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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