WDPN31 PGTW 210300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 01W (SANVU) WARNING NR 007// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 10.0N 156.2E INITIAL INTENSITY: 45 KTS GEOGRAPHIC REFERENCE: 700 NM EAST-SOUTHEAST OF ANDERSEN AFB MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 02 KTS SIGNIFICANT WAVE HEIGHT: 20 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL AND WATER VAPOR IMAGERY BOTH CONFIRM THAT TROPICAL STORM SANVU (01W) HAS PEAKED AND BEGUN WHAT IS EXPECTED TO BE ITS VERY SLOW AND GRADUAL DISSIPATION OVER WATER. THE SYSTEM HAS SLOWED AS IT TRACKED AWAY FROM ITS PREVIOUS STEERING INFLUENCE OF THE NEAR EQUATORIAL RIDGE AND HAS YET TO COME UNDER THE INFLUENCE OF ITS NEXT STEERING FORCE, THE SUBTROPICAL RIDGE. THE INTENSITY OF 45KTS IS HELD STEADY FROM THE PREVIOUS FORECAST BUT ALL SUBJECTIVE AND OBJECTIVE MEASURES ARE SHOWING THE FIRST CONFIRMATIONS OF DECLINING IN VALUES. AS AN EXAMPE THE BEST PERFORMING OBJECTIVE MEASURE THIS STORM, THE CIMMS OPEN-AIIR, SHOWED A 7KT DECLINE OVER THE PAST SIX HOURS. THE JTWC DVORAK HELD STEADY BUT THE DATA T DECLINED ONE HALF A T NUMBER AND ADT RAW VALUES ARE ALSO DECLINING. THE INTENSITY ASSESSMENT IS BASED ON THE JTWC PATTERN T THE AIDT VALUE OF 43KTS AND THE OPEN-AIIR MEDIAN OF 41KTS. TS SANVU CONTINUES TO SHOW DISPERSED CLUSTERS OF CONVECTION OVER THE NORTHEAST QUADRANT THAT ARE ROBBING THE CORE OF ENERGY. A DARK LINE IN ANIMATED WATER VAPOR IMAGERY IS CLARIFYING AND ENCROACHING OVER THE NORTHWESTERN QUADRANT CONFIRMING INCREASING SUBSIDENCE UPSTREAM. OTHER THAN THE NORTHWEST QUADRANT, OUTFLOW REMAINS VIGOROUS. STORM POSITION IS FIXED ALONG TRACK UNDER THE COLDEST TOPS IN THE MIDDLE OF THREE AGENCY FIXES. THE 35NM SPREAD BETWEEN THE JTWC AND RJTD IS WIDER THAN AVERAGE BUT TO BE EXPECTED IN THE ABSENCE OF EYE. THE MOST RECENT SCATTEROMETRY PASSES WERE A MISS LEAVING LESS CERTAINTY THAN WITH THE PREVIOUS FORECAST. INITIAL WIND RADII BASIS: PERSISTENCE FROM THE 12Z SCATTEROMETRY PASSES. CURRENT STEERING MECHANISM: SANVU IS A GAP BETWEEN THE NEAR EQUATORIAL RIDGE AND TRACKING TOWARDS THE INFLUENCE OF THE SOUTHEASTERN FLANK OF THE SUBTROPICAL RIDGE. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS RJTD: T2.5 - 35 KTS KNES: T3.0 - 45 KTS CIMSS SATCON: 51 KTS AT 202101Z CIMSS ADT: 53 KTS AT 202330Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 5-10 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: HIGH INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: ALTHOUGH ONE MAY WISH IT WAS MORE EMPHATIC, THE EVIDENCE THAT TROPICAL STORM SANVU HAS BEGUN ITS DECLINE IS MOUNTING. THE SYSTEM IS JUST NOW CROSSING THE 10TH LATITUDE AND WILL SOON FEEL THE STRETCH OF INCREASING VERTICAL WINDSHEAR (VWS) AND DRY AIR ENTRAINMENT. DRY AIR IS WRAPPING INTO THE SOUTHWEST QUADRANT AND ITS EFFECTS ARE ALREADY BECOMING APPARENT IN ANIMATED VISUAL IMAGERY. VWS WILL INCREASE TO OVER 20KTS NORTH OF THE 11TH LATITUDE AND THERE IS A SHARPLY INCREASING VWS GRADIENT POLEWARD OF THAT. COMBINED WITH A LACK OF DEEP MOISTURE IN THE COLUMN THAT IS SHOWING IN BOTH TOTAL PRECIPITABLE WATER LOOPS AND MID-LEVEL MOISTURE MODELS, TS 01W HAS TOO MUCH ENVIRONMENTAL HOSTILITY TO OVERCOME TO MAKE IT TO THE MARIANAS. THE NEXT 12 HOURS WILL PROVIDE CRITICAL CONFIRMATION TO THE DOWNTREND. MODEL DISCUSSION: MODEL GUIDANCE HAS REMAINED STEADY OVER THE PAST SIX HOURS. THE JTWC TRACK FORECAST STAYS CLOSE TO THE MULTIMODEL CONSENSUS AND PREVIOUS FORECASTS WHILE THE INTENSITY FORECAST STAYS BELOW THE CONSENSUS AND CLOSER TO THE COAMPSTC HWRF AND GFS VALUES. THE STATISTICAL DYNAMICAL GUIDANCE REMAINS STUBBORNLY HIGH DUE TO A LOW ASSESSMENT OF VWS THROUGH TAU 36. ALL OTHER INTENSITY GUIDANCE ARE CLEAR ON BEGINNING A DOWNTREND AND SLOWLY DISSIPATING THE STORM OVER WATER. THE JTWC FORECAST STAYS BELOW THE INTENSITY CONSENSUS AND CLOSER TO THE COAMPS-TC AND HWRF SOLUTIONS. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN