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| #111327 (Received by flhurricane at: 5:03 AM 17.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 20 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 AM EDT SUN SEP 17 2006 SHOWER AND THUNDERSTORM ACTIVITY CONTINUES TO ORGANIZE AROUND THE CENTER OF HELENE...WITH A BROAD BANDING TYPE EYE IS SEEN IN INFRARED SATELLITE IMAGERY. GIVEN THE SATELLITE APPEARANCE...A SOMEWHAT SURPRISING...VERY LARGE 45 TO 60 NM WIDE EYE WAS DETECTED ON A RECENT 0502 UTC AQUA MICROWAVE OVERPASS. BASED ON THE MICROWAVE DATA AND SATELLITE INTENSITY ESTIMATES OF 77 KT FROM SAB AND TAFB...THE INITIAL INTENSITY IS INCREASED TO 75 KT. THE FUTURE TRACK OF HELENE APPEARS TO BE HIGHLY DEPENDENT ON HOW THE VARIOUS MODELS ARE HANDLING A COUPLE OF KEY FEATURES. THE FIRST IS THE AMOUNT OF RIDGING THAT BUILDS BETWEEN HELENE AND THE DEPARTING HURRICANE GORDON TO THE NORTH...AND THE SECOND IS THE STRENGTH OF THE TROUGH ALONG THE U.S. EAST COAST IN A COUPLE OF DAYS. IN ANY CASE...THE DYNAMICAL MODELS REMAIN IN GOOD AGREEMENT ON A GENERAL WEST-NORTHWEST TO NORTHWEST HEADING DURING THE NEXT COUPLE OF DAYS. THEREAFTER...THE MODEL SPREAD REMAINS EXTREMELY LARGE. THE GFS REMAINS ON THE LEFT SIDE OF THE GUIDANCE...AS IT WEAKENS THE TROUGH BETWEEN THE U.S. AND BERMUDA AND BUILDS A NARROW MID- TO UPPER-LEVEL RIDGE NORTH OF THE HURRICANE...ALLOWING HELENE TO MOVE MORE WESTWARD. THE REMAINDER OF THE DYNAMICAL MODELS KEEP ENOUGH OF A TROUGH NEAR BERMUDA TO IMPART A NORTHWESTWARD MOTION AT DAYS 4 AND 5. IT IS INTERESTING TO NOTE THAT THE GFDL...WHICH USES THE INITIAL CONDITIONS OF THE GFS MODEL...IS ON THE FAR RIGHT SIDE OF THE MODEL GUIDANCE. UNTIL IT BECOMES MORE CLEAR AS TO WHICH MODEL SOLUTION MAY BE CORRECT...THE OFFICIAL TRACK REMAINS NEAR THE MODEL CONSENSUS...WHICH IS A LITTLE WEST OF THE PREVIOUS FORECAST. WITH LOW SHEAR AND WARMER SSTS ALONG THE PREDICTED PATH OF HELENE...THE ENVIRONMENT APPEARS FAVORABLE FOR ADDITIONAL STRENGTHENING DURING THE NEXT FEW DAYS. THE OFFICIAL INTENSITY FORECAST FOLLOWS THE SLIGHTLY HIGHER GFDL AND FSU SUPERENSEMBLE THROUGH 72 HOURS. THEREAFTER...THE GLOBAL MODELS PREDICT THAT HELENE WILL BE APPROACHING AN AREA OF INCREASING SOUTHWESTERLY SHEAR...AND THE INTENSITY FORECAST SHOWS SOME WEAKENING AT DAYS 4 AND 5...HOWEVER NOT AS MUCH INDICATED BY THE SHIPS MODEL. FORECAST POSITIONS AND MAX WINDS INITIAL 17/0900Z 20.0N 48.2W 75 KT 12HR VT 17/1800Z 20.7N 49.3W 80 KT 24HR VT 18/0600Z 21.7N 50.5W 85 KT 36HR VT 18/1800Z 22.6N 51.7W 90 KT 48HR VT 19/0600Z 23.3N 53.2W 95 KT 72HR VT 20/0600Z 24.0N 56.0W 95 KT 96HR VT 21/0600Z 26.0N 58.0W 85 KT 120HR VT 22/0600Z 29.0N 59.0W 80 KT $$ FORECASTER BROWN/KNABB |
