ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM TROPICAL STORM HERNAN DISCUSSION NUMBER 6 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP092008 800 PM PDT THU AUG 07 2008 THE CENTER OF HERNAN IS SOMEWHAT DIFFICULT TO LOCATE THIS EVENING. RECENT MICROWAVE IMAGERY CONTINUES TO SHOW A MID-LEVEL EYE FEATURE...BUT ALSO SHOWS THAT THIS IS DISPLACED TO THE WEST OR SOUTHWEST OF THE LOW-LEVEL CENTER. ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN INDICATE 15-20 KT OF EAST-NORTHEASTERLY VERTICAL WIND SHEAR OVER HERNAN...WHICH IS THE LIKELY CAUSE OF THIS STRUCTURE. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN 55 KT...AND THAT REMAINS THE INITIAL INTENSITY. THE MICROWAVE DATA SHOWS THAT HERNAN HAS MOVED WESTWARD DURING THE PAST SEVERAL HOURS...WITH THE INITIAL MOTION A SOMEWHAT UNCERTAIN 275/11. THE STORM IS CURRENTLY ON THE SOUTH SIDE OF A MID-LEVEL RIDGE AND HEADING FOR A LARGE MID/UPPER-LEVEL TROUGH THAT COVERS THE AREA NORTH OF 22N BETWEEN 130-150W. THE LARGE-SCALE MODELS FORECAST THE TROUGH TO LIFT OUT TO THE NORTH DURING THE FORECAST PERIOD...WITH ENOUGH MID-LEVEL RIDGING DEVELOPING TO STEER HERNAN GENERALLY WEST-NORTHWESTWARD FOR THE NEXT 48-72 HR. AFTER THAT TIME...HERNAN WILL BE WEAKENING OVER COOLER SEA SURFACE TEMPERATURES...AND LOW-LEVEL FLOW SHOULD CAUSE THE CYCLONE TO TURN WESTWARD AND EVENTUALLY WEST-SOUTHWESTWARD. THE NEW FORECAST TRACK IS SIMILAR TO...BUT SOUTH OF...THE PREVIOUS FORECAST BASED ON THE CURRENT LOCATION AND MOTION...AND CALLS FOR HERNAN TO RESUME A WEST-NORTHWEST MOTION DURING THE NEXT FEW HOURS. THE NEW TRACK IS DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE IN GOOD AGREEMENT WITH THE MODEL CONSENSUS. IT SHOULD BE NOTED THAT THE MICROWAVE IMAGERY HINTS THAT THE CENTER OF HERNAN IS SOUTH OF THE ADVISORY POSITION. IF SUBSEQUENT DATA SHOW THIS IS INDEED THE CASE...AN ADDITIONAL ADJUSTMENT TO THE FORECAST TRACK MAY BE NECESSARY ON THE NEXT ADVISORY. ON THE FORECAST TRACK...THE CENTER OF HERNAN WOULD REACH THE 26C SEA SURFACE TEMPERATURE ISOTHERM IN ABOUT 36 HR. THE ECMWF...GFS...AND NOGAPS FORECAST A GRADUAL DECREASE IN SHEAR DURING THIS TIME... WHICH COULD ALLOW GRADUAL STRENGTHENING. ON THE OTHER HAND...THE UKMET SHOWS LITTLE CHANGE OR EVEN SOME INCREASE IN THE SHEAR. THE VARIOUS INTENSITY GUIDANCE MODELS CALL FOR SOME STRENGTHENING WITH PEAK INTENSITIES IN THE 60-70 KT RANGE. GIVEN THE CURRENT STRUCTURE AND SHEAR...THE NEW INTENSITY FORECAST CALL FOR A SLOWER INTENSIFICATION RATE THAN THE PREVIOUS FORECAST...WITH HERNAN REACHING HURRICANE STRENGTH IN ABOUT 24 HR. AFTER 36 HR...THE FORECAST MOTION OVER COOLER WATER SHOULD CAUSE WEAKENING...WITH THE CYCLONE EXPECTED TO DECAY TO A NON-CONVECTIVE LOW IN ABOUT 120 HR. FORECAST POSITIONS AND MAX WINDS INITIAL 08/0300Z 14.9N 117.8W 55 KT 12HR VT 08/1200Z 15.3N 119.4W 60 KT 24HR VT 09/0000Z 15.8N 121.2W 65 KT 36HR VT 09/1200Z 16.2N 122.8W 65 KT 48HR VT 10/0000Z 16.8N 124.5W 60 KT 72HR VT 11/0000Z 18.0N 128.0W 45 KT 96HR VT 12/0000Z 18.5N 130.5W 30 KT 120HR VT 13/0000Z 18.0N 134.0W 20 KT...REMNANT LOW $$ FORECASTER BEVEN NNNN
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