WDPN31 PGTW 241500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 30W (IN-FA) WARNING NR 005// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 10.6N 165.9E INITIAL INTENSITY: 50 KTS GEOGRAPHIC REFERENCE: 160 NM NORTHWEST OF KWAJALEIN MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 02 KTS SIGNIFICANT WAVE HEIGHT: 16 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DEEP CONVECTION OBSCURING A COMPACT LOW-LEVEL CIRCULATION CENTER (LLCC). UPPER-LEVEL OUTFLOW REMAINS HEALTHY IN ALL DIRECTIONS, THOUGH THE CONVECTIVE STRUCTURE HAS NOT APPRECIABLY IMPROVED OVER THE PAST SIX HOURS. A 241039Z ASCAT-C PASS REVEALED GALE-FORCE WINDS EXTENDING A SHORT DISTANCE FROM THE CENTER, BUT IT MAY NOT HAVE RESOLVED THE MAXIMUM WINDS. THE WIND FIELD WAS ALSO SOMEWHAT ASYMMETRIC. TROPICAL STORM (TS) 30W IS TRAVERSING A GENERALLY FAVORABLE ENVIRONMENT OF VERY WARM SEA SURFACE TEMPERATURES (SST) OF 29-30 DEGREES CELSIUS AND MODERATE RADIAL OUTFLOW ALOFT; HOWEVER, MODERATE MID-LEVEL SOUTHEASTERLY SHEAR MAY BE INHIBITING MORE RAPID DEVELOPMENT. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE SCATTEROMETER DATA. THE INITIAL INTENSITY IS HELD AT 50 KTS WITH MEDIUM CONFIDENCE, BASED ON A BLEND OF THE CIMSS OBJECTIVE INTENSITY ESTIMATES AND SET SLIGHTLY ABOVE THE SUBJECTIVE DVORAK FIXES. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T3.0 - 45 KTS RCTP: T2.5 - 35 KTS PHFO: T3.0 - 45 KTS CIMSS SATCON: 41 KTS AT 240738Z CIMSS ADT: 47 KTS AT 241140Z CIMSS AIDT: 38 KTS AT 241140Z CIMSS D-MINT: 45 KTS AT 241044Z CIMSS D-PRINT: 53 KTS AT 241140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 15-20 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM 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 30W MAY HAVE BEGUN ITS ANTICIPATED NORTH-NORTHWESTWARD MOTION. THE CURRENT WEAK STEERING ENVIRONMENT, PRODUCED BY COMPETING INFLUENCES FROM A NEAR-EQUATORIAL RIDGE (NER) TO THE SOUTHEAST AND A SUBTROPICAL RIDGE (STR) TO THE NORTHWEST, WILL TRANSITION OVER THE NEXT 24 HOURS. AS THE NER BUILDS, IT WILL BECOME THE PRIMARY STEERING INFLUENCE AND DRIVE THE SYSTEM NORTH-NORTHWESTWARD AT A RELATIVELY SLOW SPEED OF 5-6 KTS. NEAR TAU 48, A DEEP TROUGH PASSING OVER THE KOREAN PENINSULA AND APPROACHING HONSHU WILL CAUSE THE STR TO REORIENT EASTWARD AND CONSOLIDATE NORTH OF TS 30W BY TAU 96. AS A RESULT, THE SYSTEM WILL SLOW TO 3-4 KTS FROM TAU 48 TO TAU 72 AND TURN SHARPLY WESTWARD. DURING TAU 96-120, THE NER AND STR WILL MERGE INTO A STRONGER RIDGE. CONCURRENTLY, A HYBRID SYSTEM WILL FORM EAST OF THE DATE LINE AND EXPAND EASTWARD, FURTHER ENHANCING THE WESTWARD STEERING FLOW. THESE FEATURES WILL CAUSE TS 30W TO ACCELERATE WESTWARD DURING THE LATTER PART OF THE FORECAST PERIOD. ALTHOUGH THE INTENSITY HAS CHANGED LITTLE OVER THE PAST SIX HOURS AND MODERATE MID-LEVEL SHEAR PERSISTS, VERY WARM SST AND MODERATE DIVERGENCE ALOFT WILL SUPPORT FURTHER DEVELOPMENT. THE FORECAST CALLS FOR STEADY INTENSIFICATION THROUGH TAU 12, ALLOWING TIME FOR THE COMPACT CIRCULATION TO BECOME MORE AXISYMMETRIC. RAPID INTENSIFICATION (RI) IS FORECAST THEREAFTER, WITH TS 30W REACHING 90 KTS BY TAU 36. BETWEEN TAU 36 AND TAU 72, THE SLOW TRANSLATION SPEED MAY PROMOTE OCEAN MIXING AND UPWELLING OF COOLER SUBSURFACE WATERS, LIMITING FURTHER INTENSIFICATION AND POSSIBLY INDUCING WEAKENING. ONCE THE SYSTEM GAINS SUFFICIENT FORWARD SPEED AFTER TAU 72, THE GENERALLY FAVORABLE ENVIRONMENT OF LOW TO MODERATE VERTICAL WIND SHEAR (VWS) AND VERY WARM SST WILL SUPPORT REINTENSIFICATION. MODEL DISCUSSION: TRACK GUIDANCE IS GENERALLY SIMILAR TO THE PREVIOUS CYCLE, WITH JGSM, HAFS-A, GFS, AND THE GEFS ENSEMBLE MEAN DEPICTING A SHORT-LIVED NORTHEASTWARD TURN NEAR TAU 36, FOLLOWED BY A SHARP WESTWARD TURN NEAR TAU 72. THIS SCENARIO ASSUMES THE STR WILL REORIENT EASTWARD MORE SLOWLY THAN FORECAST. ECMWF AND THE AI-BASED GUIDANCE DEPICT A MORE IMMEDIATE SHARP WESTWARD TURN. THE JTWC TRACK FORECAST HAS CHANGED LITTLE AND IS PLACED NEAR THE CENTER OF THE WESTWARD-TRACKING GUIDANCE ENVELOPE. THIS TRACK LIES SOUTH OF THE ECMWF SOLUTION BUT NEAR THE GOOGLE DEEPMIND GUIDANCE. INTENSITY GUIDANCE SHOWS A RANGE OF INITIAL INTENSIFICATION RATES THROUGH TAU 48, FOLLOWED BY SLIGHT WEAKENING OR SLOW INTENSIFICATION FROM TAU 48 TO TAU 72, AND STEADY INTENSIFICATION THEREAFTER. THE JTWC INTENSITY FORECAST IS SET NEAR THE LOWER END OF THE ENVELOPE AND CLOSEST TO DEEPMIND THROUGH TAU 36, NEAR THE CENTER OF THE SPREAD FROM TAU 48 TO TAU 72, AND NEAR THE UPPER END FROM TAU 72 TO TAU 120. HAFS-A AND COAMPS-TC STRENGTHEN THE SYSTEM TO 105-115 KTS BY TAU 48, WHICH IS POSSIBLE IF THE COMPACT CIRCULATION CAN QUICKLY COMPLETE AXISYMMETRIZATION. DEEPMIND HAS TRENDED STRONGER THIS CYCLE, WITH 60 PERCENT OF ITS ENSEMBLE MEMBERS ATTAINING SUPER TYPHOON INTENSITY BY TAU 120. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN