS TURN THE
CYCLONE WEST-NORTHWESTWARD DURING THE NEXT 12-24 HOURS... IN
BETWEEN AN UPPER-LEVEL LOW JUST SOUTH OF THE TROPICAL CYCLONE AND A
BUILDING MID-TROPOSPHERIC RIDGE TO THE NORTH.  IN ABOUT 2 DAYS...A
LARGE DEEP-LAYERED TROUGH MOVING INTO THE WESTERN ATLANTIC SHOULD
TURN ISAAC NORTHWARD ALLOWING THE CYCLONE TO RECURVE INTO THE
WESTERLIES AHEAD OF THE TROUGH.  THERE REMAINS A SIGNIFICANT
SPREAD IN THE GUIDANCE DURING RECURVATURE.  THE GFS AND NOGAPS
REMAIN ON THE WESTERN SIDE OF THE GUIDANCE WHILE THE UKMET IS TO
THE EAST.  GIVEN THIS LARGE SPREAD...THE OFFICIAL FORECAST TRACK
WILL REMAIN CLOSE TO THE MODEL CONSENSUS...WHICH HAPPENS TO BE VERY
NEAR THE PREVIOUS NHC FORECAST.

ISAAC CONTINUES TO BE AFFECTED BY MODERATE SOUTHERLY SHEAR. 
HOWEVER...THIS SHEAR IS FORECAST TO DECREASE OVER THE NEXT COUPLE
OF DAYS.  THE INTENSITY FORECAST IS MORE PROBLEMATIC SINCE ISAAC IS
PREDICTED TO MOVE OVER WATERS THAT HAVE BEEN COOLED BY
HURRICANES GORDON AND HELENE...WHICH MAY HINDER STRENGTHENING. 
THEREAFTER...INCREASING SOUTHWESTERLY SHEAR WILL LIKELY BE AN
ADDITIONAL INHIBITING FACTOR.  THE OFFICIAL FORECAST CALLS FOR ONLY
MODEST STRENGTHENING AND IS IN BETWEEN THE SHIPS AND FSU
SUPERENSEMBLE.  IT SHOULD BE NOTED THAT THE GFDL MAKES ISAAC A
HURRICANE IN ABOUT 48 HOURS.  HOWEVER...THIS MODEL SOMETIMES
OVERESTIMATES THE INTENSIFICATION OF TROPICAL CYCLONES IN A SHEARED
ENVIRONMENT.
  
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/0300Z 29.2N  55.2W    40 KT
 12HR VT     29/1200Z 30.0N  56.0W    40 KT
 24HR VT     30/0000Z 30.9N  57.3W    45 KT
 36HR VT     30/1200Z 31.9N  58.5W    50 KT
 48HR VT     01/0000Z 33.5N  59.6W    50 KT
 72HR VT     02/0000Z 38.0N  59.5W    50 KT...BECOMING EXTRATROPICAL
 96HR VT     03/0000Z 44.0N  53.5W    50 KT...EXTRATROPICAL
120HR VT     04/0000Z 49.0N  46.0W    50 KT...EXTRATROPICAL
 
$$
FORECASTER BROWN/KNABB
 
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