MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN) WARNING NR 001// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 18W (EIGHTEEN), LOCATED APPROXIMATELY 245 NM SOUTH-SOUTHEAST OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWESTWARD AT 03 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY INDICATES THE CENTRAL CONVECTION REMAINS SHEARED SOUTHWESTWARD OF AN EXPOSED LOW LEVEL CIRCULATION (LLC). THE ASSOCIATED CONVECTION HAS BEEN FLARING BUT HAS NOW BEGUN TO BUILD OVER THE LLC. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE WELL DEFINED LLC IN THE ABOVE ANIMATION AND CONCENTRIC WITH AGENCY FIXES. THE INITIAL INTENSITY IS BASED ON THE DVORAK ESTIMATES OF T1.5/25KTS FROM PGTW AND RJTD AND IS CONSISTENT WITH RECENT ASCAT PASSES. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM IS SPLIT UNDER STRONG (25-30KT) NORTHEASTERLY VERTICAL WIND SHEAR (VWS) ON THE SOUTHERN HALF (OVER THE CONVECTION) AND LIGHT (10-15KT) VWS ON THE NORTHERN HALF. THE VWS, HOWEVER, IS OFFSET BY STRONG EQUATORWARD OUTFLOW. SEA SURFACE TEMPERATURES IN THE AREA REMAIN CONDUCIVE AT 29-30C. TD 18W IS IN A COL BETWEEN THE SUBTROPICAL RIDGE (STR) TO THE NORTHWEST AND THE STR TO THE NORTHEAST AND REMAINS QUASI-STATIONARY. 3. FORECAST REASONING. A. THIS IS THE FIRST WARNING ON THIS SYSTEM AND ESTABLISHES THE FORECAST PHILOSOPHY. B. TD 18W WILL BEGIN TO SLOWLY TRACK NORTHWESTWARD AS THE STR BECOMES THE PREDOMINANT STEERING MECHANISM. AFTER TAU 24, IT WILL TURN MORE POLEWARD AS IT ROUNDS THE STR AXIS, CRESTING IT NEAR TAU 48. AFTERWARD, THE TD WILL TRACK MORE NORTH-NORTHEASTWARD ON THE POLEWARD SIDE OF THE STR. BY TAU 72, THE CYCLONE WILL BE IN THE EAST CHINA SEA JUST SOUTH OF CHEJU ISLAND, SOUTH KOREA. THE VWS WILL BECOME MORE FAVORABLE AS THE SYSTEM TRACKS POLEWARD, ENHANCING GRADUAL INTENSIFICATION TO A PEAK OF 55KTS BY TAU 72, AIDED BY ENHANCED POLEWARD OUTFLOW ASSOCIATED WITH THE PREVAILING WESTERLIES. NUMERICAL MODELS HAVE COME INTO BETTER AGREEMENT WITH EGRR ON THE LEFT MARGIN OF THE ENVELOPE THAT SPREADS OUT TO 260NM BY TAU 72. HOWEVER, THERE IS LOW CONFIDENCE IN THIS PORTION OF THE JTWC FORECAST DUE TO THE UNCERTAINTY IN THE INITIAL STORM MOTION. C. AFTER TAU 72, TD 18W WILL ENTER THE SEA OF JAPAN (SOJ) AND ACCELERATE NORTHEASTWARD, CROSSING THE TSUGARU STRAIT AND RETURNING BACK INTO THE PACIFIC OCEAN JUST SOUTH OF HOKKAIDO, JAPAN, BY TAU 120. INCREASING VWS AND COOLING SSTS IN THE SOJ WILL GRADUALLY ERODE THE SYSTEM AND BY TAU 120, REDUCE IT TO 30KTS. CONCURRENTLY, THE SYSTEM WILL BEGIN EXTRA-TROPICAL TRANSITION BY TAU 72 AS IT GETS EMBEDDED IN THE BAROCLINIC ZONE. NUMERICAL MODELS ARE ALSO IN GENERAL AGREEMENT BUT SPREADS OUT TO OVER 500NM BY TAU 120, LENDING LOW CONFIDENCE IN THE EXTENDED PORTION OF THE JTWC TRACK FORECAST.// NNNN NNNN