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| #111541 (Received by flhurricane at: 4:49 AM 18.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 24 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 AM EDT MON SEP 18 2006 THE INITIAL MOTION IS 340/6. WATER VAPOR IMAGERY SHOWS A SMALL MID- TO UPPER-LEVEL LOW LOCATED A FEW HUNDRED MILES TO THE NORTH-NORTHWEST OF THE HURRICANE...AND THIS UPPER LOW MAY BE RESPONSIBLE FOR THE MORE NORTHWARD CURRENT MOTION. AS THIS LOW...ALONG WITH HURRICANE GORDON...LIFT OUT OVER THE NEXT DAY OR SO...RIDGING SHOULD DEVELOP TO THE NORTH OF HELENE. THIS RIDGING IS EXPECTED TO FORCE HELENE ON MORE OF A WESTWARD TRACK OVER THE NEXT DAY OR TWO...ALTHOUGH IT IS WORTH NOTING THAT THE GUIDANCE HAS HAD A DECIDED SHORT-TERM WESTWARD BIAS OVER THE LAST COUPLE OF DAYS. THE LONGER RANGE PROGNOSIS IS LESS CLEAR. A DEEP-LAYER LOW CURRENTLY EXTENDS SOUTHWARD FROM THE WESTERN ATLANTIC INTO THE BAHAMAS...AND A SECOND LARGE TROUGH IS MOVING THROUGH THE NORTHERN PLAINS. MODELS ARE IN GENERAL AGREEMENT ON WEAKENING THE WESTERN ATLANTIC LOW AND BRINGING THE SECOND TROUGH THROUGH NEW ENGLAND IN 4 TO 5 DAYS... ALLOWING SOME RIDGING TO DEVELOP IN THE WESTERN ATLANTIC BY DAY 5. HOWEVER...THE SPECIFIC INTERACTIONS BETWEEN HELENE AND THESE VARIOUS FEATURES VARY WIDELY AMONG THE MODELS. ON THE RIGHT SIDE OF THE GUIDANCE SUITE...THE GFDL AND CANADIAN MODELS HAVE HELENE LIFTED NORTH BY THE WESTERN ATLANTIC LOW JUST ENOUGH TO THEN BE PICKED UP BY THE PLAINS TROUGH. ON THE LEFT ARE THE UKMET AND NOGAPS...WHICH KEEP HELENE FARTHER SOUTH INITIALLY AND END UP WITH IT CAPTURED OR SLOWED BY THE WESTERN ATLANTIC RIDGE ON DAY 5. THE GFS...WHICH HAD BEEN LEANING LEFT...HAS NOW SHIFTED TO THE CENTER OF THE GUIDANCE ENVELOPE. THE 00Z RUN OF THE GFS FORECASTS THE WESTERN ATLANTIC RIDGE TO BE WEAKER AND DISPLACED TO THE SOUTH OF THE PREVIOUS RUN...AND THIS IS ENOUGH TO RESULT IN A RATHER LARGE NORTHWARD SHIFT IN THE TRACK OF HELENE IN THE MODEL. CONVERSELY THE ECMWF...WHICH HAD BEEN SHOWING A RAPID RECURVATURE...HAS SHIFTED LEFT AND IS NOW PRETTY CLOSE TO THE GFS. THE BOTTOM LINE OF ALL THIS IS THAT WHAT HAPPENS IN THE NEXT 24 TO 48 HOURS...THAT IS...HOW MUCH LATITUDE HELENE CAN GAIN...COULD MAKE ALL THE DIFFERENCE BETWEEN A QUICK RECURVATURE AND AN AGONIZINGLY SLOW PATH THROUGH THE WESTERN ATLANTIC. THE OFFICIAL FORECAST HAS BEEN SHIFTED A LITTLE TO THE NORTHEAST OF THE PREVIOUS TRACK...BUT IS STILL TO THE LEFT OF THE GFS/ECMWF AND THE DYNAMICAL MODEL CONSENSUS. IT IS PERHAPS OBVIOUS BY NOW THAT I CONSIDER THE TRACK FORECAST TO HAVE MORE THAN THE NORMAL AMOUNT OF UNCERTAINTY. EYEWALL CONVECTION IS A LITTLE COLDER AND MORE SYMMETRIC THAN IT WAS 6 HOURS AGO. SUBJECTIVE DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB ARE 102 KT...WHILE THE CIMSS/ADT ESTIMATES ARE A LITTLE HIGHER AND HAVE BEEN INCREASING. THE ADVISORY ESTIMATE IS INCREASED TO 105 KT. THE OUTFLOW PATTERN REMAINS VERY STRONG...ALTHOUGH THERE HAS BEEN SOME FLATTENING ON THE WESTERN SIDE LIKELY ASSOCIATED WITH THE SMALL UPPER LOW. THIS LOW COMPLICATES THE SHORT-TERM INTENSITY FORECAST. ALTHOUGH AT THE MOMENT IT MAY BE IMPEDING THE OUTFLOW... WATER VAPOR IMAGERY SUGGESTS THAT VORTICITY FROM THIS LOW IS BEING ENTRAINED INTO HELENE...AND THIS KIND OF TROUGH INTERACTION CAN SOMETIMES PROVIDE A KICK START FOR ADDITIONAL INTENSIFICATION. THE OFFICIAL FORECAST IS SLIGHTLY ABOVE THE SHIPS AND GFDL GUIDANCE. ALL INDICATIONS ARE THAT HELENE WILL BE AT OR NEAR MAJOR HURRICANE STATUS THROUGH THE FORECAST PERIOD. FORECAST POSITIONS AND MAX WINDS INITIAL 18/0900Z 22.3N 49.6W 105 KT 12HR VT 18/1800Z 23.1N 50.4W 110 KT 24HR VT 19/0600Z 23.7N 51.7W 115 KT 36HR VT 19/1800Z 24.0N 53.3W 115 KT 48HR VT 20/0600Z 24.3N 55.0W 110 KT 72HR VT 21/0600Z 26.0N 57.5W 105 KT 96HR VT 22/0600Z 28.2N 58.9W 100 KT 120HR VT 23/0600Z 30.0N 60.0W 95 KT $$ FORECASTER FRANKLIN |
