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| #456612 (Received by flhurricane at: 4:54 AM 05.Sep.2011) TCDAT2 HURRICANE KATIA DISCUSSION NUMBER 29 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122011 500 AM AST MON SEP 05 2011 THE INNER CORE CONVECTIVE STRUCTURE OF KATIA APPEARS TO HAVE GONE THROUGH ANOTHER ROUND OF CHANGES...WITH A NET DECREASE IN ORGANIZATION NOTED DURING THE PAST SEVERAL HOURS. AN EYE THAT WAS EVIDENT EARLIER IS NO LONGER APPARENT...AND THERE ARE SIGNS THAT DRY AIR HAS BEEN ROTATING AROUND THE CIRCULATION. UPPER-LEVEL OUTFLOW IS GOOD TO EXCELLENT...WITH A FAVORABLE OUTFLOW SINK TO THE NORTHEAST. A BLEND OF DVORAK T AND CI NUMBERS FROM TAFB AND SAB IS LOWER THAN 6 HOURS AGO...SO THE CURRENT INTENSITY ESTIMATE IS LOWERED TO 85 KT. THE CENTER OF KATIA HAS BEEN WOBBLING. SMOOTHING OF RECENT FIXES YIELDS AN INITIAL MOTION ESTIMATE OF 305/10. LARGE-SCALE MODELS SHOW KATIA ROUNDING THE SOUTHWEST PERIPHERY OF A MID-LEVEL RIDGE EAST OF BERMUDA...HEADING TOWARD A WEAKNESS OVER THE WESTERN ATLANTIC. THIS SYNOPTIC PATTERN SHOULD STEER THE HURRICANE ON A GENERAL NORTHWESTWARD COURSE WITH SOME DECELERATION OVER THE NEXT FEW DAYS. LATER IN THE FORECAST PERIOD...FURTHER WESTWARD PROGRESS IS LIKELY TO BE BLOCKED AS KATIA IS CAUGHT UP IN DEEP-LAYER SOUTHERLY FLOW IN BETWEEN A LARGE CUT-OFF LOW OVER THE EASTERN UNITED STATES AND THE RIDGE TO THE EAST. ONCE KATIA GAINS ENOUGH LATITUDE...GLOBAL MODELS SHOW THE HURRICANE BECOMING EMBEDDED IN STRONG MID-LATITUDE WESTERLY FLOW AND ACCELERATING NORTHEASTWARD. THE SPREAD IN THE TRACK GUIDANCE HAS DECREASED SUBSTANTIALLY SINCE YESTERDAY AND NOW FAVORS A RECURVATURE SCENARIO ROUGHLY ALONG 70W...WITH EVEN THE LEFT-LEANING UKMET/ECMWF MODELS NOW SHIFTING SIGNIFICANTLY TO THE EAST. THE MAIN FORECAST CHALLENGE IS HOW QUICKLY KATIA WILL TURN NORTHWARD AND THEN NORTHEASTWARD...WITH MAJOR TIMING DIFFERENCES NOTED AMONGST THE VARIOUS MODELS. THE OFFICIAL TRACK IS NEARLY IDENTICAL TO THE PREVIOUS ONE THROUGH 72 HOURS BUT IS ADJUSTED TO THE RIGHT AND IS MUCH FASTER TOWARD THE END OF THE PERIOD BUT NOT AS FAST AS THE MULTI-MODEL CONSENSUS. ACCORDING TO THE GLOBAL MODELS...KATIA WILL BE MOVING THROUGH AN ENVIRONMENT CHARACTERIZED BY ANTICYCLONIC UPPER-LEVEL FLOW AND WEAK VERTICAL SHEAR THROUGH AT LEAST 72 HOURS. THE PRIMARY NEGATIVE FACTORS THAT COULD INHIBIT THE INTENSIFICATION PROCESS DURING THIS TIME ARE LOW MID-LEVEL RELATIVE HUMIDITY VALUES...A LOWERING OF THE OCEANIC HEAT CONTENT ALONG THE FORECAST TRACK...AND THE POSSIBILITY OF A MORE STABLE ENVIRONMENT DUE TO WARMING ALOFT. AFTER 96 HOURS...INCREASING SHEAR AND THE PROXIMITY OF KATIA TO A STRONG GRADIENT OF SEA SURFACE TEMPERATURES WILL LIKELY CONTRIBUTE TO WEAKENING. THE OFFICIAL INTENSITY FORECAST HAS BEEN LOWERED EARLY IN THE FORECAST PERIOD BASED ON CURRENT TRENDS AND ALSO AT THE END OF THE PERIOD AS A RESULT OF ADJUSTMENTS TO THE TRACK FORECAST. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 23.9N 62.0W 85 KT 100 MPH 12H 05/1800Z 25.0N 63.2W 90 KT 105 MPH 24H 06/0600Z 26.3N 64.8W 95 KT 110 MPH 36H 06/1800Z 27.5N 66.2W 100 KT 115 MPH 48H 07/0600Z 28.6N 67.6W 105 KT 120 MPH 72H 08/0600Z 31.4N 70.1W 100 KT 115 MPH 96H 09/0600Z 35.5N 69.5W 90 KT 105 MPH 120H 10/0600Z 38.5N 64.0W 75 KT 85 MPH $$ FORECASTER KIMBERLAIN |
