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| #113779 (Received by flhurricane at: 4:40 PM 30.Sep.2006) TCDAT4 HURRICANE ISAAC DISCUSSION NUMBER 13 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092006 500 PM EDT SAT SEP 30 2006 CONVECTION HAS CONTINUED TO INCREASE IN THE EYEWALL OF ISSAC THIS AFTERNOON. IN ADDITION THE EYE HAS BECOME A LITTLE BETTER-DEFINED WITH AN EXPANDING OUTFLOW PATTERN IN ALL QUADRANTS EXCEPT TO THE SOUTHWEST. DVORAK T-NUMBERS FROM SAB/AFWA ARE INCREASING... AND THE LOCAL 3 HOUR OBJECTIVE T-NUMBERS ARE NEAR T4.2... EQUATING TO 70 KT. THE INITIAL INTENSITY WILL BE SET AT 70 KT FOR THIS ADVISORY IN AGREEMENT WITH THE ABOVE ESTIMATES. ISAAC STILL HAS A LITTLE MORE TIME TO INTENSIFY BEFORE SOUTHWESTERLY SHEAR ASSOCIATED WITH AN UPPER TROUGH IMPACTS THE HURRICANE. STEADY WEAKENING SHOULD BEGIN BY LATE TOMORROW AS THE SHEAR BECOMES STRONG AND WATERS COOL BELOW 26C. THE OFFICIAL FORECAST IS SIMILAR TO THE PREVIOUS ONE AND IS CLOSE TO ICON... THE INTENSITY CONSENSUS OF GFDL AND SHIPS. GFDL CONTINUES TO INSIST ON STRENGTHENING AFTER RECURVATURE BUT THIS MAY BE OVERDONE DUE TO VERY STRONG SHEAR FORECAST TO OCCUR BY ALL GLOBAL MODELS. THESE GLOBAL MODELS ALSO EXPECT ISAAC TO BE A POWERFUL EXTRATROPICAL LOW UNTIL IT IS EVENTUALLY ABSORBED BY A LARGER MIDDLE-LATITUDE CYCLONE. THE CYCLONE HAS SPED UP AND IS NOW MOVING ABOUT 310/9... THOUGH A SHORTER-TERM MOTION IS FASTER. THE FORECAST PHILOSOPHY IS THE SAME AS BEFORE. ISAAC IS EXPECTED TO MOVE IN BETWEEN A RIDGE TO THE EAST OVER THE CENTRAL ATLANTIC AND A LARGE TROUGH OVER THE WESTERN ATLANTIC. AT THIS POINT THE BIGGEST QUESTION IS HOW CLOSE WILL EXTRATROPICAL ISAAC GET TO NEWFOUNDLAND. THE ECMWF...GFS AND GFS ENSEMBLE MEAN MODELS WHIP EXTRATROPICAL ISAAC AROUND A LARGER EXTRATROPICAL LOW AND ARE... CONSEQUENTLY... FARTHER TO THE LEFT THAN THE OTHER MODELS... WHICH KEEP ISAAC MORE SEPARATE FROM THE LOW. GLOBAL MODELS DO AGREE THAT ITS REMNANTS WILL FORM A POWERFUL STORM OVER THE NORTH ATLANTIC AND MIGHT BE SIMILAR TO THE STRENGTH OF EXTRATROPICAL FLORENCE A FEW WEEKS AGO. THE OFFICIAL FORECAST IS CLOSE TO THE PREVIOUS FORECAST TRACK WITH THE MODELS NOW HONED IN ON RECURVATURE ALONG ABOUT 61W... AND SHOWS SOME THREAT TO NEWFOUNDLAND IN ABOUT 2-3 DAYS AS A STRONG EXTRATROPICAL SYSTEM. FORECAST POSITIONS AND MAX WINDS INITIAL 30/2100Z 31.6N 59.5W 70 KT 12HR VT 01/0600Z 32.6N 60.3W 75 KT 24HR VT 01/1800Z 35.0N 60.8W 75 KT 36HR VT 02/0600Z 39.0N 60.0W 70 KT...BECOMING EXTRATROPICAL 48HR VT 02/1800Z 44.0N 56.8W 60 KT...EXTRATROPICAL 72HR VT 03/1800Z 52.0N 46.0W 45 KT...EXTRATROPICAL 96HR VT 04/1800Z...ABSORBED $$ FORECASTER BLAKE/STEWART |
