WDPN32 PGTW 050300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (ATSANI) WARNING NR 027// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 23W (ATSANI), LOCATED APPROXIMATELY 381 NM SOUTH OF KADENA AB, HAS TRACKED WESTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS SPIRAL BANDS OF CONVECTION WRAPPING INTO A LOW LEVEL CIRCULATION CENTER (LLCC) THAT IS OBSCURED BY DENSE CONVECTION OVER THE CENTER OF THE SYSTEM. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE LOW LEVEL CIRCULATION FEATURES IN THE MSI LOOP AS WELL AS IN A 041934Z SSMI 37 GHZ IMAGE. THE INITIAL INTENSITY OF 55 KTS IS PLACED WITH FAIR CONFIDENCE BASED ON A PGTW DVORAK CURRENT INTENSITY ESTIMATE OF T3.5 (55 KTS) WHICH IS IN AGREEMENT WITH A 041717 CIMSS SATELLITE CONSENSUS INTENSITY ESTIMATE OF 56 KTS. THE ENVIRONMENT IS FAVORABLE FOR INTENSIFICATION WITH LOW (5-10) KTS VERTICAL WIND SHEAR (VWS), WARM (28-29 CELSIUS) SEA SURFACE TEMPERATURES AND ROBUST POLEWARD AND MODERATE EQUATORWARD OUTFLOW ALOFT. THE STEERING ENVIRONMENT HAS IMPROVED OVER THE LAST 6 HOURS AND THE SYSTEM IS NOW TRACKING WEST-SOUTHWESTWARD ALONG THE PERIPHERY OF THE SUBTROPICAL RIDE POSITIONED TO THE NORTHEAST. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 23W HAS BEGUN ITS WESTWARD MOVEMENT AS THE STR TO THE NORTHEAST BUILDS IN TO THE NORTH, AND IT IS FORECAST THAT THE SYSTEM WILL ACCELERATE ALONG THIS GENERAL WEST-NORTHWEST MOTION THROUGH TAU 48. THE SYSTEM IS FORECAST TO GRADUALLY INTENSIFY TO 70 KTS BY THIS TIME DUE TO CONTINUED LOW (10-15 KTS) VWS, WARM SST AND UPPER LEVEL SUPPORT. AFTER TAU 48, THE SYSTEM WILL SLOW SOMEWHAT AS THE STR WEAKENS SLIGHTLY AND REORIENTS TO A MORE SOUTHWEST TO NORTHEAST ORIENTATION. BY TAU 72 THE PRIMARY STEERING MECHANISM WILL SHIFT TO THE STR LOCATED OVER CENTRAL CHINA, WHICH WILL SERVE TO SHIFT THE TRACK MORE SOUTHWESTWARD. THIS NEW TRACK WILL BRING THE SYSTEM INTO HIGH (30-35 KTS) VWS WHICH WILL WEAKEN THE SYSTEM INTENSITY TO 40 KTS BY TAU 72. NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT, HOWEVER NAVGEM AND AFUM DIVERGE SIGNIFICANTLY FROM THE MULTI-MODEL CONSENSUS. SPECIFICALLY, NAVGEM DRIVES THE VORTEX NORTHWARD WHILE AFUM BRINGS THE SYSTEM SOUTH TOWARDS LUZON. AS SUCH, THIS INITIAL PORTION OF THE JTWC FORECAST TRACK IS PLACED WITH GOOD CONFIDENCE AND IS LAID SLIGHTLY SOUTH OF THE JTWC MULTI-MODEL CONSENSUS IN ORDER TO OFFSET THE UNREALISTIC TRACKS FROM THESE MODELS. C. IN THE EXTENDED TAUS, THE SYSTEM WILL ENTER THE NORTHERN PORTION OF THE SOUTH CHINA SEA BY TAU 72 ON A GENERALLY WEST- SOUTHWESTWARD TRACK ALONG THE SOUTHEAST PERIPHERY OF THE STR OVER CENTRAL CHINA. SIMULTANEOUSLY, THE COMBINATION OF INCREASED VWS, CONVERGENT UPPER-LEVEL FLOW, ENTRAINMENT OF DRIER AIR FROM THE NORTH, AND COOLER SSTS WILL COMBINE TO STEADILY WEAKEN THE SYSTEM TO 25 KTS BY TAU 96. AS THE SYSTEM WEAKENS, IT WILL COME UNDER THE STEERING INFLUENCE OF THE LOW TO MID-LEVEL FLOW FROM THE NORTHEAST, LEADING TO A SOUTHWESTWARD TRACK THAT WILL LEAD TO LANDFALL AFTER SHORTLY AFTER TAU 96 ALONG THE SOUTH CENTRAL COAST OF VIETNAM. INTERACTION WITH THE UNDERLYING TERRAIN WILL LEAD TO FURTHER WEAKENING AND FULL DISSIPATION OVER LAND BY TAU 120. AFTER DISCOUNTING THE AFOREMENTIONED NAVGEM AND AFUM OUTLIERS, THERE REMAINS GOOD AGREEMENT IN NUMERICAL MODEL SOLUTIONS AND AS SUCH THE JTWC FORECAST TRACK IS PLACED WITH GOOD CONFIDENCE ALONG THE MULTI-MODEL CONSENSUS THROUGH THE REMAINDER OF THE FORECAST PERIOD.// NNNN