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| #110929 (Received by flhurricane at: 4:58 AM 15.Sep.2006) TCDAT3 TROPICAL STORM HELENE DISCUSSION NUMBER 12 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006 500 AM EDT FRI SEP 15 2006 HELENE DOES NOT LOOK TOO MUCH DIFFERENT THAN IT DID SIX HOURS AGO...EXCEPT FOR THE DEEP CONVECTION BUBBLING IN SLIGHTLY DIFFERENT PORTIONS OF THE CIRCULATION. PERHAPS SOME CONVECTION HAS RECENTLY INCREASED A LITTLE NEAR THE LOW-LEVEL CENTER...BUT EVEN THAT SEEMS TO BE ON THE DECREASE AGAIN. THE ADVISORY INTENSITY REMAINS 40 KT BASED ON A BLEND OF THE 06Z DVORAK ESTIMATES. HELENE APPEARS TO BE INGESTING SOME DRY AIR...AND IT MIGHT TAKE A WHILE FOR INNER CORE CONVECTION TO BECOME PERSISTENT. THEREFORE...THE OFFICIAL INTENSITY FORECAST IS SLIGHTLY BELOW ALL OF THE AVAILABLE GUIDANCE AND IS ALSO A SLIGHT DOWNWARD ADJUSTMENT FROM THE PREVIOUS ADVISORY. HELENE IS STILL FORECAST TO BECOME A HURRICANE IN A COUPLE OF DAYS...JUST NOT QUITE AS STRONG AS THE SHIPS AND GFDL ARE SUGGESTING. HELENE CONTINUES WEST-NORTHWESTWARD AT ABOUT 13 KT ALONG A HEADING BETWEEN 285 AND 290 DEGREES...WITH STEERING PROVIDED BY THE SUBTROPICAL RIDGE TO ITS NORTH. THE DYNAMICAL MODELS ARE IN REASONABLE AGREEMENT ON A WEST-NORTHWESTWARD TO NORTHWESTWARD MOTION DURING THE NEXT 48 TO 72 HOURS. AFTER THAT THE MODELS DIVERGE FAIRLY SIGNIFICANTLY...BOUNDED BY THE NOGAPS ON THE LEFT AND TURNING THE CYCLONE WESTWARD...AND THE GFDL ON THE RIGHT AND RACING HELENE NORTHWARD. THE DISAGREEMENT SEEMS TO STEM FROM HOW EACH MODEL HANDLES A MID- TO UPPER-LEVEL TROUGH CURRENTLY OVER THE EASTERN UNITED STATES THAT WILL BE MOVING SLOWLY EASTWARD DURING THE NEXT SEVERAL DAYS. IF THE NOGAPS IS CORRECT...ENOUGH RIDGING WILL REMAIN BETWEEN THAT TROUGH AND HELENE TO ALLOW THE TROPICAL CYCLONE TO TURN WESTWARD. THE GFDL AND GFS...HOWEVER...FORECAST HELENE TO GAIN ENOUGH LATITUDE BY ROUNDING THE RIDGE SUCH THAT THE APPROACHING TROUGH WILL PICK UP HELENE. I AM NOT YET CONFIDENT THE RIDGE TO THE NORTH OF HELENE WILL BREAK DOWN THAT FAST...SO THE NEW OFFICIAL TRACK FORECAST LIES ON THE LEFT SIDE OF THE MODEL GUIDANCE ENVELOPE...AND IT IS ESSENTIALLY IDENTICAL TO THE PREVIOUS ADVISORY. FORECAST POSITIONS AND MAX WINDS INITIAL 15/0900Z 15.0N 40.3W 40 KT 12HR VT 15/1800Z 15.9N 42.1W 45 KT 24HR VT 16/0600Z 17.0N 44.2W 50 KT 36HR VT 16/1800Z 18.0N 46.0W 55 KT 48HR VT 17/0600Z 19.0N 47.4W 65 KT 72HR VT 18/0600Z 21.0N 50.0W 75 KT 96HR VT 19/0600Z 23.0N 52.5W 80 KT 120HR VT 20/0600Z 24.5N 54.5W 85 KT $$ FORECASTER KNABB |
