WDPN32 PGTW 051500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (ATSANI) WARNING NR 
029//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 23W (ATSANI), LOCATED APPROXIMATELY 406 NM 
SOUTH-SOUTHWEST OF KADENA AB, OKINAWA, HAS TRACKED WEST-
NORTHWESTWARD AT 13 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED 
ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY-EXPOSED 
LOW LEVEL CIRCULATION CENTER (LLCC) WITH THE ASSOCIATED DEEP 
CONVECTION BEING SHEARED OVER THE WESTERN SEMICIRCLE. A TIMELY 
051142Z ASCAT-A BULLSEYE IMAGE REVEALS A DEFINED LLCC WITH A SWATH 
OF 45-50 KT WINDS IN THE NORTHERN QUADRANT. BASED ON THE FULLY-
EXPOSED NATURE AND AFOREMENTIONED ASCAT IMAGE, THERE IS GOOD 
CONFIDENCE IN THE INITIAL POSITION. THE INITIAL INTENSITY IS 
ASSESSED AT 50 KTS BASED ON THE ASCAT DATA, A 051009Z SATCON 
ESTIMATE OF 51 KTS, AND DVORAK CURRENT INTENSITY ESTIMATES OF T3.0 
(45 KTS) BY PGTW AND RJTD. THE 34- AND 50-KT WIND RADII WERE 
ASSESSED USING RECENT DATA. ENVIRONMENTAL ANALYSIS REVEALS AN 
OVERALL CONDUCIVE ENVIRONMENT FOR DEVELOPMENT CHARACTERIZED BY LOW 
(5-10 KT) VERTICAL WIND SHEAR (VWS), ESTABLISHED POLEWARD AND 
EQUATORWARD OUTFLOW, AND WARM (28-29 CELSIUS) SEA SURFACE 
TEMPERATURES (SST). TS 23W IS TRACKING GENERALLY WEST-NORTHWESTWARD 
ALONG THE SOUTHEASTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR).
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 36 AS IT TRANSITS ALONG THE SOUTHEASTERN PERIPHERY OF 
THE STR. DURING THIS TIME, THE SYSTEM SHOULD INTENSIFY TO 65 KTS BY 
TAU 24 DUE TO PERSISTENT FAVORABLE ENVIRONMENTAL CONDITIONS. 
AFTERWARDS, INCREASING VWS WILL BEGIN TO IMPACT THE SYSTEM CAUSING 
IT TO BEGIN TO GRADUALLY WEAKEN. AFTER TAU 36, 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 48 WHERE IT WILL CONTINUE TO WEAKEN 
TO 25 KTS BY TAU 72. NUMERICAL MODEL GUIDANCE IS IN TIGHT AGREEMENT 
THROUGH TAU 36 WITH THE EXCEPTION OF THREE NOTABLE OUTLIERS (NAVGEM, 
GFS, AND GALWEM). NAVGEM AND GALWEM CONTINUE TO DIVERGE TO THE NORTH 
AND SOUTH RESPECTIVELY. HOWEVER, THE LATEST MODEL RUNS DEPICT THE 
GFS SOLUTION TRACKING NORTHWARD AFTER TAU 12 THEN TURNING WESTWARD 
AFTER TAU 36. WITH CONSIDERATION TO THESE OUTLIERS, THE JTWC 
FORECAST TRACK IS POSITIONED NEAR THE MULTI-MODEL CONSENSUS THROUGH 
TAU 36 WITH FAIR CONFIDENCE. AFTERWARDS, VARIATIONS IN ALONG TRACK 
SPEEDS REDUCE THE CONFIDENCE THROUGH TAU 72.
   C. IN THE EXTENDED FORECAST, TS 23W SHOULD CONTINUE TO TRACK 
SOUTHWESTWARD TOWARDS SOUTHERN VIETNAM. AS IT WEAKENS, THE STEERING 
WILL BE INFLUENCED BY LOW- TO MID-LEVEL FLOW FROM THE NORTHEAST. 
CONTINUED HIGH VWS AND COOLING SST WILL STEADILY WEAKEN THE SYSTEM 
PRIOR TO LANDFALL OVER THE SOUTH CENTRAL COAST OF VIETNAM. COMBINED 
WITH THE UNFAVORABLE ENVIRONMENT, TERRAIN INTERACTION WILL CAUSE THE 
SYSTEM TO FULLY DISSIPATE OVER LAND BY TAU 96. IN ADDITION TO THE 
AFOREMENTIONED OUTLIERS, INCREASING ALONG-TRACK SPREAD PLACES FAIR 
AGREEMENT IN THE EXTENDED RANGE NUMERICAL MODEL GUIDANCE AND 
THEREFORE ONLY FAIR CONFIDENCE IN THE JTWC FORECAST TRACK WHICH IS 
PLACED NEAR THE MULTI-MODEL CONSENSUS.//
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