WDPN31 PGTW 190900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 30W (FENGSHEN) WARNING NR 006// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 14.8N 119.7E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 76 NM WEST OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 16 KTS SIGNIFICANT WAVE HEIGHT: 17 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL STORM (TS) 30W HAS NOW MOVED OFFSHORE INTO THE SOUTH CHINA SEA AFTER PASSING SOUTH OF MANILA AND IS PRIMED FOR RAPID INTENSIFICATION. TRACKING OF THE LOW LEVEL CIRCULATION CENTER (LLCC) HAS BEEN A CHALLENGE AS THE SYSTEM PASSED ACROSS LUZON AND LOST A WELL-DEFINED LLCC. SURFACE OBSERVATIONS FROM THE MANILA REGION, SUBIC BAY, CLARK AB AND IBA HAVE PROVEN TO BE HIGHLY BENEFICIAL TO PLACEMENT OF THE LLCC. SURFACE WINDS AT THE LAST THREE ABOVE ARE NOW SOUTHERLY OR SOUTHWESTERLY, PROVIDING HIGH CONFIDENCE THAT THE LLCC IS SOMEWHERE TO THE WEST OF ALL THREE. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A FAST-DEVELOPING LLCC WITH DEEP CONVECTION AND VORTICAL HOT TOWERS CONGEALING INTO BANDING FEATURES ALONG THE SOUTHERN AND EASTERN FLANKS OF THE LLCC. RADAR DATA HAS BEEN INCONCLUSIVE ALL DAY BUT IS NOW BEGINNING TO TIGHTEN UP AND SHOWING WHAT COULD BE A MID-LEVEL VORTEX BEGINNING TO TAKE SHAPE JUST WEST OF IBA. SOME OF THE MESOSCALE MODELS SUGGESTED THIS VERY SCENARIO, WITH A MID-LEVEL ROTATION DISPLACED NORTH OF THE LLCC DUE TO SOME MODERATE SOUTHERLY MID-LEVEL SHEAR. THE INITIAL POSITION IS ASSESSED WITH LOW CONFIDENCE DUE TO THE LACK OF RECENT MICROWAVE IMAGERY OR RELIABLE SCATTEROMETER DATA AND IS BASED MOSTLY ON THE SURFACE OBSERVATIONS AND MSI LOOP. THE INITIAL INTENSITY IS ASSESSED WITH LOW CONFIDENCE AS WELL AND COULD BE A BIT HIGHER, WITH THE BULK OF AGENCY FIXES RIDING ABOUT 10 KNOTS HIGHER AND THE AVAILABLE OBJECTIVE ESTIMATES AVERAGING RIGHT AROUND 35 KNOTS. THE ENVIRONMENT HAS IMPROVED NOW THAT THE SYSTEM IS MOVING OFFSHORE, WITH ZESTY WATERS TO FEED OFF, STRONG RADIAL OUTFLOW AND LOW DEEP-LAYER SHEAR. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE NORTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS RJTD: T2.0 - 30 KTS RCTP: T3.0 - 45 KTS KNES: T3.0 - 45 KTS DEMS: T3.0 - 45 KTS CIMSS SATCON: 38 KTS AT 190400Z CIMSS D-MINT: 32 KTS AT 190524Z CIMSS D-PRINT: 32 KTS AT 190600Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 0-5 KTS SST: 29-30 CELSIUS OUTFLOW: STRONG RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: LOW INITIAL INTENSITY: LOW INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TS 30W IS FORECAST TO TRACK RELATIVELY QUICKLY TOWARDS THE NORTHWEST ALONG THE SOUTHWESTERN SIDE OF THE STR TO THE NORTHEAST. THE SYSTEM WILL CONTINUE TRACKING NORTHWESTWARD FOR ABOUT 36 HOURS, BEFORE IT MOVES INTO A WEAK STEERING ENVIRONMENT, CAUGHT BETWEEN COMPETING RIDGES TO THE NORTHEAST AND THE WEST. AT THE SAME TIME, THE SYSTEM WILL SMACK HEADLONG INTO STRONG NORTHEASTERLY FLOW FUNNELING SOUTH OUT OF THE TAIWAN STRAIT. AS THE SYSTEM CONTINUES TO PUSH SLOWLY WESTWARD, THESE NORTHEASTERLY WINDS WILL STRENGTHEN AS THEY ARE TRAPPED BETWEEN TS 30W AND COASTAL CHINA, AND AS THEY DO SO, WILL EFFECTIVELY BLOCK TS 30W FROM PROGRESSING FURTHER TO THE NORTHWEST. BY TAU 48, THE SYSTEM WILL HAVE REACHED ITS FURTHER NORTHWESTWARD POINT AND QUICKLY TURN SOUTHWESTWARD, AHEAD OF THE LOW-LEVEL NORTHEASTERLY FLOW PUSHING OUT OF CHINA. THE SYSTEM IS EXPECTED TO MAKE LANDFALL ALONG THE COAST OF CENTRAL VIETNAM PRIOR TO TAU 96, BEFORE PROGRESSING INTO EASTERN CAMBODIA BY THE END OF THE FORECAST PERIOD. NOW THAT THE SYSTEM HAS MOVED OVER WATER AND MOVING FURTHER AWAY FROM LUZON, STRONG RADIAL OUTFLOW ALOFT, WARM WATERS AND LOW SHEAR WILL ALLOW FOR RAPID VORTEX SYMMETRIZATION AND INTENSIFICATION OVER THE NEXT 24 HOURS. THE FORECAST CALLS FOR THE SYSTEM TO REACH PEAK INTENSITY WITHIN 36 HOURS, THOUGH A FASTER RATE OF INTENSIFICATION AND A HIGHER PEAK INTENSITY ARE CERTAINLY A POSSIBILITY IF THE SYSTEM CAN QUICKLY ORGANIZE. AFTER TAU 48 HOWEVER, SHEAR IS EXPECTED TO INCREASE AND THE INFLUX OF RELATIVELY COOL, DRY AND STABLE AIR ASSOCIATED WITH THE NORTHEASTERLY FLOW INTO THE WESTERN FLANK OF THE SYSTEM WILL MARK THE ONSET OF A RAPID WEAKENING PHASE. BY THE TIME IT MAKES LANDFALL ALONG THE COAST OF VIETNAM THE SYSTEM WILL BE A MINIMAL TROPICAL STORM, WITH A RELATIVELY SHALLOW VORTEX, HAVING BEEN ENGULFED BY DRY AIR AND SHATTERED BY MODERATE EASTERLY SHEAR. AFTER LANDFALL, THE SYSTEM WILL RAPID WEAKEN AND DISSIPATE NO LATER THAN TAU 120 AND LIKELY MUCH EARLIER. MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, WHICH HAS BEEN DISCARDED FROM THE CONSENSUS DUE TO ITS HIGHLY UNLIKELY WESTWARD TRACK, THE REMAINDER OF THE GUIDANCE PACKAGE IS IN RELATIVELY GOOD AGREEMENT, AT LEAST THROUGH TAU 48. AT THAT POINT IN THE FORECAST, CROSS-TRACK SPREAD IS JUST 75NM BETWEEN THE JGSM AND GOOGLE DEEP MIND (FNV3) MODEL ON THE NORTHERN EDGE, AND THE GEFS MEAN ON THE SOUTH. ALONG-TRACK SPREAD UP TO THIS POINT IN THE FORECAST IS MINIMAL. BUT THINGS GET HAIRY AFTER TAU 48, WITH SIGNIFICANT AND RAPID ALONG- AND CROSS-TRACK DISPERSION IN THE MODEL GUIDANCE. AT TAU 72, THERE IS A 250NM ALONG-TRACK SPREAD BETWEEN THE JGSM WHICH SHOWS LANDFALL IN VIETNAM AT THIS POINT AND THE EC-AIFS WHICH LAGS FAR BEHIND, LINGERING IN THE SCS. THE SITUATION ONLY GETS WORSE GOING FORWARD, SUCH THAT BY TAU 120, THERE IS OVER 500NM OF ALONG-TRACK SPREAD BETWEEN THE GALWEM IN THE GULF OF THAILAND AND THE ECMWF JUST ONSHORE IN SOUTHERN VIETNAM. ENSEMBLE MODELS ALL SHOW SIMILAR TRACK SHAPES BUT ALSO DIFFER FAIRLY WIDELY ON THE SPEED OF THE TRACK AFTER TAU 48, LEADING TO SIGNIFICANT ALONG-TRACK SPREAD AFTER THAT POINT IN THE FORECAST. THE JTWC FORECAST IS POSITIONED ON THE NORTHERN SIDE OF THE GUIDANCE ENVELOPE, CLOSE TO THE FNV3 TRACKER TO TAU 48, THEN CLOSER TO THE ECMWF DETERMINISTIC THEREAFTER. CONFIDENCE IS HIGH TO TAU 48, BUT LOW THEREAFTER DUE TO THE LARGE AMOUNT OF UNCERTAINTY IN THE MODEL GUIDANCE AFTER TAU 72. INTENSITY GUIDANCE IS IN GOOD AGREEMENT, WITH ALL CONSENSUS MEMBERS INDICATING THE PEAK INTENSITY OCCURRING AT TAU 48, WITH A RANGE BETWEEN 50 AND 65 KNOTS, FOLLOWED BY RAPID WEAKENING. TWO RAPID INTENSIFICATION (RI) AIDS (RIPA AND FRIA) ARE TRIPPED, AND THE JTWC FORECAST IS PLACED NEAR THE RIPA FORECAST TO TAU 24, THEN NEAR THE CONSENSUS MEAN THEREAFTER. CONFIDENCE IS MEDIUM IN THE NEAR-TERM AND LOW IN THE LONG-TERM. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN