Tropical Storm Dorian - 10am CDT Thursday Advisory

By: National Hurricane Center
By: National Hurricane Center

TROPICAL STORM DORIAN DISCUSSION NUMBER 6
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
1100 AM AST THU JUL 25 2013

CONVENTIONAL AND MICROWAVE SATELLITE IMAGERY INDICATE THAT THERE HAS BEEN NO SIGNIFICANT CHANGE IN THE OVERALL CONVECTIVE PATTERN OR STRUCTURE OF DORIAN SINCE THE PREVIOUS ADVISORY. THE COMPACT TROPICAL CYCLONE HAS MAINTAINED A SMALL BUT SYMMETRIC OUTFLOW PATTERN...ALONG WITH GOOD LOW-LEVEL EQUATORIAL INFLOW. SATELLITE INTENSITY ESTIMATES ARE 55 KT FROM TAFB AND AND 45 KT FROM TAFB... SO THE INITIAL INTENSITY IS BEING MAINTAINED AT 50 KT. THE INITIAL POSITION IS A LITTLE SOUTH OF THE CONVENTIONAL SATELLITE FIX POSITIONS BASED ON MICROWAVE IMAGERY OVER THE PAST 6 HOURS.

THE INITIAL MOTION ESTIMATE IS 285/15. THERE IS ESSENTIALLY NO CHANGE TO PREVIOUS FORECAST TRACK OR REASONING. THE NHC MODEL GUIDANCE CONTINUES TO BE IN EXCELLENT AGREEMENT ON DORIAN MOVING IN A GENERAL WEST-NORTHWESTWARD DIRECTION THROUGHOUT THE FORECAST PERIOD. THE MAIN DIFFERENCE BETWEEN THE MODELS CONTINUES TO BE THE FORWARD SPEED OF DORIAN...WITH THE ECMWF AND HWRF BEING THE FASTEST MODELS WITH A WEAKER CYCLONE...WHILE THE GFS AND GFS-ENSEMBLE MEAN MODELS ARE THE SLOWEST DUE TO THEIR FORECASTING A STRONGER VORTEX. BASED ON THE BETTER PERFORMANCE OF THE GFS MODEL INTENSITY FORECASTS OVER THE PAST FEW DAYS...THE OFFICIAL FORECAST LEANS TOWARD THE LATTER SOLUTION AND SHOWS A GRADUAL DECREASE IN FORWARD SPEED TO BEGIN BY 48-72 HOURS AS DORIAN APPROACHES A WEAKNESS IN THE SUBTROPICAL RIDGE. THE NEW TRACK FORECAST IS JUST AN EXTENSION OF THE PREVIOUS ADVISORY TRACK...AND IS VERY CLOSE TO BUT SLOWER THAN THE CONSENSUS MODEL TVCN.

DORIAN APPEARS TO HAVE SURVIVED PASSAGE OVER THE REGION OF COOLEST SSTS OVERNIGHT AND IS NOW MOVING OVER AND TOWARD WARMER WATERS. ALTHOUGH THE AFOREMENTIONED EQUATORIAL INFLOW OF WARM UNSTABLE AIR LIKELY HELPED OFFSET THE COOLING BENEATH DORIAN...THAT LIFELINE IS FORECAST BY ALL OF THE MODELS TO BE CUT OFF DURING THE NEXT 24 HOURS...WHICH WILL RESULT IN THE COMPACT CYCLONE MOVING THROUGH A REGION OF DECREASING MID-LEVEL MOISTURE AND MORE STABLE AIR... PRODUCING SOME SLIGHT WEAKENING FOR THE NEXT 24 HOURS OR SO. AFTER THAT...THE VERTICAL SHEAR IS EXPECTED TO REMAIN AT OR BELOW 10-15 KT...WHICH WOULD FAVOR AT LEAST SOME GRADUAL INTENSIFICATION IF THE INNER CORE CONVECTION CAN REMAIN INTACT. THE OFFICIAL INTENSITY FORECAST LIES BETWEEN THE SHIPS AND LGEM SOLUTIONS...AND ASSUMES DORIAN WILL SURVIVE ITS TREK THROUGH UNFAVORABLE THERMODYNAMIC CONDITIONS DURING THE NEXT 2 DAYS.


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