MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 24W (SON-TINH) WARNING NR 17// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 24W (SON-TINH), LOCATED APPROXIMATELY 205 NM SOUTH- SOUTHEAST OF HANOI, VIETNAM, HAS TRACKED WEST-NORTHWESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY (EIR) REVEALS THE 20 NM EYE HAS FILLED IN OVER THE PAST SIX HOURS. A 271425Z AMSU MICROWAVE IMAGE SHOWS THAT TY 24W HAD TWO CONCENTRIC EYE WALLS AT THE TIME. THE INITIAL POSITION IS BASED ON THE ANIMATED EIR AND FIXES FROM PGTW AND RJTD WITH GOOD CONFIDENCE. THE INITIAL INTENSITY IS ASSESSED AT 105 KNOTS BASED ON DVORAK ESTIMATES OF 102 KNOTS FROM PGTW, KNES AND RJTD. 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. ANIMATED WATER VAPOR IMAGERY REVEALS THE PREVIOUS BURST OF ENHANCED OUTFLOW ASSOCIATED WITH A PASSING MID-LATITUDE TROUGH IS BEGINNING TO SUBSIDE, THOUGH STILL ROBUST. THE CYCLONE IS NOW TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) ANCHORED JUST SOUTH OF TAIWAN. OBSERVED WINDS AT HUE, VIETNAM, 100 NM SOUTH OF THE CURRENT POSITION, HAVE BEEN PREDOMINANTLY FROM THE SOUTHWEST AT 5-15 KNOTS OVER THE PAST SIX HOURS. 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 PRIOR TO TAU 24. AFTER LANDFALL, THE CYCLONE WILL DEFLECT POLEWARD AS THE STR FLATTENS IN RESPONSE TO THE AFOREMENTIONED TROUGH. THE SYSTEM IS FORECAST TO MAINTAIN ITS INTENSITY OVER THE NEXT 12 HOURS GIVEN THE AMPLE OUTFLOW AND PERENNIALLY WARM WATERS OF THE GULF OF TONKIN. AFTER THIS TIME, 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 IS FORECAST TO DISSIPATE BY TAU 72 NEAR THE CHINESE BORDER DUE TO LAND INTERACTION AND INCREASING SHEAR. THE NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT UP TO TAU 36, AFTER WHICH THE MODELS SPREAD DUE TO UNCERTAINTIES IN THE INTERACTION OF THE SYSTEM WITH THE MID-LATITUDE WESTERLIES AND TERRAIN EFFECTS. THERE IS HIGH CONFIDENCE IN THE TRACK FORECAST UP TO TAU 36. AFTER TAU 36, SOME OF THE GUIDANCE SUGGESTS THE REMNANTS OF TY 24W MAY CURVE MORE SHARPLY, AND RE- EMERGE BACK OVER THE GULF OF TONKIN. IN THIS ALTERNATE SCENARIO, THE SYSTEM WOULD BE CLOSELY MONITORED FOR SIGNS OF REGENERATION.// NNNN NNNN