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| #455333 (Received by flhurricane at: 4:40 AM 01.Sep.2011) TCDAT2 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 |
