ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM TROPICAL STORM IRWIN DISCUSSION NUMBER 9 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 2 PM PDT SAT AUG 27 2005 MOST OF THE DEEP CONVECTION HAS EITHER WEAKENED OR BEEN SHEARED OFF WELL TO THE WEST OF THE LOW-LEVEL CIRCULATION CENTER...AND IRWIN COULD VERY WELL BE A DEPRESSION. INSTEAD...I HAVE OPTED TO MAINTAIN THE INTENSITY AT 35 KT BASED ON CONSENSUS DVORAK SATELLITE CURRENT INTENSITY ESTIMATES OF 35 KT FROM TAFB...SAB...AND AFWA...AND SOME 35-40 KT WIND VECTORS NOTED IN A 27/1332Z QUIKSCAT OVERPASS. IN ADDITION... A SMALL BAND OF MODERATE CONVECTION WAS NOTED RE-DEVELOPING IN THE NORTHWEST QUADRANT...WHICH COULD BE A PRECURSOR TO ANOTHER BURST OF DEEP CONVECTION FORMING. HOWEVER... IF CONVECTION DOES NOT REDEVELOP...THEN IRWIN WILL LIKELY HAVE TO BE DOWNGRADED TO A DEPRESSION ON THE NEXT ADVISORY. THE INITIAL MOTION IS 270/8. THERE IS NO SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING. IRWIN IS EXPECTED TO BE STEERED GENERALLY WESTWARD BY DEEP-LAYER EASTERLY FLOW PRESENT ON THE SOUTH SIDE OF A LARGE MID-LEVEL SUBTROPICAL RIDGE. THE 12Z GFS AND GFS-ENSEMBLE MODELS TAKE IRWIN WEST-SOUTHWESTWARD AFTER 36 HOURS...WHEREAS THE REST OF THE MODELS MOVE THE CYCLONE WESTWARD AND THEN DISSIPATE IT AFTER 48 HOURS. GIVEN THAT IRWIN WILL BE OVER 26-27C SSTS THROUGHOUT THE FORECAST PERIOD... I HAVE KEPT THE SYSTEM A TROPICAL CYCLONE THROUGH 120 HOURS. THIS IS CONSISTENT WITH THE SHIPS INTENSITY MODEL... WHICH KEEPS IRWIN AS A 25-KT DEPRESSION AFTER 72 HOURS UNDERNEATH GRADUALLY DECREASING VERTICAL SHEAR. HOWEVER...IT IS ENTIRELY POSSIBLE THAT IRWIN MAY DEGENERATE INTO A NON-CONVECTIVE REMNANT LOW AFTER 48 HOURS WHEN THE CYCLONE WILL BE MOVING THROUGH SOME FAIRLY DRY MID-LEVEL AIR. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 27/2100Z 17.4N 113.4W 35 KT 12HR VT 28/0600Z 17.4N 114.6W 35 KT 24HR VT 28/1800Z 17.4N 116.1W 35 KT 36HR VT 29/0600Z 17.4N 117.4W 30 KT 48HR VT 29/1800Z 17.3N 118.8W 30 KT 72HR VT 30/1800Z 17.2N 121.2W 25 KT 96HR VT 31/1800Z 17.0N 124.5W 25 KT 120HR VT 01/1800Z 17.0N 128.0W 25 KT $$ NNNN
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Page last modified: Saturday, 27-Aug-2005 20:55:13 UTC