WDPN32 PGTW 052100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (ATSANI) WARNING NR 
030//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 23W (ATSANI), LOCATED APPROXIMATELY 277
NM SOUTH-SOUTHEAST OF TAIPEI, TAIWAN, HAS TRACKED WESTWARD AT 11
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) 
SATELLITE IMAGERY DEPICTS A PARTIALLY EXPOSED LOW LEVEL CIRCULATION 
CENTER (LLCC) WITH THE ASSOCIATED DEEP CONVECTION BEING SHEARED OVER 
THE WESTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH FAIR
CONFIDENCE AND IS BASED ON THE POSITION OF THE PARTIALLY EXPOSED 
LLCC AS WELL AS THE LOW LEVEL CONVECTIVE BANDING PRESENT IN A 
051700Z ATMS 183 GHZ IMAGE. THE INITIAL INTENSITY IS ASSESSED AT 45 
KTS WITH HIGH CONFIDENCE AND IS BASED ON A DVORAK CURRENT INTENSITY 
ESTIMATE OF T3.0 (45 KTS) FROM PGTW AND RJTD AND A 051659Z CIMSS 
SATELLITE CONSENSUS INTENSITY ESTIMATE OF 46 KTS. 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 24 AS IT TRANSITS ALONG THE SOUTHEASTERN PERIPHERY OF 
THE STR. DURING THIS TIME, THE SYSTEM SHOULD INTENSIFY TO 50 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 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 48 WHERE IT WILL CONTINUE TO WEAKEN 
TO 25 KTS BY TAU 72 AS IT BEGINS TO DISSIPATE OVER WATER. 
NUMERICAL MODEL GUIDANCE IS IN TIGHT AGREEMENT THROUGH TAU 36, 
HOWEVER TRACK AND INTENSITY GUIDANCE DIVERGES SIGNIFICANTLY 
THEREAFTER. SPECIFICALLY, DETERMINISTIC GFS, UKMET AND ECMWF MODEL 
TRACKS BRING THE VORTEX GENERALLY NORTHWESTWARD AFTER TAU 24 AND 
DISSIPATE THE SYSTEM BY TAUS 36-60. THE JGSM MODEL PROVIDES A 
SLIGHTLY DIFFERENT TRACK THAT CONTINUES WESTWARD THROUGH TAU 48 AT 
WHICH TIME THE VORTEX DISSIPATES. THE MAJORITY OF ENSEMBLE MEMBERS 
FROM ECMWF, GFS AND UKMET, AS WELL AS THE TRACK FROM NAVGEM ALL 
TRACK THE SYSTEM SOUTHWESTWARD TOWARDS THE COAST OF CENTRAL VIETNAM 
THROUGH TAU 72. THIS HIGH DEGREE OF SPREAD IN BOTH ALONG AND ACROSS 
TRACK MODEL SOLUTIONS PROVIDES OVERALL POOR CONFIDENCE IN THE 
FORECAST TRACK WHICH IS PLACED NEAR THE JTWC MULTI-MODEL CONSENSUS. 
   C. IN THE EXTENDED FORECAST, TS 23W WILL CONTINUE TO TRACK 
SOUTHWESTWARD AND MAKE LANDFALL ALONG THE COAST OF CENTRAL VIETNAM 
BYTAU 96. AT THIS TIME, THE SYSTEM WILL COMPLETE DISSIPATION OVER 
LAND DUE TO INTERACTION WITH THE UNDERLYING TERRAIN. THERE REMAINS A 
LARGE SPREAD IN MODEL SOLUTIONS BOTH ALONG AND ACROSS TRACK. THIS 
SPREAD LENDS OVERALL POOR CONFIDENCE IN THE OVERALL JTWC FORECAST 
TRACK, WHICH IS PLACED NEAR THE MULTI-MODEL CONSENSUS.//
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