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Hurricane KATIA (Text)

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HURRICANE KATIA DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
500 AM AST THU SEP 01 2011

AN AMSR-E OVERPASS AT 0451 UTC RATHER UNEXPECTEDLY SHOWED A PARTLY
EXPOSED LOW-LEVEL CENTER WITH ONLY A FRAGMENT OF AN EYEWALL TO THE
NORTH AND NORTHEAST.  ANALYSES FROM CIMSS AT THE UNIVERSITY OF
WISCONSIN SHOW ABOUT 10 KT OF SOUTHERLY VERTICAL WIND SHEAR...BUT
THAT BY ITSELF DOES NOT ADEQUATELY EXPLAIN THE OBSERVED STRUCTURE.
IT MAY BE THAT DRY AIR PREVIOUSLY SEEN NEAR THE STORM HAS WORKED ITS
WAY INTO THE CORE AS HINTED AT IN AMSR-E TOTAL PRECIPITABLE WATER
DATA.  SATELLITE INTENSITY ESTIMATES ARE 77 KT FROM TAFB AND 65 KT
FROM SAB...AND THE INITIAL INTENSITY REMAINS 65 KT FOR THIS
ADVISORY.

THE INITIAL MOTION IS SLIGHTLY TO THE LEFT OF THE PREVIOUS
ADVISORY...280/17.  KATIA IS ON THE SOUTH SIDE OF A DEEP-LAYER
RIDGE OVER THE CENTRAL ATLANTIC.  WATER VAPOR IMAGERY SHOWS A
MID/UPPER-LEVEL LOW NEAR 29N55W WITH A TROUGH EXTENDING TO 15N60W...
AND A SECOND MID/UPPER-LEVEL LOW NEAR 34N71W.  THESE FEATURES HAVE
CREATED A SIZABLE WEAKNESS IN THE SUBTROPICAL RIDGE OVER THE
WESTERN ATLANTIC...AND THE DYNAMICAL MODELS FORECAST KATIA TO MOVE
GENERALLY WEST-NORTHWESTWARD INTO THE WEAKNESS.  THERE IS SOME
SPREAD IN THE GUIDANCE...ESPECIALLY BY 120 HOURS.  THE GFDL...
UKMET...CANADIAN...AND NOGAPS MODELS ARE ON THE LEFT SIDE OF THE
GUIDANCE ENVELOPE...WHILE THE GFS AND ECMWF ARE ON THE RIGHT SIDE. 
THE NEW FORECAST TRACK IS ALONG THE LEFT SIDE OF THE GUIDANCE
ENVELOPE FOR THE FIRST 24 HOURS BASED ON THE CURRENT MOTION...THEN
THEREAFTER LIES IN THE CENTER OF THE ENVELOPE NEAR THE VARIOUS
CONSENSUS MODELS.  THE NEW TRACK IS A LITTLE TO THE WEST OF THE
PREVIOUS TRACK.
 
IT IS UNCLEAR WHETHER THE CURRENT CONVECTIVE TREND IS TEMPORARY OR A
SIGN OF ARRESTED DEVELOPMENT.  THE EARLY PART OF THE INTENSITY
FORECAST IS BASED ON THE FORMER POSSIBILITY AND SHOWS A SLOW
STRENGTHENING FOR THE FIRST 24-36 HR.  AFTER THAT TIME...THERE
REMAINS SOME UNCERTAINTY REGARDING THE UPPER-LEVEL WINDS NEAR
KATIA.  THE GFS...NOGAPS...AND CANADIAN MODELS SHOW A VERY
FAVORABLE PATTERN...WHILE THE UKMET AND ECMWF FORECAST SOME
SOUTHWESTERLY SHEAR.  DUE TO THE UNCERTAINTY...THE LATER PART OF
THE INTENSITY FORECAST IS THE SAME AS IN THE PREVIOUS ADVISORY. 
HOWEVER...THE GFDL AND GFDN MODELS BOTH FORECAST KATIA TO INTENSIFY
TO STRONGER THAN 120 KT...WHICH IS A POSSIBILITY IF THE MORE
FAVORABLE UPPER-LEVEL WINDS OCCUR.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  01/0900Z 15.2N  45.9W   65 KT  75 MPH
 12H  01/1800Z 15.7N  48.1W   70 KT  80 MPH
 24H  02/0600Z 16.5N  50.5W   80 KT  90 MPH
 36H  02/1800Z 17.6N  52.4W   90 KT 105 MPH
 48H  03/0600Z 18.6N  54.1W   95 KT 110 MPH
 72H  04/0600Z 20.5N  57.0W  100 KT 115 MPH
 96H  05/0600Z 22.0N  60.0W  105 KT 120 MPH
120H  06/0600Z 23.5N  63.0W  105 KT 120 MPH
 
$$
FORECASTER BEVEN
 
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