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| #47607 (Received by flhurricane at: 11:14 PM 21.Sep.2005) TCDAT3 HURRICANE RITA DISCUSSION NUMBER 18 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 11 PM EDT WED SEP 21 2005 RITA HAS CONTINUED TO STRENGTHEN SINCE THE LAST ADVISORY. A DROPSONDE IN THE EYE OF RITA AROUND 21/2309Z RECORDED A PRESSURE OF 899 MB WITH A SURFACE WIND OF 32 KT. ANYTHING BELOW 10 KT IS USUALLY CONSIDERED A VALID PRESSURE. HOWEVER...THE GENERAL THUMB RULE IS TO DECREASE THE PRESSURE 1 MB FOR EVERY 10 KT ABOVE THAT WIND SPEED. IN THIS CASE... RITA'S CENTRAL PRESSURE IS ESTIMATED TO BE 897 MB...MAKING IT THE THIRD MOST INTENSE HURRICANE IN TERMS OF PRESSURE FOR THE ATLANTIC BASIN. THE LAST RECON PASS ONLY INDICATED 2 REPORTS OF 700 MB FLIGHT-LEVEL WINDS OF 157 KT IN THE NORTHEAST QUADRANT. HOWEVER... ODT VALUES OVER THE PAST 2 HOURS HAVE AVERAGED BETWEEN T7.3/149 KT AND T7.4/152 KT... SO THE INITIAL INTENSITY HAS BEEN INCREASED TO 150 KT. THE PRESSURE-WIND RELATIONSHIP FOR AN 897 MB PRESSURE IS 160 KT. THE INITIAL MOTION ESTIMATE IS 280/08. RITA HAS ACTUALLY MADE A LARGE WOBBLE AND SLOWED TO A 285/06 KT MOTION THE PAST 6 HOURS. HOWEVER...SUCH MOTION CHANGES...WHICH ARE NOT UNUSUAL FOR EXPLOSIVELY DEEPENING TROPICAL CYCLONES AS THE INNER CORE WIND FIELD AND CONVECTIVE PATTERN REORGANIZES...ARE CONSIDERED TEMPORARY AND RITA IS EXPECTED TO SHORTLY RESUME A MORE WESTWARD MOTION. THE 18Z NHC MODEL GUIDANCE IS A LITTLE MORE CONVERGENT THAN PREVIOUS MODEL RUNS... WITH THE GFS AND GFDL MODELS DOING THEIR USUAL AFTERNOON EASTWARD SHIFT. THOSE MODELS NOW BRING RITA ACROSS THE HOUSTON-GALVESTON AREA IN ABOUT 72 HOURS. HOWEVER...THE OTHER MODELS SEEM TO HAVE STABILIZED THEIR FORECAST TRACKS FARTHER WEST WITH THE CONSENSUS HAVING SHIFTED A LITTLE MORE TO THE RIGHT. THE OFFICIAL FORECAST TRACK WAS ALSO SHIFTED TO THE RIGHT...BUT NOT AS FAR AS THE GFS/GFDL MODELS...SINCE IT NOW APPEARS THAT THE GLOBAL MODELS HAVE A REASONABLE HANDLE ON WEAKENING THE MID-LEVEL RIDGE ACROSS TEXAS AND THE GULF COAST BASED 22/00Z UPPER-AIR DATA INDICATING 40 METER HEIGHT FALLS ACROSS THIS REGION DURING THE PAST 24 HOURS. THE INTENSITY FORECAST IS SOMEWHAT PROBLEMATIC. THE UPPER-LEVEL OUTFLOW PATTERN IS PERFECT WITH A POLEWARD OUTFLOW CHANNEL CONVERGING INTO AN UPPER-LOW NORTHEAST OF THE LESSER ANTILLES... AN EQUATORWARD OUTFLOW CHANNEL CONVERGING INTO AN UPPER-LOW OVER THE BAY OF CAMPECHE...AND A THIRD WEAK OUTFLOW CHANNEL DEVELOPING TO THE NORTHWEST. THIS EXTREMELY FAVORABLE PATTERN...COMBINED WITH 30-31C SSTS...HAS ALLOWED RITA'S EXPLOSIVE DEEPENING TO OCCUR. THE EYE WILL BE PASSING OVER THE WARM GULF LOOP CURRENT DURING THE NEXT 12 HOURS...SO SOME ADDITIONAL STRENGTHENING IS POSSIBLE... IF AN EYEWALL REPLACEMENT CYCLE DOES NOT INHIBIT THE INTENSIFICATION PROCESS. BY 36-48 HOURS...THE GLOBAL MODELS ARE IN GOOD AGREEMENT THAT THE CURRENT THREE OUTFLOW CHANNEL PATTERN WILL BE REPLACED BY MAINLY A LARGE POLEWARD OUTFLOW PATTERN. THIS SHOULD INDUCE SOME STEADY WEAKENING...BUT THAT TYPE OF OUTFLOW PATTERN...COUPLED WITH EXPECTED LOW VERTICAL SHEAR CONDITIONS...IS STILL SUFFICIENT TO SUPPORT A CATEGORY 4 HURRICANE UNTIL LANDFALL OCCURS. OF COURSE... INTERNAL DYNAMICS ALSO PLAY A ROLE IN WHAT THE INTENSITY OF A HURRICANE WILL BE... AND WE HAVE NO SKILL IN FORECASTING EYEWALL REPLACEMENT CYCLES BEYOND ABOUT 6-12 HOURS...AT BEST. THE INTENSITY FORECAST IS SIMILAR TO THE TREND OF THE SHIPS MODEL...ONLY SLIGHTLY HIGHER AFTER 24 HOURS DUE TO LOWER VERTICAL SHEAR INDICATED BY THE NOGAPS...CANADIAN...UKMET...AND ECMWF MODELS. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 22/0300Z 24.6N 87.2W 150 KT 12HR VT 22/1200Z 24.9N 88.7W 155 KT 24HR VT 23/0000Z 25.5N 90.5W 150 KT 36HR VT 23/1200Z 26.3N 92.4W 145 KT 48HR VT 24/0000Z 27.5N 94.2W 135 KT 72HR VT 25/0000Z 31.0N 96.4W 65 KT...INLAND 96HR VT 26/0000Z 33.5N 96.5W 30 KT...INLAND 120HR VT 27/0000Z 35.0N 96.5W 25 KT...DISSIAPTING INLAND |
