WDPN33 PGTW 080900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 14W (CHAN-HOM) WARNING NR 006// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 33.0N 154.9E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 793 NM EAST-SOUTHEAST OF MISAWA, JAPAN MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 15 KTS SIGNIFICANT WAVE HEIGHT: 16 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC), WITH THE WESTERN PERIPHERY OF THE SYSTEM OBSCURED BY A POCKET OF DEEP CONVECTION AND ASSOCIATED CIRRUS BLOWOFF, AND THE EASTERN SEMICIRCLE FULLY EXPOSED. ANIMATED TOTAL PRECIPITABLE WATER (TPW) IMAGERY, AS WELL AS A 080656Z SSMIS 91GHZ MICROWAVE IMAGE, REVEAL A LARGE WEDGE OF EXTREMELY DRY MID-LEVEL AIR ENTRAINED ALONG THE EASTERN FLANK. ANIMATED WATER VAPOR IMAGERY AND ATMOSPHERIC MOTION VECTOR (AMV) DATA VERIFY THE PRESENCE OF AN UPPER-LEVEL LOW POSITIONED JUST SOUTHWEST OF THE SURFACE LLCC, USHERING IN THE DRY MID-LEVEL AIR WHILE INDUCING CONVERGENCE ALOFT OVER THE SOUTHEASTERN PORTION OF TS 14W. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN THE MSI. THE INITIAL INTENSITY IS ASSESSED WITH HIGH CONFIDENCE BASED ON A BLEND OF THE SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED BELOW. THE ENVIRONMENT IS MARGINAL FOR INTENSIFICATION. SSTS ARE GENERALLY SUFFICIENT TO SUPPORT DEVELOPMENT, WHILE DEEP-LAYER SHEAR IS LOW. HOWEVER, THE PRESENCE OF THE UPPER-LEVEL LOW IN CLOSE PROXIMITY AND THE ASSOCIATED DRY AIR ENTRAINMENT AND UPPER CONVERGENCE ARE OFFSETTING THE OTHERWISE FAVORABLE FACTORS. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF THE MID-LEVEL SUBTROPICAL RIDGE (STR) CENTERED SOUTHWEST OF THE ALEUTIANS. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS RJTD: T1.5 - 25 KTS RCTP: T3.0 - 45 KTS CIMSS SATCON: 40 KTS AT 080510Z CIMSS ADT: 35 KTS AT 080600Z CIMSS AIDT: 34 KTS AT 080600Z CIMSS D-MINT: 35 KTS AT 080656Z CIMSS D-PRINT: 33 KTS AT 080710Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 10-15 KTS SST: 27-28 CELSIUS OUTFLOW: WEAK RADIAL OTHER FACTORS: UPPER-LEVEL LOW POSITIONED TO THE SOUTHWEST; CONVERGENCE ALOFT; DRY AIR ENTRAINMENT. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TROPICAL STORM (TS) 14W WILL CONTINUE TO TRACK WESTWARD ALONG THE SOUTHERN FLANK OF A LOW- TO MID-LEVEL STR FOR THE NEXT 12 TO 24 HOURS. THE SYSTEM WILL BEGIN TO DECELERATE AFTER TAU 12, AS THE RIDGE BEGINS TO ERODE FROM THE WEST DUE TO AN APPROACHING UPPER-LEVEL TROUGH MOVING EAST FROM JAPAN. AFTER TAU 24, THE SYSTEM SLOWS DOWN TO 5-6 KNOTS WHILE TURNING NORTHWESTWARD AS A WEAKNESS DEVELOPS IN THE RIDGE OVER HOKKAIDO, WITH THE RIDGE REORIENTING ALONG A NORTHWEST-TO-SOUTHEAST AXIS. BY TAU 72, THE TROUGH AND ASSOCIATED WEAKNESS IN THE RIDGE MOVE EAST AND ARE REPLACED BY A NEW RIDGE DEVELOPING OVER FAR EASTERN RUSSIA, WHICH WILL DRIVE TS 14W ONTO A WESTWARD TRACK ACROSS THE NORTHERN PORTION OF HONSHU. LANDFALL ALONG THE COAST OF HONSHU, IN THE GENERAL VICINITY OF SENDAI, JAPAN, IS EXPECTED NEAR TAU 72. THEREAFTER, THE SYSTEM RAPIDLY MOVES INTO THE SEA OF JAPAN (SOJ) ALONG THE STRONG GRADIENT ON THE SOUTHERN SIDE OF THE BUILDING RIDGE OVER EASTERN RUSSIA. AS THE SYSTEM WEAKENS AND SHALLOWS OUT IN THE EXTENDED FORECAST, THE STEERING GRADIENT WEAKENS, ALLOWING THE SYSTEM TO DECELERATE THROUGH TAU 120. REGARDING INTENSITY, TS 14W WILL FACE MARGINAL TO MARGINALLY UNFAVORABLE CONDITIONS FOR THE NEXT 12 TO 24 HOURS, AS IT MOVES NORTHWESTWARD GENERALLY IN ALIGNMENT WITH THE UPPER-LEVEL LOW. AROUND TAU 24, THE UPPER LOW OPENS UP INTO A TROUGH AND BEGINS TO DECOUPLE FROM TS 14W, ALLOWING FOR MODEST IMPROVEMENT IN THE OVERALL ENVIRONMENT. BY TAU 48, ANOTHER UPPER-LEVEL LOW DEVELOPS NEAR SENDAI, WITH A TROUGH EXTENDING SOUTHEASTWARD TO THE SOUTH OF TS 14W, OPENING UP A ROBUST, NEAR-JET-LEVEL POLEWARD OUTFLOW CHANNEL. TS 14W WILL TAP INTO THIS OUTFLOW CHANNEL AND INTENSIFY TO A PEAK OF 50 KNOTS BY TAU 60. FURTHER EVOLUTION OF THE UPPER-LEVEL PATTERN PLACES TS 14W ON THE NORTHWESTERN (CONVERGENT) SIDE OF THE UPPER LOW BY TAU 72, AND THE SYSTEM WILL WEAKEN SLIGHTLY AS IT APPROACHES LANDFALL. THE SYSTEM WILL BEGIN A RAPID SUBTROPICAL TRANSITION (STT) AS IT CROSSES HONSHU AND MOVES INTO THE SOJ, COMPLETING STT NO LATER THAN TAU 120. THE SYSTEM WILL STEADILY WEAKEN AS IT UNDERGOES STT. MODEL DISCUSSION: TRACK GUIDANCE IS IN OVERALL GOOD AGREEMENT ON THE TRACK SCENARIO. ALL GUIDANCE POINTS TO A WESTWARD TRACK THROUGH TAU 24, FOLLOWED BY A NORTHWESTWARD TRAJECTORY THROUGH LANDFALL AT TAU 72. CROSS-TRACK SPREAD GRADUALLY INCREASES TO 250NM AT TAU 72, WITH GALWEM POSITIONED ON THE NORTHERN EDGE OF THE ENVELOPE AND THE ECMWF ON THE SOUTHERN FLANK. THE ECMWF IS A SIGNIFICANT SOUTHERN OUTLIER, TRACKING THE SYSTEM OVER THE VICINITY OF IWAKI, DISPLACED NEARLY 100NM FROM THE PRIMARY MODEL CLUSTER. ENSEMBLE GUIDANCE DEPICTS A FAIRLY SIGNIFICANT AMOUNT OF UNCERTAINTY THROUGH TAU 72, WITH MEMBER DISPERSION EXTENDING FROM HOKKAIDO IN THE NORTH TO SOUTH OF TOKYO IN THE SOUTH. THE JTWC FORECAST IS PLACED NEAR THE EXPERIMENTAL AI CONSENSUS MEAN WITH MEDIUM CONFIDENCE, DECREASED DUE TO THE LARGE ENSEMBLE MEMBER DISPERSION. IN THE EXTENDED RANGE, BOTH THE CONSENSUS PACKAGE AND THE SUPER ENSEMBLE SHOW INCREASED SPREAD AND DISPERSION, LENDING LOW CONFIDENCE TO THE JTWC FORECAST TRACK, WHICH IS POSITIONED NEAR THE AI CONSENSUS MEAN. INTENSITY GUIDANCE IS IN GOOD AGREEMENT OVERALL. HOWEVER, THE JTWC FORECAST IS PLACED LOWER THAN ALL THE GUIDANCE SAVE FOR GOOGLE DEEPMIND THROUGH TAU 36, BEFORE JOINING THE CONSENSUS ENVELOPE (WHICH PEAKS AT 50 KNOTS AT TAU 60) THROUGH THE REMAINDER OF THE FORECAST. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN