MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (MEGI) WARNING NR 03// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 20W (MEGI), LOCATED APPROXIMATELY 960 NM EAST- SOUTHEAST OF TAIPEI, TAIWAN, HAS TRACKED WESTWARD AT 15 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS AN EXPANSIVE SYSTEM WITH MONSOON DEPRESSION CHARACTERISTICS THAT IS SLOWLY CONSOLIDATING. FORMATIVE BANDS HAVE DEEPENED BUT REMAIN FRAGMENTED. THE INITIAL POSITION IS BASED ON A LOW REFLECTIVITY CIRCULATION FEATURE IN THE 230935Z AMSU-B MICROWAVE IMAGE. THE INITIAL INTENSITY OF 35 KNOTS IS BASED ON AN OVERALL ASSESSMENT OF AGENCY DVORAK ESTIMATES RANGING FROM T2.0 TO T3.0. THERE IS LOW CONFIDENCE IN THE INITIAL POSITION GIVEN THE FORMATIVE NATURE OF THE CYCLONE AND THE POSSIBILITY OF MULTIPLE CIRCULATIONS. UPPER LEVEL ANALYSIS INDICATES THE STORM IS IN THE VICINITY OF AN ANTICYCLONIC POINT SOURCE WITH LOW VERTICAL WIND SHEAR AND EXCELLENT DUAL OUTFLOW. ADDITIONALLY, SEA SURFACE TEMPERATURES AT 30C ARE EXCEPTIONALLY WARM AND CONDUCIVE FOR FURTHER DEVELOPMENT. THE CYCLONE IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A MID-LEVEL REFLECTION OF THE SUBTROPICAL RIDGE (STR) TO THE NORTH-NORTHEAST. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO FORECAST PHILOSOPHY FROM THE PREVIOUS PROGNOSTIC REASONING BULLETIN. B. TS 20W IS EXPECTED TO CONTINUE ON A WEST-NORTHWESTWARD TRAJECTORY THROUGHOUT ITS LIFESPAN, STEERED BY THE STR THAT IS FORECAST TO BUILD. ENVIRONMENTAL CONDITIONS WILL REMAIN FAVORABLE FOR STEADY INTENSIFICATION, REACHING A PEAK OF 90 KNOTS BY TAU 72 AS THE SYSTEM DRIFTS ACROSS THE WARM PHILIPPINE SEA. C. IN THE EXTENDED PERIOD, TS MEGI WILL CROSS SOUTHERN TAIWAN AND THE TAIWAN STRAIT PRIOR TO MAKING LANDFALL IN EASTERN CHINA SHORTLY AFTER TAU 96. LAND INTERACTION WILL BE THE PRIMARY CAUSE OF ITS RAPID WEAKENING DURING THIS STAGE. SANS THE UNCERTAINTY IN THE INITIAL POSITION, MODEL GUIDANCE REMAINS IN GENERALLY GOOD AGREEMENT, LENDING HIGH CONFIDENCE TO THE JTWC TRACK FORECAST.// NNNN NNNN