WDPN31 PGTW 271500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 24W (SON-TINH) WARNING NR 16// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON 24W (SON-TINH), LOCATED APPROXIMATELY 265 NM SOUTHEAST OF HANOI, VIETNAM, HAS TRACKED WEST-NORTHWESTWARD AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS THE SYSTEM HAS UNDERGONE RAPID INTENSIFICATION (RI) OVER THE PAST SIX HOURS AS CONVECTIVE BANDS DEEPENED AND WRAPPED TIGHTER INTO THE LOW LEVEL CIRCULATION CENTER. THE RI IS REFLECTED IN A 270948Z SSMI-S MICROWAVE IMAGE THAT DEPICTS TWO CONCENTRIC, ALBEIT BROKEN, EYE WALLS. THE INITIAL POSITION IS BASED ON THE EYE FEATURE WITH HIGH CONFIDENCE. THE INITIAL INTENSITY IS BASED ON DVORAK ESTIMATES FROM PGTW, KNES, AND RJTD RANGING FROM 102 TO 115 KNOTS. UPPER-LEVEL ANALYSIS INDICATES THE SYSTEM IS DIRECTLY BENEATH A RIDGE AXIS IN AN AREA OF LIGHT TO MODERATE (10-15 KNOTS) VERTICAL WIND SHEAR. JUST TO THE NORTH ARE PROGRESSIVELY INCREASING AND DIVERGENT WINDS ASSOCIATED WITH THE STRONG WESTERLIES. THIS IS EVIDENT ON ANIMATED WATER VAPOR IMAGERY THAT SHOWS THE RECENT APPEARANCE OF TRANSVERSE BANDING AND A DRAMATIC INCREASE IN POLEWARD OUTFLOW IN THIS AREA. THIS OUTFLOW APPEARS TO BE INDUCED BY A MID-LATITUDE TROUGH PASSING ACROSS THE YELLOW SEA. THE CYCLONE IS NOW TRACKING ALONG THE SOUTHWEST EXTENSION OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) ANCHORED JUST SOUTH OF TAIWAN. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. TY SON-TINH WILL CONTINUE TO TRACK WEST-NORTHWESTWARD UNTIL IT MAKES LANDFALL ALONG THE NORTHERN COAST OF VIETNAM SHORTLY AFTER TAU 24. AFTER LANDFALL, THE CYCLONE WILL DEFLECT POLEWARD AS THE STR FLATTENS IN RESPONSE TO TO THE AFOREMENTIONED TROUGH. THE ROBUST POLEWARD OUTFLOW IN ADDITION TO THE PERENNIALLY WARM WATERS OF THE GULF OF TONKIN WILL CONTINUE TO FUEL THE SYSTEM AND ALLOW FOR MORE INTENSIFICATION OVER THE SHORT TERM. HOWEVER, BY TAU 12, THE WARM MOIST AIR FROM THE EQUATOR WILL BE RESTRICTED BY THE INDOCHINESE PENINSULA AND THE INFLUX OF RELATIVELY COOLER AND DRIER CONTINENTAL AIR FROM CHINA WILL BEGIN TO WEAKEN THE SYSTEM. NEVERTHELESS, TY 24W WILL STILL BE AT A MODEST TROPICAL STORM STRENGTH WHEN IT APPROACHES HANOI. THE SYSTEM WILL EVENTUALLY DISSIPATE CLOSE TO THE CHINESE BORDER BY TAU 72 AS IT ATTEMPTS TO CURVE BACK INTO THE GULF. THE NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT UP TO TAU 36, AFTER WHICH THE MODELS SPREAD DUE TO UNCERTAINTIES IN THE INTERACTION WITH THE TERRAIN AND VARIOUS SOLUTIONS TO THE WEAKENING STR. THERE IS HIGH CONFIDENCE IN THE TRACK FORECAST UP TO TAU 36. // NNNN