WDPN32 PGTW 281500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 007// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 25.2N 166.5E INITIAL INTENSITY: 45 KTS GEOGRAPHIC REFERENCE: 354 NM NORTH OF WAKE ISLAND MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 22 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL STORM 21W (ETAU) WITH A FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH DEEP CONVECTION ATTEMPTING TO BUILD CLOSER TO THE CENTER. THE STRONG WESTERLY SHEAR ACTING ON THE SYSTEM HAS CONTINUED TO CAUSE THE CONVECTION TO BE DISPLACED TO THE EAST. A 280959Z METOP-B ASCAT PASS REVEALED A HIGHLY ASYMMETRIC WIND FIELD WITH THE 34 KNOT WINDS ENTIRELY CONFINED TO THE EASTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 45 KTS AND THE INITIAL WIND RADII ARE ASSESSED WITH HIGH CONFIDENCE BASED ON THE ASCAT PASS SHOWING A SWATH OF 40 KTS TO THE EAST AND THE KNOWN SLIGHT LOW BIAS OF THE SENSOR AT THESE INTENSITIES. ENVIRONMENTAL ANALYSIS INDICATES THAT 21W IS PLACED WITHIN A MARGINAL ENVIRONMENT CHARACTERIZED BY STRONG OUTFLOW ALOFT AND WARM (29-30 C) SEA SURFACE TEMPERATURES OFFSET BY HIGH (30-35 KTS) WESTERLY SHEAR AND DRY AIR ENTRAINMENT. INITIAL WIND RADII BASIS: 280959Z METOP-B ASCAT SCATTEROMETER DATA CURRENT STEERING MECHANISM: THE WESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T2.5 - 35 KTS RCTP: T2.5 - 35 KTS KNES: T2.5 - 35 KTS PHFO: T3.0 - 45 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 30-35 KTS SST: 29-30 CELSIUS OUTFLOW: STRONG POLEWARD OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE WEST ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TS 21W (ETAU) IS FORECAST TO TRACK NORTH- NORTHWESTWARD ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST THROUGH TAU 48. NEAR TAU 48, AN INCOMING UPPER-LEVEL TROUGH IS EXPECTED TO DIG IN FROM THE NORTH, BREAKING DOWN THE STEERING RIDGE AND CAUSING TRACK SPEEDS TO SLOW. BINARY INTERACTION WITH TY 20W (BANG-LANG) WILL CAUSE 21W TO BE TUGGED NORTHEASTWARD THROUGH AFTER TAU 72. A DEVELOPING FRONTAL BOUNDARY IS EXPECTED TO FORM NEAR THE VERY END OF THE FORECAST PERIOD, WHICH COULD CAUSE TRACK SPEEDS TO BE FASTER IF THE FRONTAL BOUNDARY PICKS UP THE SYSTEM SOONER THAN ANTICIPATED. SUBTROPICAL TRANSITION IS FORECAST TO BEGIN AT TAU 60 WITH COMPLETION BY TAU 72 AS THE SYSTEM CROSSES THE 26 C ISOTHERM, BECOMES ENGULFED BY DRY AIR, AND INTERACTS WITH THE MID- LATITUDE TROUGH. IN TERMS OF INTENSITY, 21W IS FORECAST TO MAINTAIN ITS CURRENT INTENSITY THROUGH TAU 24, PRIMARILY DRIVEN BY WARM SST AND STRONG POLEWARD OUTFLOW. AFTER TAU 24, DRY AIR, HIGH SHEAR, AND QUICKLY LOWERING OCEAN HEAT CONTENT WILL CAUSE 21W TO WEAKEN THROUGH THE REMAINDER OF THE FORECAST PERIOD. MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN MODERATE AGREEMENT THROUGH TAU 72 WITH THE PRIMARY OUTLIER BEING THE JGSM, WHICH DEPICTS A STRAIGHT NORTHWARD TRACK CROSSING THE 40N LATITUDE AT TAU 72. DISCOUNTING JGSM, THE REMAINDER OF GUIDANCE GENERALLY SHOWS A SLOW TURN TO THE EAST OR NORTHEAST AFTER TAU 48, ONCE THE SYSTEM BEGINS TO INTERACT WITH 20W. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE MODIFIED CONSENSUS WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE AGREES ON A STEADY INTENSITY THROUGH TAU 24 FOLLOWED BY GRADUAL WEAKENING THROUGH TAU 72 WITH NO MAJOR OUTLIERS PRESENT. AS A RESULT, THE JTWC INTENSITY FORECAST IS PLACED NEAR THE CONSENSUS WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM INTENSITY 00-72 HR: MEDIUM// NNNN