Tropical Storm Dorian - 4pm Wednesday Advisory

By: National Hurricane Center
By: National Hurricane Center

TROPICAL STORM DORIAN DISCUSSION NUMBER 3
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
500 PM EDT WED JUL 24 2013

CONVENTIONAL AND MICROWAVE SATELLITE IMAGERY INDICATE THAT THE CONVECTIVE STRUCTURE OF DORIAN HAS CHANGED LITTLE SINCE THE PREVIOUS ADVISORY. HOWEVER...UPPER-LEVEL ANTICYCLONIC OUTFLOW HAS IMPROVED IN ALL QUADRANTS AS THE VERTICAL SHEAR HAS DIMINISHED SIGNIFICANTLY. THE INITIAL INTENSITY OF 45 KT IS BASED ON A SATELLITE INTENSITY ESTIMATE OF T3.0/45 KT FROM TAFB.

THE INITIAL MOTION ESTIMATE IS 285/17 KT. DORIAN IS EXPECTED TO REMAIN SOUTH OF AND MOVE WEST-NORTHWESTWARD TO WESTWARD IN TANDEM WITH A STRONG SUBTROPICAL RIDGE LOCATED WELL TO THE NORTH. THE NHC MODEL GUIDANCE CONTINUES TO REMAIN TIGHTLY PACKED AND IN EXCELLENT AGREEMENT ON THIS SCENARIO. THE ONLY DIFFERENCES IN THE MODELS ARE THE FORWARD SPEED...WHICH IS DUE TO THE STRENGTH AND VERTICAL DEPTH OF DORIAN. THE GFS...UKMET...AND CANADIAN MODELS MAINTAIN A ROBUST AND MORE VERTICALLY DEEP CYCLONE...WHICH GETS STEERED MORE SLOWLY BY THE DEEPER MEAN FLOW. IN CONTRAST...THE HWRF AND ECMWF MODELS WEAKEN DORIAN SIGNIFICANTLY AS THE CYCLONE PASSES OVER COOLER WATER DURING THE NEXT 48 HOURS...WHICH RESULTS IN A SHALLOWER AND WEAKER VORTEX BEING STEERED MORE RAPIDLY WESTWARD BY THE STRONGER EASTERLY TRADE WIND FLOW. GIVEN THAT DORIAN WILL BE MOVING OVER COOLER WATER AT A BRISK 18 KT FOR THE NEXT 24-36 HOURS...MY FEELING IS THAT THE CYCLONE WILL NOT WEAKEN AS MUCH AS THE SOME OF THE MODELS ARE FORECASTING...AND AS A RESULT...THE OFFICIAL TRACK FORECAST LEANS TOWARD THE SLOWER GFS...UKMET...AND CANADIAN MODEL SOLUTIONS.

DORIAN IS BEGINNING TO MOVE OVER SUB-26C SSTS...AND THE CYCLONE SHOULD REMAIN OVER COOLER WATERS NEAR 25C FOR THE NEXT 12 HOURS OR SO...WHICH COULD INDUCE SOME SLIGHT WEAKENING. HOWEVER...BY 36-48 HOURS...DORIAN WILL BE MOVING BACK OVER SSTS WARMER THAN 26C AND IN A VERY LOW VERTICAL SHEAR ENVIRONMENT...WHICH SHOULD ALLOW FOR SOME RE-STRENGTHENING TO OCCUR. THE LATEST RUNS OF THE GFS AND ECMWF ARE SHOWING A LESS HOSTILE UPPER-AIR PATTERN NEAR THE END OF THE FORECAST PERIOD...WITH MORE RIDGING AND LESS INFLUENCE FROM AN UPPER-LEVEL TROUGH OVER THE BAHAMAS. IN CONSIDERATION OF THAT...THE FORECAST INTENSITIES AT DAYS 4-5 HAVE BEEN INCREASED RELATIVE TO THE PREVIOUS ADVISORY.


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