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| #199649 (Received by flhurricane at: 5:11 AM 07.Jul.2008) TCDAT2 HURRICANE BERTHA DISCUSSION NUMBER 17 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 500 AM EDT MON JUL 07 2008 IN THE HOURS LEADING UP TO 00Z...SEVERAL CONSECUTIVE MICROWAVE OVERPASSES OCCURRED ON ROUGHLY AN HOURLY BASIS...AND ALMOST MADE IT SEEM LIKE WE WERE RECEIVING RADAR FIXES. THOSE IMAGES REVEALED A WELL-DEFINED AND VERTICALLY-ALIGNED EYE FEATURE...AND MORE RECENTLY AN IMAGE FROM AMSU TAKEN AT 0517Z ALSO DEPICTED AN EYE. THE MICROWAVE EYE DIAMETER APPEARS TO BE ABOUT 15-20 N MI. GOES INFRARED IMAGERY HAS INTERMITTENTLY EXHIBITED HINTS OF AN EYE...ESPECIALLY AROUND 06Z WHEN SUBJECTIVE DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB WERE BOTH 65 KT. OBJECTIVE DVORAK ESTIMATES REMAIN SLIGHTLY BELOW THE HURRICANE THRESHOLD...BUT GIVEN THE MICROWAVE SIGNATURES THE SYSTEM IS UPGRADED TO A HURRICANE... THE FIRST OF THE 2008 ATLANTIC SEASON...WITH AN INTENSITY OF 65 KT. WHILE A HURRICANE DOES NOT FORM EVERY YEAR IN JULY IN THE ATLANTIC BASIN...IT HAS CERTAINLY HAPPENED BEFORE...INCLUDING IN 1996 WHEN ANOTHER HURRICANE BERTHA FORMED...COINCIDENTALLY ALSO ON JULY 7. EXTRAPOLATION OF THE EARLIER SERIES OF MICROWAVE IMAGES MATCHES UP QUITE WELL WITH MORE RECENT GEOSTATIONARY FIXES TO YIELD AN INITIAL MOTION ESTIMATE OF 285/17. THE SUBTROPICAL RIDGE TO THE NORTH OF BERTHA IS PROVIDING THE STEERING...BUT THAT FEATURE IS FORECAST BY THE DYNAMICAL MODELS TO GRADUALLY WEAKEN OVER THE WESTERN ATLANTIC DURING THE NEXT FEW DAYS...ALLOWING BERTHA'S TRACK TO STEADILY BEND TO THE RIGHT...BUT ALSO SLOW DOWN SUBSTANTIALLY. ALL OF THE MODELS GENERALLY AGREE ON THIS SCENARIO...BUT THERE REMAINS A HEALTHY AMOUNT OF SPREAD REGARDING AT WHAT LONGITUDE BERTHA WILL TURN. THE NEW OFFICIAL TRACK FORECAST IS EDGED SLIGHTLY TO THE RIGHT TOWARD THE CONSENSUS AND IS A TAD SLOWER AT THE END. GIVEN THE WEAK STEERING CURRENTS AND LACK OF A HIGH-AMPLITUDE TROUGH OVER THE WESTERN ATLANTIC DURING THE NEXT SEVERAL DAYS...IT IS STILL NOT GUARANTEED THAT BERTHA WILL RECURVE. THE UPPER-LEVEL WINDS NEAR BERTHA ARE NOT FORECAST TO CHANGE APPRECIABLY DURING THE NEXT COUPLE OF DAYS...WHILE UNDERLYING SEA-SURFACE TEMPERATURES WARM A LITTLE MORE...SO SOME ADDITIONAL STRENGTHENING IS FORECAST ALONG THE LINES OF THE SHIPS AND LGEM GUIDANCE. AT 3 TO 5 DAYS...THE GLOBAL MODELS DISAGREE ON THE EVOLUTION OF AN UPPER-LEVEL TROUGH IN THE VICINITY OF THE TROPICAL CYCLONE...AND SO IT REMAINS UNCLEAR HOW MUCH WIND SHEAR WILL AFFECT BERTHA THEN. THE NEW OFFICIAL INTENSITY FORECAST PEAKS AT 80 KT WITHIN THE NEXT COUPLE OF DAYS...BUT THEN SHOWS GRADUAL WEAKENING DUE TO THE POTENTIAL FOR INCREASING SHEAR...AND DUE TO COOLER WATERS AS BERTHA PROGRESSES NORTHWARD ALONG THE FORECAST TRACK. FORECAST POSITIONS AND MAX WINDS INITIAL 07/0900Z 19.3N 50.2W 65 KT 12HR VT 07/1800Z 19.9N 52.5W 70 KT 24HR VT 08/0600Z 20.8N 55.0W 75 KT 36HR VT 08/1800Z 21.7N 57.1W 80 KT 48HR VT 09/0600Z 22.7N 59.0W 80 KT 72HR VT 10/0600Z 25.0N 62.0W 75 KT 96HR VT 11/0600Z 28.0N 64.0W 70 KT 120HR VT 12/0600Z 30.5N 64.5W 70 KT $$ FORECASTER KNABB |
