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| #110900 (Received by flhurricane at: 10:52 PM 14.Sep.2006) TCDAT3 TROPICAL STORM HELENE DISCUSSION NUMBER 11 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 1100 PM EDT THU SEP 14 2006 THE INITIAL MOTION REMAINS 285/13...BASED PRIMARILY ON MICROWAVE SATELLITE FIX POSITIONS. THE 18Z NHC MODEL GUIDANCE HAS LESS SPREAD NOW THAN PREVIOUS ADVISORIES...WITH THE GFS AND NOGAPS MODELS HAVING SHIFTED MORE WESTWARD AND CLOSER TO THE UKMET 12Z SOLUTION. THE NEW 18Z UKMET RUN IS ALSO MUCH SLOWER NOW AND VERY SIMILAR TO THE 18Z NOGAPS MODEL RUN. THE KEY PLAYER DURING THE FORECAST CYCLE IS THE MID-LEVEL SHORTWAVE TROUGH CURRENTLY OVER THE NORTHEASTERN U.S. THAT IS FORECAST BY ALL OF THE MODELS TO DIG SOUTHEASTWARD AND CUTOFF INTO A LOW PRESSURE SYSTEM. HOWEVER...THE UKMET AND NOGAPS MODELS ARE NOW WEAKENING THE LOW AND LIFTING IT OUT SOONER THAN THE GFS...ECMWF... GFDL...AND CANADIAN MODELS. GIVEN THAT THE MID-LATITUDE FLOW ACROSS THE NORTHERN U.S. AND THE NORTH ATLANTIC IS FORECAST BY ALL THE MODELS TO BE FAST ZONAL FLOW...LESS AMPLIFICATION OF THE TROUGH SEEMS MORE REASONABLE...WHICH SHOULD DECREASE THE EFFECT OF ANY NORTHWARD STEERING FLOW ON THE WAST SIDE OF THE TROUGH. THE RESULT IS THAT THE STEERING FLOW SHOULD WEAKEN CONSIDERABLY BY 96 AND 120 HOURS AS MID-LEVEL RIDGING BUILDS SLOWLY WESTWARD TO THE NORTH OF HELENE. THE OFFICIAL FORECAST TRACK IS A LITTLE SOUTH OF THE PREVIOUS TRACK AND IS CLOSE TO THE GUNA AND CONU MODEL CONSENSUS FORECAST TRACKS. HELENE HAS BEEN HAVING A DIFFICULT TIME MAINTAINING CENTRAL DEEP CONVECTION FOR MORE THAN A JUST FEW HOURS AT A TIME DUE TO A LARGE SLUG OF DRY AIR PUNCHING INTO THE SOUTHERN AND EASTERN QUADRANTS OF THE CYCLONE. SINCE THIS PATTERN SEEMS TO BE UNLIKELY TO CHANGE ANY TIME SOON...THE INTENSITY FORECAST WAS DECREASED SLIGHTLY DURING THE FIRST 72 HOURS SIMILAR TO THE SHIPS MODEL. AFTER THAT... A SLIGHTLY FASTER RATE OF INTENSIFICATION IS FORECAST AS PER THE GFDL AND FSU MODELS...WHICH SEEMS REASONABLE SINCE HELENE WILL BE MOVING OVER 28C AND WARMER SSTS AND ALSO BE UNDERNEATH A VERY FAVORABLE UPPER-LEVEL OUTFLOW REGIME. FORECAST POSITIONS AND MAX WINDS INITIAL 15/0300Z 14.5N 38.9W 40 KT 12HR VT 15/1200Z 15.3N 40.8W 45 KT 24HR VT 16/0000Z 16.4N 43.2W 50 KT 36HR VT 16/1200Z 17.5N 45.2W 60 KT 48HR VT 17/0000Z 18.5N 46.7W 70 KT 72HR VT 18/0000Z 20.3N 49.4W 80 KT 96HR VT 19/0000Z 22.4N 51.9W 90 KT 120HR VT 20/0000Z 24.5N 54.0W 90 KT $$ FORECASTER STEWART |
