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| #111948 (Received by flhurricane at: 10:46 PM 19.Sep.2006) TCDAT3 HURRICANE HELENE DISCUSSION NUMBER 31 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 1100 PM EDT TUE SEP 19 2006 SATELLITE INTENSITY ESTIMATES AVERAGE NEAR 100 KT FROM TAFB...SAB AND AFWA. HOWEVER...A 2039 UTC TRMM OVERPASS INDICATED THAT HELENE WAS IN THE MIDDLE OF AN EYEWALL REPLACEMENT CYCLE WHICH SUGGESTS THE SLIGHT FILLING OF THE CYCLONE IS NOT YET FINISHED. IN ADDITION RECENT INFRARED SATELLITE IMAGERY SHOWS SOME ASYMMETRIES IN THE DEEP CONVECTION WITH A FAIRLY RAGGED EYE AS WELL. THE INITIAL INTENSITY IS KEPT AT 95 KT FOR THIS ADVISORY. THE INITIAL MOTION IS 275 DEGREES AT 8 KT...HOWEVER A TURN TOWARD THE WEST-NORTHWEST AND NORTHWEST IS EXPECTED SOON. THE TRACK MODEL GUIDANCE IS TIGHTLY CLUSTERED THROUGH 48-72 HOURS AND VARIES WITH FORWARD SPEED THEREAFTER. THE GFDL MODEL AND GUNA CONSENSUS WERE FOLLOWED CLOSELY FOR THIS PACKAGE SINCE THESE MODELS HAVE EXHIBITED THE BEST PERFORMANCE THUS FAR WITH HELENE. THE OFFICIAL FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY AND INDICATES A QUICK RECURVATURE TO THE NORTH IN THE NEXT 12-24 HOURS...AND ACCELERATION TO THE NORTHEAST AFTER 48 HOURS ON THE SOUTHEAST SIDE OF A LARGE 500 MB TROUGH MOVING OFF THE U.S. EAST COAST. HELENE REMAINS IN A LOW SHEAR ENVIRONMENT FOR NOW WITH STRONG UPPER TROPOSPHERIC OUTFLOW OVER ALL BUT THE SOUTHWEST PORTION OF THE CIRCULATION. ONCE THE EYEWALL REPLACEMENT IS COMPLETED THERE IS THE POTENTIAL FOR SOME STRENGTHENING AND HELENE COULD REGAIN MAJOR HURRICANE STATUS IN THE NEXT 12-24 HOURS. BEYOND 36 HOURS INCREASING SOUTHWESTERLY SHEAR IS LIKELY TO BRING ABOUT GRADUAL WEAKENING. BY 96 HOURS...GLOBAL MODELS...SUCH AS THE GFS...SHOW CONSIDERABLE COLD AIR ADVECTION OVER THE SOUTHWESTERN SEMICIRCLE OF THE CYCLONE...JUSTIFYING A FORECAST OF TRANSITION TO A STRONG EXTRATROPICAL STORM AROUND THAT TIME. FORECAST POSITIONS AND MAX WINDS INITIAL 20/0300Z 24.6N 54.8W 95 KT 12HR VT 20/1200Z 25.5N 55.7W 100 KT 24HR VT 21/0000Z 27.5N 56.8W 100 KT 36HR VT 21/1200Z 30.0N 56.8W 95 KT 48HR VT 22/0000Z 32.7N 56.0W 95 KT 72HR VT 23/0000Z 39.0N 50.0W 85 KT 96HR VT 24/0000Z 45.0N 40.0W 75 KT...EXTRATROPICAL 120HR VT 25/0000Z 49.0N 30.0W 60 KT...EXTRATROPICAL $$ FORECASTER COBB/PASCH |
