[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 020848
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


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