WDPN32 PGTW 302100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 26W (CHOI-WAN) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 16.2N 150.0E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 331 NM EAST-NORTHEAST OF ANDERSEN AFB MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 08 KTS SIGNIFICANT WAVE HEIGHT: 15 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL STORM (TS) 26W IS GRADUALLY INTENSIFYING. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS CURVED BANDING DISPLACED WEST OF A PARTIALLY OBSCURED, ELONGATED LOW-LEVEL CIRCULATION CENTER (LLCC). THE ASSOCIATED CONVECTION IS PARTICULARLY DEEP, WITH CLOUD TOP TEMPERATURES AS COLD AS -90 DEGREES CELSIUS, WHILE THE CURVATURE OF THE CONVECTIVE BANDING CONTINUES TO IMPROVE. A 301448Z AMSR3 PASS INDICATES THAT THE VORTEX IS TILTED WESTWARD WITH HEIGHT. THIS STRUCTURE IS CONSISTENT WITH EASTERLY MID-LEVEL SHEAR OF NEAR 20 KTS, WHICH IS PREVENTING THE SYSTEM FROM BECOMING VERTICALLY ALIGNED. DESPITE THE SHEAR, THE ENVIRONMENT IS FAVORABLE, CHARACTERIZED BY VERY WARM SEA SURFACE TEMPERATURES (SST) OF 30 DEGREES CELSIUS AND VERY HIGH OHC NEAR 140 KJ PER CM2, MODERATE RADIAL OUTFLOW ALOFT, AND PLENTIFUL DEEP-LAYER MOISTURE. THE INITIAL POSITION IS PLACED WITH LOW CONFIDENCE, HEDGED EAST OF THE MID-LEVEL CIRCULATION CENTER TO ACCOUNT FOR VORTEX TILT. AN AGED 301015Z ASCAT-B PASS REVEALED A COMPACT PATCH OF GALE-FORCE WINDS ISOLATED TO THE NORTHWEST QUADRANT, EMBEDDED WITHIN A MUCH BROADER 20- TO 25-KT WIND FIELD ACROSS THE REMAINING QUADRANTS. GIVEN THE GAP SINCE THAT PASS AND SUBSEQUENT CONSOLIDATION, IT REMAINS UNCERTAIN WHETHER THE OBSERVED STRUCTURAL IMPROVEMENT HAS ONLY EXPANDED THE AREAL FOOTPRINT OF GALES OR ALSO INCREASED THE MAXIMUM SUSTAINED WINDS. ACCORDINGLY, THE INITIAL INTENSITY HAS BEEN HELD AT 35 KTS WITH MEDIUM CONFIDENCE, SUPPORTED BY AIDT AND THE T2.5 DVORAK ESTIMATE FROM PGTW. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A LOW- TO MID-LEVEL SUBTROPICAL RIDGE CENTERED FAR TO THE NORTHEAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T1.5 - 25 KTS RCTP: T2.0 - 30 KTS CIMSS ADT: 41 KTS AT 301430Z CIMSS AIDT: 35 KTS AT 301430Z CIMSS D-MINT: 26 KTS AT 301540Z CIMSS D-PRINT: 30 KTS AT 301710Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 15-20 KTS SST: 30-31 CELSIUS OUTFLOW: MODERATE RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: LOW INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE FORECAST WIND RADII HAVE BEEN SIGNIFICANTLY EXPANDED BEYOND TAU 48. FORECAST DISCUSSION: TS 26W IS MOVING WEST-NORTHWESTWARD ALONG THE SOUTHERN PERIPHERY OF A LOW- TO MID-LEVEL STR CENTERED NEAR 31N 169E. THIS MOTION WILL CONTINUE FOR THE NEXT 24 HOURS. AFTER THAT TIME, A PROGRESSIVE SHORTWAVE TROUGH RACING DOWNSTREAM OF NORTHERN HONSHU AND HOKKAIDO WILL ERODE THE STR. THE PRIMARY STEERING INFLUENCE WILL WEAKEN MARKEDLY, AND TS 26W WILL BECOME NEARLY STATIONARY. THE SYSTEM WILL THEN TURN POLEWARD NEAR TAU 36 AS THE DEEP-LAYER STR REORIENTS, AND THE WESTERN FLANK OF THE RIDGE BECOMES THE DOMINANT STEERING MECHANISM. TOWARD THE END OF THE FORECAST PERIOD, THE TRAJECTORY WILL NUDGE WESTWARD AS THE RIDGE BUILDS SLIGHTLY WESTWARD. THE FORECAST CALLS FOR GRADUAL TO STEADY INTENSIFICATION FOR THE FIRST 24 HOURS, GIVING TIME FOR THE STORM TO BECOME VERTICALLY ALIGNED AND MORE SYMMETRIC. AFTER TAU 24, THE PERSISTENT MID-LEVEL SHEAR WILL EASE. COUPLED WITH VERY WARM SST AND VERY HIGH OHC, THE LOWER SHEAR AND RICH DEEP-LAYER MOISTURE WILL ENABLE TS 26W TO UNDERGO SIGNIFICANT INTENSIFICATION. BETWEEN TAU 72 AND TAU 96, THE SYSTEM WILL MOVE OVER LOWER OHC, WHICH WILL LIMIT CONTINUED INTENSIFICATION. THE PROGRESSIVELY COOLER SST AND LOWER OHC WILL BECOME INCREASINGLY UNFAVORABLE FOR MAINTAINING PEAK INTENSITY, LEADING TO GRADUAL WEAKENING AFTER TAU 96. THE LOW-LEVEL CIRCULATION ASSOCIATED WITH TS 26W IS LARGE. THE WIND FIELD WILL EXPAND THROUGH THE MAJORITY OF THE FORECAST PERIOD, WITH IMPACTS EXTENDING FAR FROM THE CENTER. MODEL DISCUSSION: THE JTWC FORECAST TRACK HAS CHANGED VERY LITTLE THIS CYCLE. TRACK GUIDANCE IS COMING INTO BETTER AGREEMENT ON THE LOCATION AND TIMING OF THE POLEWARD TURN, WITH DYNAMICAL MODELS GENERALLY SHIFTING WESTWARD AND THE AI MODELS SHIFTING EASTWARD TOWARD A COMMON SOLUTION. AT THE POINT OF RECURVATURE, THE SPATIAL SPREAD AMONG THE GUIDANCE IS APPROXIMATELY 125 NM. NAVGEM AND HAFS-A ARE FASTEST IN DEPICTING THE POLEWARD TURN, LIKELY ATTRIBUTED TO THEIR TENDENCY TO SIMULATE STRONGER CONVECTIVE FEEDBACKS. ECMWF AND GOOGLE DEEPMIND ARE THE WESTERNMOST SOLUTIONS, DELAYING THE TURN BY 6-12 HOURS RELATIVE TO THE AI CONSENSUS. THE FORECAST TRACK IS POSITIONED NEAR THE MEAN OF THE AI MODELS THROUGH TAU 120 WITH MEDIUM CONFIDENCE, WHICH IS NOW ONLY 50 NM WEST OF THE JTWC CONSENSUS TRACKER (CONW) AT TAU 60. SUBSEQUENT FORECASTS RETAIN SOME FLEXIBILITY TO SHIFT THE TRACK SLIGHTLY FARTHER SOUTH AND WEST BEFORE THE SHARP POLEWARD TURN. MULTIPLE RAPID INTENSIFICATION (RI) AIDS HAVE TRIGGERED THIS CYCLE. SHIPS INDICATES A 48 PERCENT PROBABILITY OF A 65-KT INCREASE IN THE NEXT 72 HOURS. GOOGLE DEEPMIND ALSO PREDICTS SIGNIFICANT INTENSIFICATION. GIVEN THE MODEL SUPPORT AND FAVORABLE ENVIRONMENT AHEAD OF THE STORM, THE FORECAST CALLS FOR RI TO COMMENCE AFTER TAU 24. THE JTWC INTENSITY FORECAST IS SET AT THE UPPER END OF THE GUIDANCE ENVELOPE WITH MEDIUM CONFIDENCE, LYING CLOSEST TO THE DEEPMIND SOLUTION. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN