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| #455571 (Received by flhurricane at: 4:50 AM 02.Sep.2011) TCDAT2 TROPICAL STORM KATIA DISCUSSION NUMBER 17 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122011 500 AM AST FRI SEP 02 2011 ALTHOUGH KATIA HAS PRODUCED A RATHER LARGE AND INTENSE AREA OF THUNDERSTORMS OVERNIGHT...ASCAT DATA FROM AROUND 0100 UTC SHOWED THAT THE CENTER WAS NEAR THE SOUTHERN EDGE OF THE CONVECTION. THIS STRUCTURE IS DUE TO SOUTHWESTERLY SHEAR...WHICH IS QUITE EVIDENT FROM THE CIRRUS CLOUD MOTIONS MOVING DIRECTLY INTO THE CORE OF THE CYCLONE. THE ASCAT PASS...HOWEVER...DID SUPPORT AN INTENSITY OF AT LEAST 60 KT AFTER ACCOUNTING FOR THE LOW BIAS OF THE INSTRUMENT. GIVEN THE STRUCTURE TONIGHT...AND SATELLITE CLASSIFICATIONS BELOW HURRICANE FORCE...THE INITIAL WINDS ARE KEPT AT 60 KT. THE INTENSITY FORECAST IS NOT AN EASY ONE THIS MORNING. A LARGE MID/UPPER-LEVEL TROUGH TO THE NORTHWEST OF THE CYCLONE SHOULD CONTINUE TO SHEAR KATIA FOR AT LEAST THE NEXT DAY OR SO. THE GLOBAL MODELS ARE IN RATHER POOR AGREEMENT ON WHEN THIS SHEAR COULD ABATE...WITH THE GFS SHOWING LIGHTER SHEAR BY LATE TOMORROW...WHILE THE ECMWF KEEPS SOME SOUTHERLY SHEAR FOR THE NEXT SEVERAL DAYS. THE MODELS HAVE GENERALLY UNDERDONE THE AMOUNT OF THE SHEAR FOR THIS SYSTEM...SO THE NEW FORECAST WILL BE DECREASED FROM THE PREVIOUS ONE AND PUTS MORE WEIGHT ON THE LOWER STATISTICAL GUIDANCE. THE LARGE-SCALE ENVIRONMENT IS FORECAST BY MOST OF THE GUIDANCE TO BECOME MORE FAVORABLE FOR STRENGTHENING BY EARLY NEXT WEEK...WHICH WOULD SEEM TO BE THE BEST CHANCE OF KATIA BECOMING SIGNIFICANTLY STRONGER. AN EXTRAPOLATION FROM THE PREVIOUS ADVISORY AND A TIMELY AMSR-E MICROWAVE PASS AT 0533 UTC GIVES AN INITIAL MOTION IS 295/13. A TURN TO THE NORTHWEST IS FORECAST BY MOST OF THE GLOBAL GUIDANCE LATER TODAY AS KATIA ROUNDS THE SOUTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE...WITH THAT TRACK GENERALLY CONTINUING FOR THE NEXT SEVERAL DAYS. THE ONE NOTABLE OUTLIER TO THIS SOLUTION IS THE UKMET MODEL...WHICH SHOWS A MUCH STRONGER RIDGE BUILDING OVER THE WESTERN ATLANTIC OCEAN AND KEEPS A WEST-NORTHWESTWARD TRACK. THAT MODEL HAS SHOWN A WESTWARD BIAS IN EARLIER SIMILAR SITUATIONS...SO THE NHC FORECAST WILL DISREGARD THAT SOLUTION AND LEAN MORE CLOSELY ON THE USUALLY RELIABLE ECMWF AND GFS MODELS. AT THE END OF THE FORECAST PERIOD...ALL MODELS FORECAST A RIDGE TO BUILD IN OVER THE WESTERN ATLANTIC...WHICH WOULD FORCE KATIA TO TURN TOWARD THE WEST OR WEST-NORTHWEST. THE NEW FORECAST IS CLOSE TO THE PREVIOUS ONE...BUT IS SHIFTED A BIT TO THE NORTH AT DAY 5. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 17.0N 51.8W 60 KT 70 MPH 12H 02/1800Z 17.7N 53.1W 60 KT 70 MPH 24H 03/0600Z 18.9N 54.8W 60 KT 70 MPH 36H 03/1800Z 19.9N 56.2W 65 KT 75 MPH 48H 04/0600Z 21.0N 57.6W 70 KT 80 MPH 72H 05/0600Z 23.3N 60.6W 80 KT 90 MPH 96H 06/0600Z 25.5N 63.5W 90 KT 105 MPH 120H 07/0600Z 27.0N 66.5W 100 KT 115 MPH $$ FORECASTER BLAKE |
