MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (MEGI) WARNING NR 17// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 20W (MEGI), LOCATED APPROXIMATELY 152 NM SOUTHEAST OF TAIPEI, TAIWAN, HAS TRACKED NORTHWESTWARD AT 13 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A 35-NM RAGGED EYE WITH TIGHTLY-CURVED BANDING WRAPPING INTO THE LOW-LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS BASED ON THE EYE IN THE MSI LOOP WHICH LINES UP WELL WITH A LOW REFLECTIVITY MICROWAVE EYE FEATURE IN THE 262231Z 36GHZ GPM PASS. THE INITIAL INTENSITY IS ASSESSED AT 100 KNOTS AND IS BASED ON DVORAK CURRENT INTENSITY ESTIMATES OF T5.5 FROM ALL AGENCIES. UPPER-LEVEL ANALYSIS INDICATES EXCELLENT RADIAL OUTFLOW WITH ENHANCED POLEWARD OUTFLOW INTO A MIDLATITUDE SHORTWAVE TROUGH TO THE NORTH. HOWEVER, MOST RECENT WATER VAPOR SATELLITE IMAGERY INDICATES SUBSIDENCE ALONG THE NORTHWEST QUADRANT, INDICATING THE SYSTEM IS APPROACHING AN AREA OF INCREASING VERTICAL WIND SHEAR (VWS). TY MEGI IS TRACKING WEST-NORTHWESTWARD ALONG THE PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. TY 20W IS FORECAST TO CONTINUE TRACKING WEST-NORTHWESTWARD, DRAG THROUGH CENTRAL TAIWAN SHORTLY AFTER TAU 06, THEN CROSS THE TAIWAN STRAIT BEFORE MAKING A FINAL LANDFALL IN EASTERN CHINA JUST BEFORE TAU 24. AFTERWARDS, A SECONDARY STR OVER NORTHEASTERN CHINA WILL ASSUME STEERING AND CAUSE THE CYCLONE TO TRACK ON A FLATTER WESTWARD TRAJECTORY. A GRADUAL WEAKENING IS EXPECTED OVER THE NEXT FEW HOURS AS INCREASED VERTICAL WIND SHEAR (VWS) ASSOCIATED WITH THE LEADING EDGE OF THE SECONDARY STR IMPACTS THE SYSTEM. AFTER LANDFALL INTO TAIWAN, RAPID WEAKENING IS EXPECTED AS LAND INTERACTION, COMBINED WITH INCREASING VWS, ERODES THE SYSTEM. TY MEGI WILL EVENTUALLY DISSIPATE INLAND NORTH OF HONG-KONG BY TAU 48. THE DYNAMIC GUIDANCE REMAINS IN TIGHT AGREEMENT WITH SOME SPREADING AFTER LANDFALL IN CHINA. OVERALL, THERE IS HIGH CONFIDENCE IN THE JTWC TRACK FORECAST.// NNNN NNNN