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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