WDPN33 PGTW 150900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NESAT) WARNING NR 003// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 18.5N 124.0E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 291 NM NORTHEAST OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: WESTWARD AT 22 KTS SIGNIFICANT WAVE HEIGHT: 13 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A LOW LEVEL CIRCULATION CENTER (LLCC) WHICH IS PARTIALLY EXPOSED ON ITS EASTERN EXTENT AND MULTIPLE CENTERS OF ROTATION, OR SPINNERS, ROTATING AROUND THE CENTRAL LLCC. ANIMATED MSI ALSO REVEALS A WELL-DEFINED MID-LEVEL VORTEX, WITH PERSISTENT CONVECTION FLARING UP NEAR THE ASSESSED CENTER. THE MOST RECENT FRAMES OF ENHANCED INFRARED SUGGEST DEVELOPMENT OF VORTICAL HOT TOWERS NEAR THE MID-LEVEL ROTATION CENTER, WITH CLOUD TOP TEMPERATURES DOWN TO -85C. LATE RECEIPT OF A 150733Z SSMIS COLOR 91GHZ MICROWAVE IMAGE CONFIRMED THE PRESENCE OF THE DISPLACED LLCC TO THE EAST OF THE DEEP CONVECTION, AND PROVIDED HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY IS ASSESSED ABOVE THE AVAILABLE AGENCY AND OBJECTIVE FIXES WITH LOW CONFIDENCE. ADT IS USING THE SHEAR TECHNIQUE, AND THE POSITION IS WELL TO THE EAST OF THE BEST TRACK AND HENCE IS UNRELIABLE AT PRESENT, THOUGH THE AIDT IS ASSESSED AT 34 KNOTS. THE SYSTEM IS MOVING WESTWARD ALONG THE SOUTHERN SIDE OF A DEEP SUBTROPICAL RIDGE (STR) TO THE NORTH. TRACK SPEED INDICATES 22 KNOTS, BUT THIS IS DUE TO A RELOCATION OF THE CENTER AND NOT THE ACTUAL FORWARD TRACK SPEED, ESTIMATED TO BE MORE IN THE 10-15 KNOT RANGE. THE MOST RECENT MSI SUGGESTS THE SYSTEM HAS SLOWED AN ADDITIONAL MARGIN SINCE THE BEST TRACK. THE OVERALL ENVIRONMENT IS FAVORABLE FOR DEVELOPMENT WITH SHEAR ASSESSED AT 10-15 KNOTS FROM THE EAST, WARM SSTS AND MODERATE WESTWARD OUTFLOW, BUT MID-LEVEL DRY AIR IS WRAPPING IN FROM THE SOUTH AND EAST, INHIBITING CONVECTIVE CONSOLIDATION IN THOSE SECTORS AND SLOWING DEVELOPMENT. INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE NORTH. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS RJTD: T2.0 - 30 KTS KNES: T2.0 - 30 KTS CIMSS ADT: 31 KTS AT 150550Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE WESTWARD OTHER FACTORS: MID-LEVEL DRY AIR ENTRAINMENT ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH 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: NOW TROPICAL STORM NESAT HAS BEEN RELOCATED WITH THIS FORECAST, MUCH CLOSER TO THE NORTHEAST TIP OF LUZON THAN PREVIOUSLY EXPECTED BY THIS POINT IN THE FORECAST. THUS TRACK SPEED APPEARS TO HAVE JUMPED, BUT THIS WILL SMOOTH OUT WITH THE NEXT POSITION AND IS NOT REPRESENTATIVE OF THE ACTUAL TRACK SPEED. OVER THE NEXT 24 HOURS, THE STEERING RIDGE TO THE NORTH WILL SLIDE TO THE EAST AND THE SOUTHWEST EDGE WILL BECOME ORIENTED NORTHWEST TO SOUTHEAST. AT THE SAME TIME, PASSAGE OF A TROUGH OVER EASTERN CHINA WILL INDUCE A WEAKNESS IN THE RIDGE AND THE COMBINATION OF THESE FACTORS MEANS TS 23W WILL ASSUME A TRACK TOWARDS THE WEST-NORTHWEST AFTER TAU 12, SKIRTING THE NORTHERN COAST OF LUZON. AROUND TAU 36 THE SYSTEM WILL REACH AN INFLECTION POINT, WITH A STRENGTHENING STR TO THE WEST TAKING OVER THE STEERING REGIME AND INDUCING STRONG NORTHEASTERLY WINDS THAT WILL FUNNEL DOWN THE TAIWAN STRAIGHT. BY TAU 48, TS 23W WILL TURN SOUTHWESTWARD IN RESPONSE TO THE SHIFT IN STEERING RIDGE AND THE STRONG PUSH OF NORTHEASTERLY WINDS DOWN THE STRAIT. THE STEERING PATTERN CHANGES ONCE AGAIN BY TAU 72, WITH THE RIDGE TO THE NORTH SLIDING EASTWARD, ALLOWING TS 23W TO BEGIN A TURN TOWARDS THE WEST-NORTHWEST BY TAU 120 AS IT SKIRTS THE SOUTH SIDE OF HAINAN. IN THE NEAR-TERM, IF THE HOT TOWERS CURRENTLY FIRING NEAR THE MID-LEVEL VORTEX CAN MOVE UPSHEAR, THE SYSTEM IS LIKELY TO INTENSIFY A SIGNIFICANT AMOUNT PRIOR TO REACHING THE NORTHEAST TIP OF LUZON. THE PACE OF INTENSIFICATION WILL SLOW A BIT AS IT SLIDES PAST LUZON DUE TO INTERRUPTION OF THE INFLOW DUE TO TERRAIN INTERACTION. BUT ONCE BACK OVER OPEN WATERS, THE PRESENCE OF AN UPPER-LEVEL POINT SOURCE PROVIDING EXHAUST ALOFT AND THE IMPARTATION OF ENHANCED VORTICITY FROM THE LOW-LEVEL NORTHEAST SURGE, WHICH REMAINS RELATIVELY MOIST THROUGH THE MID-LEVELS, WILL ALLOW FOR FURTHER INTENSIFICATION TO A PEAK OF 80 KTS BY TAU 72. THEREAFTER THE SYSTEM WILL START TO FEEL THE EFFECTS OF INCREASING SHEAR, AND DRY AIR ENTRAINMENT PUSHING INTO THE SOUTHERN SIDE OFF THE COAST OF VIETNAM AND BEGIN TO WEAKEN THROUGH THE END OF THE FORECAST PERIOD. MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN FAIRLY GOOD AGREEMENT ON THE OVERALL TRACK SCENARIO. GFS AND HWRF DO NOT SHOW THE CURVE TO THE WEST-NORTHWEST HOWEVER AND MARK THE SOUTHERN SIDE OF THE ENVELOPE, TAKING THE SYSTEM OVER THE COAST OF NORTHERN LUZON. THEY REJOIN THE PACK BY TAU 48, LEADING TO A CROSS-TRACK SPREAD OF 90NM THROUGH TAU 72. AT THIS POINT THE MODELS BEGIN TO DIVERGE A BIT, ESPECIALLY IN THE ALONG-TRACK DIRECTION, WITH THE ECMWF IN PARTICULAR SHOWING A MUCH FASTER TRACK SPEED AFTER TAU 48, WELL AHEAD OF THE REST OF THE GUIDANCE. CROSS-TRACK SPREAD MARGINALLY INCREASES TO 175NM BY TAU 120, FROM CENTRAL VIETNAM (UKMET ENSEMBLE) TO CENTRAL HAINAN (NVGM). THE JTWC TRACK LIES ALONG THE SOUTHERN SIDE OF THE ENVELOPE THROUGH TAU 36, NEAR THE CONSENSUS MEAN TO TAU 72 AND THEN ON THE NORTHERN EDGE THROUGH TAU 120. CONFIDENCE IS MEDIUM IN THE NEAR-TERM, TRENDING TO LOW THEREAFTER. INTENSITY GUIDANCE IS MIXED, WITH A VERY LARGE SPREAD EXTENDING FROM 50 KNOTS ON THE LOW SIDE (CTCX) TO 90 KNOTS ON THE HIGH SIDE (HWRF). THE JTWC FORECAST MOST CLOSELY TRACKS THE HWRF SOLUTION BUT REMAINS ABOUT 10 KNOTS LOWER THAN THE PEAK. ENSEMBLE PROBABILITIES INDICATE A 10 PERCENT CHANCE OF RAPID INTENSIFICATION BETWEEN TAU 24 AND 48. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN