Hurricane KATIA

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HURRICANE KATIA DISCUSSION NUMBER  37
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
500 AM AST WED SEP 07 2011
 
THE CLOUD PATTERN OF KATIA HAS BECOME A LITTLE LESS ORGANIZED
OVERNIGHT. CLOUD TOP TEMPERATURES HAVE GENERALLY BEEN WARMING AND
DRY AIR APPEARS TO WRAPPING AROUND THE WESTERN SEMICIRCLE OF THE
CIRCULATION...THE LATTER GIVING THE CYCLONE A SOMEWHAT HOLLOW
APPEARANCE. IN ADDITION...THERE APPEARS TO BE NORTHWESTERLY SHEAR
OVER THE HURRICANE...AS EVIDENCED BY CIRRUS CLOUD MOTIONS AND
CONFIRMED BY UW-CIMSS ANALYSES. DVORAK T- AND CI-NUMBERS FROM TAFB
AND SAB ARE LOWER...AND A REASONABLE BLEND OF THESE DATA GIVES AN
INTENSITY ESTIMATE OF 80 KT. AN EYEWALL REPLACEMENT CYCLE THAT
BEGAN 24-36 HOURS APPEARS TO HAVE BEEN INTERRUPTED...WITH EARLIER
MICROWAVE IMAGERY SHOWING AN OUTER EYEWALL AT LARGE RADIUS THAT WAS
NOT CONTRACTING MUCH. THIS STRUCTURE...ALONG WITH THE MODERATE
SHEAR...SUGGESTS NOT MUCH CHANGE IN STRENGTH...FOLLOWED BY ONLY A
SLOW WEAKENING DURING THE NEXT FEW DAYS AS KATIA REMAINS OVER
RELATIVELY WARM WATERS. AFTER 72 HOURS...KATIA WILL QUICKLY MOVE
ALONG A STRONG GRADIENT OF SSTS AND ENCOUNTER AN INCREASINGLY
BAROCLINIC ENVIRONMENT...BOTH OF WHICH SHOULD RESULT IN A RAPID
TRANSITION TO AN INTENSE EXTRATROPICAL CYCLONE. POST-TROPICAL
CYCLONE KATIA IS EXPECTED TO BE A SIGNIFICANT WEATHER SYSTEM OVER
THE FAR NORTH ATLANTIC FOR AT LEAST A COUPLE OF DAYS. THE OFFICIAL
NHC WIND SPEED FORECAST IS LOWERED IN ACCORDANCE WITH THE LATEST
INTENSITY GUIDANCE AND IS IN BEST AGREEMENT WITH THE MODEL
CONSENSUS.
 
THE INITIAL MOTION IS A FAIRLY SLOW 315/08. GLOBAL MODELS SHOW KATIA
GRADUALLY TURNING FROM NORTHWESTWARD TO NORTHWARD INTO A WEAKNESS
IN THE SUBTROPICAL RIDGE OVER THE WESTERN ATLANTIC DURING THE NEXT
24-36 HOURS. AFTER THAT...THE HURRICANE IS EXPECTED TO GRADUALLY
RECURVE INTO THE WESTERLIES AND ACCELERATE NORTHEASTWARD ON THE
SOUTH SIDE OF A BROAD TROUGH COVERING THE FAR NORTH ATLANTIC. THE
TRACK GUIDANCE HAS CONSISTENTLY BEEN HANDLING THE STEERING FLOW
AROUND THE STORM WELL FOR SEVERAL FORECAST CYCLES AND IS TIGHTLY
CLUSTERED...GIVING HIGH CONFIDENCE TO THE FORECAST. ONLY MINOR
CHANGES WERE MADE TO THE PREVIOUS FORECAST TRACK...WITH THE CURRENT
NHC TRACK AGAIN CLOSE TO THE MULTI-MODEL CONSENSUS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  07/0900Z 28.7N  67.9W   80 KT  90 MPH
 12H  07/1800Z 29.8N  69.0W   80 KT  90 MPH
 24H  08/0600Z 31.7N  69.6W   75 KT  85 MPH
 36H  08/1800Z 34.0N  69.5W   75 KT  85 MPH
 48H  09/0600Z 36.4N  67.9W   70 KT  80 MPH
 72H  10/0600Z 40.9N  58.2W   65 KT  75 MPH
 96H  11/0600Z 47.0N  38.0W   65 KT  75 MPH...POST-TROP/EXTRATROP
120H  12/0600Z 59.0N  17.5W   65 KT  75 MPH...POST-TROP/EXTRATROP
 
$$
FORECASTER KIMBERLAIN/PASCH
 
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