000 WTPZ44 KNHC 091457 TCDEP4 HURRICANE HERNAN DISCUSSION NUMBER 12 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP092008 800 AM PDT SAT AUG 09 2008 THE EYE OF HERNAN HAS BECOME DRAMATICALLY MORE DISTINCT IN GOES INFRARED IMAGERY DURING THE PAST SEVERAL HOURS...AND IT IS NOW SURROUNDED BY A SOLID RING OF CLOUD TOPS COLDER THAN -70C. AMSR-E MICROWAVE IMAGERY AT 0937 UTC DEPICTED A SINGLE COMPLETE EYEWALL AND AN EYE DIAMETER OF 20-25 N MI. THERE SEEMS LITTLE DOUBT THAT HERNAN HAS BECOME A MAJOR HURRICANE... HAVING UNDERGONE RAPID INTENSIFICATION SINCE YESTERDAY. THIS PROVIDES YET ANOTHER EXAMPLE OF THE INABILITY OF MODEL GUIDANCE...AND THEREFORE THE HUMAN FORECASTER...TO CAPTURE AND CONVEY THE TIMING AND MAGNITUDE OF THESE EPISODES WITH A DETERMINISTIC INTENSITY FORECAST. THE EXACT INITIAL INTENSITY IS A LITTLE UNCERTAIN. SUBJECTIVE DVORAK ESTIMATES AT 12Z FROM TAFB AND SAB WERE BOTH 102 KT...WHILE OBJECTIVE ESTIMATES ARE ABOUT 115 KT. THE ADVISORY INTENSITY COMPROMISES AT 105 KT...BUT WITHOUT ACTUAL WIND DATA TO KNOW ANY BETTER...IT IS POSSIBLE HERNAN IS A LITTLE STRONGER THAN THIS ESTIMATE. UNDERLYING SEA-SURFACE TEMPERATURES ARE ABOUT 27 CELSIUS NOW...AND ALONG THE FORECAST TRACK THE HURRICANE WILL CROSS THE 26 CELSIUS ISOTHERM IN ROUGHLY 24 HOURS...SO A GRADUAL WEAKENING TREND IS FORECAST TO BEGIN AROUND THAT TIME. A FASTER DECLINE IS FORECAST IN A COUPLE OF DAYS ONCE THE CYCLONE REACHES SUBSTANTIALLY COOLER WATERS. THE OFFICIAL FORECAST IS ABOVE THE OBJECTIVE GUIDANCE FOR THE NEXT DAY OR TWO...THEN SHOWS A FASTER WEAKENING THAN THE MODELS DUE TO THE SMALL SIZE OF THE CYCLONE. HERNAN HAS TURNED A LITTLE MORE TO THE RIGHT...AND ONE MUST GIVE SOME OF THE MODELS CREDIT FOR CORRECTLY FORECASTING THIS BEND YESTERDAY. THE INITIAL MOTION ESTIMATE IS 295/8...BUT A GRADUAL TURN BACK TO THE LEFT IS FORECAST BY ALL OF THE DYNAMICAL MODELS AS THE SUBTROPICAL RIDGE BUILDS WESTWARD OVER THE EASTERN PACIFIC DURING THE NEXT COUPLE OF DAYS. BEYOND THAT TIME...A WEAKENING HERNAN IS EXPECTED TO BECOME A MORE SHALLOW SYSTEM AND TURN WESTWARD AND EVENTUALLY WEST-SOUTHWESTWARD IN THE LOW- TO MID-LEVEL FLOW. THE NEW OFFICIAL TRACK FORECAST IS ALMOST IDENTICAL TO THE PREVIOUS ONE AND IS CLOSE TO THE MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INITIAL 09/1500Z 15.7N 123.3W 105 KT 12HR VT 10/0000Z 16.2N 124.4W 105 KT 24HR VT 10/1200Z 16.8N 125.9W 95 KT 36HR VT 11/0000Z 17.3N 127.3W 80 KT 48HR VT 11/1200Z 17.6N 128.7W 65 KT 72HR VT 12/1200Z 18.0N 131.0W 45 KT 96HR VT 13/1200Z 17.5N 133.5W 35 KT 120HR VT 14/1200Z 16.5N 136.0W 30 KT $$ FORECASTER KNABB