WDPN32 PGTW 060300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (ATSANI) WARNING 
NR 031//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 23W (ATSANI), LOCATED APPROXIMATELY 265 NM 
SOUTH OF TAIPEI, TAIWAN, HAS TRACKED WESTWARD AT 11 KNOTS OVER THE 
PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) 
DEPICTS BUILDING CONVECTION THAT IS NOW OBSCURING A LOW LEVEL 
CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS PLACED WITH 
GOOD CONFIDENCE BASED ON LOW LEVEL SPIRAL BANDS THAT WRAP INTO THE 
LLCC IN THE MSI LOOP AS WELL AS A MICROWAVE EYE FEATURE PRESENT IN 
A 052308Z SSMIS 37 GHZ IMAGE. THE INITIAL INTENSITY IS ASSESSED AT 
45 KTS WITH FAIR CONFIDENCE BASED ON A DVORAK CURRENT INTENSITY 
ESTIMATE OF T3.0 (45 KTS) FROM PGTW AND RJTD AS WELL AS A 052241Z 
CIMSS SATELLITE CONSENSUS INTENSITY ESTIMATE OF 45 KTS. TS ATSANI 
IS TRACKING ALONG THE SOUTHERN PERIPHERY OF A SUBTROPICAL RIDGE 
(STR) POSITIONED TO THE NORTHEAST AND THROUGH A FAVORABLE 
ENVIRONMENT WITH WARM (27-28 CELSIUS) SEA SURFACE TEMPERATURES 
(SST), ESTABLISHED POLEWARD AND EQUATORWARD OUTFLOW ALOFT AND LOW 
(10-15 KTS) VERTICAL WIND SHEAR (VWS).
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TS 23W IS EXPECTED TO TRACK GENERALLY WEST-NORTHWESTWARD 
THROUGH TAU 24 AS IT TRANSITS ALONG THE SOUTHWESTERN PERIPHERY OF 
THE STR. DURING THIS TIME, THE SYSTEM SHOULD INTENSIFY TO 55 KTS BY 
TAU 12 DUE TO PERSISTENT FAVORABLE ENVIRONMENTAL CONDITIONS. 
AFTERWARDS, INCREASING VWS WILL BEGIN TO IMPACT THE SYSTEM CAUSING 
IT TO BEGIN TO GRADUALLY WEAKEN. AFTER TAU 24, THE SYSTEM IS 
EXPECTED TO TURN TO A WEST-SOUTHWESTWARD TRACK AS A STR POSITIONED 
OVER CENTRAL CHINA TAKES OVER AS THE PRIMARY STEERING MECHANISM. 
THIS STEERING MECHANISM SHIFT WILL CAUSE THE SYSTEM TO THEN SHIFT TO 
A SOUTHWESTWARD TRACK AFTER TAU 36 WHEREUPON IT WILL BEGIN TO WEAKEN 
TO 30 KTS BY TAU 48 AS IT BEGINS TO DISSIPATE OVER WATER. THE SYSTEM
WILL THEN FURTHER WEAKEN DUE TO CONTINUED HIGH (30+ KTS) VWS AND
FULLY DISSIPATE OVER WATER BY TAU 72. NUMERICAL MODEL GUIDANCE IS IN 
TIGHT AGREEMENT THROUGH TAU 36, HOWEVER TRACK AND INTENSITY GUIDANCE 
DIVERGES SIGNIFICANTLY THEREAFTER. THIS UNCERTAINTY IN MODEL GUIDANCE
IS LIKELY DUE TO THE TIMING AND MAGNITUDE OF THE RIDGE REORIENTATION
AND YIELDS OVERALL POOR CONFIDENCE IN THE JTWC FORECAST TRACK WHICH 
IS PLACED NEAR THE JTWC MULTI-MODEL CONSENSUS.//
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