WDPN31 PGTW 211500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (JELAWAT) WARNING NR 05// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (JELAWAT), LOCATED APPROXIMATELY 535 NM EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WEST-SOUTHWESTWARD AT 07 KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED (IR) SATELLITE IMAGERY SHOWS THE SYSTEM HAS MAINTAINED CENTRAL DEEP CONVECTION SLIGHTLY WEST OF THE LOW LEVEL CIRCULATION CENTER. THE INITIAL POSITION IS BASED ON THE IR ANIMATION AND EXTRAPOLATED FROM A NOTCH FEATURE ON A 210901Z SSMI-S MICROWAVE IMAGE. THE INITIAL INTENSITY IS BASED ON CONGRUENT DVORAK ESTIMATES OF 45 KNOTS FROM PGTW AND RJTD. TS 18W LIES SLIGHTLY EQUATORWARD OF AN UPPER-LEVEL RIDGE AXIS IN AN AREA OF LIGHT EASTERLY VERTICAL WIND SHEAR AND LIMITED OUTFLOW ALOFT. THE CYCLONE CONTINUES TO TRACK ALONG THE SOUTHWESTERN EXTENSION OF THE SUBTROPICAL RIDGE (STR) ANCHORED EAST OF JAPAN. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 18W IS EXPECTED TO TRACK SLOWLY WESTWARD OVER THE NEXT 48 HOURS AS A MID-LATITUDE TROUGH APPROACHING FROM THE NORTHWEST WEAKENS THE CURRENT STEERING RIDGE AND THE CYCLONE ENTERS A WEAK STEERING ENVIRONMENT. LOW VERTICAL WIND SHEAR, PASSAGE OVER WARM WATER, AND IMPROVING OUTFLOW ALOFT AHEAD OF THE APPROACHING TROUGH WILL ALLOW TS 18W TO STEADILY INTENSIFY DURING THIS PERIOD. AFTER THE TROUGH TRANSLATES EASTWARD, SUBTROPICAL RIDGING IS EXPECTED TO BUILD IN ITS WAKE AND TURN TS 18W ON A NORTHWESTWARD TRAJECTORY. C. DURING THE EXTENDED TAUS, THE REBUILDING STR TO THE NORTHEAST IS EXPECTED TO STEER THE SYSTEM IN A NORTHWESTWARD DIRECTION AS THE PRIMARY STEERING MECHANISM GRADUALLY TRANSITIONS TO ANOTHER SUBTROPICAL RIDGE OVER EASTERN CHINA. ENVIRONMENTAL CONDITIONS ARE EXPECTED TO DRIVE CONTINUED INTENSIFICATION DURING THIS PERIOD. THE NUMERICAL MODEL GUIDANCE REMAINS WIDELY SPREAD. THE CURRENT FORECAST IS ADJUSTED LEFT OF CONSENSUS TO OFFSET UNLIKELY INTRUSION OF THE VORTEX INTO THE STR, AS DEPICTED BY A HANDFUL OF TRACKERS INCLUDING GFS AND ECMWF. THERE IS LOW CONFIDENCE IN THE CURRENT TRACK FORECAST. // NNNN