WDPN31 PGTW 141500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING NR 009// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 23.6N 157.6E INITIAL INTENSITY: 40 KTS GEOGRAPHIC REFERENCE: 201 NM EAST OF MINAMI TORI SHIMA MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 25 KTS SIGNIFICANT WAVE HEIGHT: 14 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL STORM (TS) 21W IS EMBEDDED UNDER STRONG SOUTHERLY FLOW WITHIN THE EASTERN PERIPHERY OF A BROAD, SUBTROPICAL UPPER-LEVEL LOW POSITIONED NEAR 24N 152E. CONSEQUENTLY, ENVIRONMENTAL CONDITIONS ARE MARGINAL WITH LOW TO MODERATE VERTICAL WIND SHEAR OFFSET BY ROBUST POLEWARD OUTFLOW. DESPITE THE MARGINAL ENVIRONMENT, A 140904Z GMI 89GHZ COLOR COMPOSITE MICROWAVE IMAGE SHOWS A CONSOLIDATED LOW-LEVEL CIRCULATION WITH DEEP CONVECTIVE BANDING WRAPPING INTO A WELL-DEFINED CENTER. THIS CENTER COINCIDES WITH A SMALL CIRCULATION EVIDENT IN THE 141027Z ASCAT-B IMAGE WITH A SMALL PATCH OF 35-45 KNOT WINDS OVER THE EASTERN QUADRANT AND WEAKER WINDS ELSEWHERE. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE ASCAT-B IMAGE. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE ASCAT DATA AND THE CIMSS SATCON ESTIMATE. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: TS 21W IS TRACKING QUICKLY POLEWARD WITHIN THE ENHANCED FLOW BETWEEN THE AFOREMENTIONED SUBTROPICAL LOW AND A SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS RJTD: T1.5 - 25 KTS CIMSS SATCON: 40 KTS AT 141024Z CIMSS ADT: 33 KTS AT 141140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 15-20 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TS 21W IS FORECAST TO TRACK NORTHWARD TO NORTH-NORTHEASTWARD THROUGH TAU 24 ALONG THE WESTERN PERIPHERY OF THE STR. ENVIRONMENTAL CONDITIONS WILL REMAIN MARGINAL AS THE SYSTEM TRACKS WITHIN THE EASTERN PERIPHERY OF THE UPPER-LEVEL SUBTROPICAL LOW WITH ROBUST POLEWARD OUTFLOW OFFSET BY DRY AIR ENTRAINMENT AND MODERATE VERTICAL WIND SHEAR. GRADUAL WEAKENING IS ANTICIPATED WITH DISSIPATION EXPECTED BY TAU 24. MODEL DISCUSSION: WITH THE EXCEPTION OF JGSM AND AFUM, NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 12 WITH GUIDANCE SPREADING OUT AFTER TAU 12 DUE TO THE COMPLEX INTERACTION AND MERGER WITH A LARGE SUBTROPICAL LOW POSITIONED NEAR 27N 154E (INVEST 91W). NUMERICAL MODELS CONTINUE TO INDICATE THAT TS 21W WILL DISSIPATE AND THE REMNANTS WILL GET ABSORBED INTO THIS SUBTROPICAL LOW BY TAU 24. THIS NEWLY-DEVELOPED SUBTROPICAL LOW COULD THEN DEEPEN AND TRACK POLEWARD WITH EXPANDING GALE-FORCE WINDS. THIS COMPLEX SYNOPTIC PATTERN AND MERGER WILL BE CLOSELY MONITORED. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM// NNNN