ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM HURRICANE HERNAN DISCUSSION NUMBER 17 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP092008 200 PM PDT SUN AUG 10 2008 A SERIES OF MICROWAVE IMAGES DURING THE PAST 24 HOURS INDICATES THAT HERNAN HAS UNDERGONE AN EYEWALL REPLACEMENT...AS THE INNER EYEWALL THAT BEGAN TO ERODE AT ABOUT 15Z YESTERDAY IS ESSENTIALLY GONE. HERNAN EXHIBITS A WELL-DEFINED EYE...WITH DIAMETER OF ABOUT 20 N MI...SURROUNDED BY CLOUD TOPS THAT ARE STILL QUITE COLD BUT HAVE BEEN GRADUALLY WARMING TODAY. SUBJECTIVE DVORAK INTENSITY ESTIMATES AT 18Z WERE A CONSENSUS 77 KT...AND THE ADVISORY INTENSITY IS ADJUSTED SLIGHTLY DOWNWARD TO 75 KT. THE INTENSITY FORECAST HINGES PRIMARILY ON WATER TEMPERATURE CHANGES ALONG THE FORECAST TRACK...SINCE DYNAMICAL MODELS SHOW WEAK VERTICAL WIND SHEAR PERSISTING THROUGH ALMOST THE ENTIRE FIVE-DAY FORECAST PERIOD. HERNAN IS CURRENTLY OVER SEA-SURFACE TEMPERATURES OF ABOUT 24.5 CELSIUS...AND WHILE THE UNDERLYING WATERS WILL GET COOLER DURING THE NEXT 24 HOURS...THEY WILL THEN GET SLIGHTLY WARMER AGAIN AND APPROACH 25 CELSIUS AT DAYS 3 THROUGH 5. AS A RESULT...HERNAN IS LIKELY TO CONTINUE TO STEADILY WEAKEN FOR THE NEXT COUPLE OF DAYS...AND THEN WEAKEN MORE GRADUALLY. THE OFFICIAL FORECAST CLOSELY FOLLOWS THE SHIPS AND LGEM GUIDANCE IN FORECASTING HERNAN TO WEAKEN TO A DEPRESSION AFTER ABOUT 72 HOURS...EVEN THOUGH THE GFDL AND HWRF SUGGEST IT COULD STILL BE A MARGINAL TROPICAL STORM AT FIVE DAYS. THE INITIAL MOTION ESTIMATE IS 295/9...BUT THE CYCLONE IS GRADUALLY TURNING TO THE LEFT AS SUBTROPICAL RIDGING BUILDS TO ITS NORTHWEST. THE TRACK FORECAST IS FAIRLY STRAIGHTFORWARD...WITH THE MODELS AGREEING WELL ON A GENERAL WEST-SOUTHWESTWARD MOTION BEGINNING IN ABOUT 48 HOURS. NEAR THE END OF THE FORECAST PERIOD...HERNAN SHOULD BE A MUCH WEAKER SYSTEM MOVING ALONG IN THE LOW- TO MID-LEVEL TRADE WIND FLOW. THE NEW TRACK IS JUST ABOUT IDENTICAL TO THE PREVIOUS ONE AND IS CLOSE TO THE MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INITIAL 10/2100Z 18.2N 127.5W 75 KT 12HR VT 11/0600Z 18.5N 128.7W 65 KT 24HR VT 11/1800Z 18.5N 130.2W 55 KT 36HR VT 12/0600Z 18.3N 131.6W 50 KT 48HR VT 12/1800Z 17.9N 133.0W 45 KT 72HR VT 13/1800Z 17.0N 135.5W 35 KT 96HR VT 14/1800Z 16.5N 138.5W 30 KT 120HR VT 15/1800Z 16.0N 142.0W 30 KT $$ FORECASTER KNABB NNNN
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