WDPN32 PGTW 031500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (ATSANI) 
WARNING NR 021//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 23W (ATSANI), LOCATED APPROXIMATELY 404 NM SOUTH 
OF KADENA AB, HAS TRACKED EASTWARD AT 05 KNOTS OVER THE PAST SIX 
HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY (EIR) SHOWS A 
QUASI-STATIONARY SYSTEM WHICH IS STEADILY BECOMING BETTER ORGANIZED 
WITH FORMATIVE DEEP CONVECTIVE BANDS AND CYCLING CONVECTIVE ACTIVITY 
NEAR THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL 
POSITION IS PLACED WITH FAIR CONFIDENCE BASED SPIRAL BAND ANALYSIS 
OF THE LOW-LEVEL BANDING FEATURES WHICH ARE EVIDENT BETWEEN THE 
FLARE UPS OF CONVECTION, SUPPORTED BY EXTRAPOLATION TO A 030921Z 
SSMIS 91GHZ MICROWAVE IMAGE. THE INITIAL INTENSITY IS ASSESSED AT 35 
KNOTS WITH FAIR CONFIDENCE AND IS HEDGED SLIGHTLY BELOW THE PGTW 
DVORAK CURRENT INTENSITY ESTIMATE OF T3.0 (45 KNOTS) IN LIGHT OF AN 
ADVANCED DVORAK TECHNIQUE (ADT) ESTIMATE OF T2.5 (35 KNOTS) AND A 
SATELLITE CONSENSUS ESTIMATE OF 38 KNOTS. THE OVERALL ENVIRONMENT 
REMAINS FAVORABLE FOR DEVELOPMENT WITH WARM (27 TO 28 DEG CELSIUS) 
SSTS, LOW (5-10 KNOTS) VWS, AND MODERATE OUTFLOW. HOWEVER, THE UPPER-
LEVEL ANTI-CYCLONE WHICH PREVIOUSLY WAS DIRECTLY OVER THE SYSTEM, 
HAS MOVED OFF TO THE NORTHEAST, RESULTING IN SOME UPPER-LEVEL 
CONVERGENCE ON THE SOUTHEASTERN SIDE OF THE ANTI-CYCLONE WHICH IS 
EVIDENT AS A SHARP EASTERN EDGE OF THE NEAR-CORE CONVECTION, 
INHIBITING DEVELOPMENT FOR THE TIME BEING. THE SYSTEM REMAINS 
ENTRENCHED IN A COL REGION WITH NO DOMINATE STEERING FLOW, LEADING 
TO MEANDERING BUT GENERALLY EASTWARD TRACK.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TS 23W WILL CONTINUE TO MEANDER SLOWLY EAST TO SOUTHEAST OVER 
THE NEXT 12 HOURS UNDER A WEAK STEERING ENVIRONMENT. OVER THE NEXT 
24 HOURS; HOWEVER, THE UPPER-LEVEL PATTERN WILL BEGIN TO SHIFT, AS 
THE STR TO THE NORTHEAST BUILDS AND EXTENDS WESTWARD, SLOWLY PUSHING 
TS 23W ONTO A MORE WESTWARD TRACK BY TAU 24. BY TAU 36, THE STR 
BUILDS EVEN FURTHER WEST, AND WILL SERVE TO ACCELERATE TS 23W INTO 
THE LUZON STRAIT BY TAU 72. THE NEAR-TERM CONVERGENCE OVER THE 
SYSTEM WILL CONTINUE THROUGH THE NEXT 6 TO 12 HOURS, LEADING TO A 
PERIOD OF STEADY INTENSITY THROUGH TAU 12. SLOW BUT STEADY WEAKENING 
IS FORECAST THEREAFTER AS THE SYSTEM STARTS TO MOVE INTO AN AREA 
WITH MORE ROBUST WESTWARD OUTFLOW, WARM SSTS AND LOW TO MODERATE 
VWS, REACHING A PEAK OF 55 KNOTS BY TAU 72. NUMERICAL MODELS ARE IN 
OVERALL GOOD AGREEMENT WITH A SPREAD OF 115NM AT TAU 72 DISCOUNTING 
THE NVGM SOLUTION, WHICH CONTINUES TO TAKE THE SYSTEM ON AN 
UNREALISTIC TRACK NORTH OVER TAIWAN AND INTO THE EAST CHINA SEA. THE 
JTWC FORECAST LIES JUST NORTH OF THE MULTI-MODEL CONSENSUS SOLUTION 
THROUGH TAU 72 WITH FAIR CONFIDENCE DUE TO THE NEAR-TERM ERRATIC 
MOTION AND RESULTANT UNCERTAINTY IN THE TIMING OF THE TURN TO THE 
WEST.  
   C. IN THE EXTENDED TAUS, AFTER TAU 72 THE SYSTEM BEGINS TO TRACK 
SOUTHWESTWARD AS THE STEERING RIDGE TO THE NORTH SHIFTS TO A MORE 
SOUTHWEST-NORTHEAST ORIENTATION. AT THE SAME TIME, THE SYSTEM BEGINS 
TO FEEL THE IMPACT OF INCREASING VWS, DECREASING SST AND CONVERGENCE 
FLOW ALOFT, LEADING TO A FAIRLY RAPID WEAKENING TREND. AS THE SYSTEM 
WEAKENS TO 35 KNOTS, IT STARTS TO SHIFT TO AN EVEN MORE SOUTHWARD 
TRACK UNDER THE INFLUENCE OF THE LOW-LEVEL NORTHEASTERLY FLOW IN THE 
SOUTH CHINA SEA. DYNAMIC MODEL GUIDANCE IS IN OVERALL GOOD AGREEMENT 
IN THE OVERALL SCENARIO, WITH SOME DIVERGENCE AS THE LOW LEVEL 
CIRCULATION BECOMES WEAKER, LEADING TO A 280 NM SPREAD BY TAU 120. 
THE NVGM AND AFUM SOLUTIONS ARE DISCOUNTED, AS THE NVGM TAKES THE 
SYSTEM INTO THE EAST CHINA SEA, WHILE THE AFUM SOLUTION TURNS 
DRAMATICALLY SOUTHWARD AFTER TAU 72, TAKING THE SYSTEM TO THE 
VICINITY OF SOUTHERN LUZON BY TAU 120. BOTH SOLUTIONS ARE CONSIDERED 
UNREALISTIC AND DISCOUNTED FROM THE MULTI-MODEL CONSENSUS. THE JTWC 
FORECAST TRACK LIES TO THE NORTH AND SLOWER THAN THE MULTI-MODEL 
CONSENSUS WITH LOW CONFIDENCE IN LIGHT OF THE INCREASED UNCERTAINTY 
SURROUNDING THE EXACT TIMING OF THE TURN SOUTHWARD AND THE OVERALL 
SPREAD IN THE GUIDANCE.// 
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