WDPN32 PGTW 062100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (ATSANI) WARNING NR 
034//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 23W (ATSANI), LOCATED APPROXIMATELY 192 NM 
SOUTH-SOUTHWEST OF TAIPEI, TAIWAN, HAS TRACKED NORTHWESTWARD AT 06 
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) 
SATELLITE IMAGERY DEPICTS A COMPACT (55-60NM CORE), CONSOLIDATING 
SYSTEM WITH TIGHTLY-CURVED BANDING WRAPPING INTO A DEFINED LOW-LEVEL 
CIRCULATION CENTER (LLCC). THERE ARE NO MICROWAVE IMAGES AVAILABLE, 
HOWEVER, RADAR IMAGERY REVEALS A WELL-ORGANIZED SYSTEM WITH AN EYE-
LIKE STRUCTURE AND SPIRAL BANDING OVER THE EASTERN SEMICIRCLE. EIR 
IMAGERY HAS YET TO SHOW THE PRESENCE OF AN EYE AND INDICATES A 
SLIGHT DEGRADATION OF STRUCTURE SINCE 062000Z. OVERALL, CONFIDENCE 
IN THE INITIAL POSITION IS GOOD. A RECENT (060947Z) SMAP IMAGE 
SHOWED MAXIMUM 10-MINUTE SUSTAINED WINDS OF 51-55 KNOTS AND THIS 
DATA ALONG WITH A PGTW DVORAK PATTERN-T ESTIMATE OF 3.5 (55 KNOTS) 
SUPPORT THE INITIAL INTENSITY ASSESSMENT OF 55 KNOTS. THIS ESTIMATE 
IS CONSISTENT WITH THE CONVECTIVE STRUCTURE ALTHOUGH OBJECTIVE AND 
SUBJECTIVE DVORAK DATA-T ESTIMATES ARE GENERALLY LOWER AT 2.5 TO 3.0 
(35-45 KNOTS) LIKELY DUE TO THE SMALL SIZE OF THE SYSTEM. LOW TO 
MODERATE (15-20 KT) VERTICAL WIND SHEAR (VWS), WARM (26-27 CELSIUS) 
SEA SURFACE TEMPERATURES (SST), AND ENHANCED POLEWARD OUTFLOW ARE 
CONTRIBUTING TO AN OVERALL MARGINALLY-FAVORABLE ENVIRONMENT AS TS 
23W TRACKS SLOWLY ALONG THE SOUTHERN PERIPHERY OF A WEAK SUBTROPICAL 
RIDGE (STR) POSITIONED TO THE NORTH.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TS 23W IS EXPECTED TO CONTINUE TRACKING SLOWLY WEST-
NORTHWESTWARD TO WESTWARD THROUGH TAU 12 UNDER A WEAKENED MID-LEVEL 
STR. AFTER TAU 12, LOW-LEVEL RIDGING ASSOCIATED WITH A NORTHEAST 
SURGE EVENT IS FORECAST TO STRENGTHEN OVER THE EAST CHINA SEA, WHICH 
WILL LEAD TO A SHARP SOUTHWESTWARD TURN AND ACCELERATED TRACK 
THROUGH TAU 72. THE SYSTEM IS FORECAST TO WEAKEN THROUGH THE 
FORECAST PERIOD AS VWS INCREASES TO 30-40 KNOTS WITH COOLER SST 
VALUES (24-26C). TS 23W SHOULD WEAKEN RAPIDLY AFTER TAU 36 WITH 
DISSIPATION EXPECTED BY TAU 72 BUT PERHAPS SOONER AS THE ENVIRONMENT 
DEGRADES. DESPITE THE COMPLEX, EVOLVING STEERING PATTERN, NUMERICAL 
MODEL GUIDANCE IS IN FAIR AGREEMENT WITH MAJOR DIFFERENCES IN THE 
TIMING OF DISSIPATION. THE BULK OF THE GUIDANCE SUPPORTS THE 
SOUTHWESTWARD TRACK ASSOCIATED WITH THE NORTHEAST SURGE EVENT. 
OVERALL, THERE IS FAIR CONFIDENCE IN THE JTWC FORECAST TRACK WHICH 
IS POSITIONED NEAR THE MULTI-MODEL CONSENSUS THROUGH TAU 36.//
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