Tropical Storm IRWIN
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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
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