WDPN31 PGTW 031500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 24W (TWENTYFOUR) WARNING NR 002// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 10.7N 136.7E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 110 NM NORTHWEST OF YAP MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 18 KTS SIGNIFICANT WAVE HEIGHT: 20 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A NEARLY SYMMETRICAL, COMPACT, CENTRAL DENSE OVERCAST (CDO) WITH VERY COLD CLOUD TOPS OF AROUND -70C ACROSS THE MAJORITY OF THE CDO. THE CDO BEGAN TO FORM OUT OF CONVECTIVE TOWERS AROUND 030900Z. THE CONVECTIVE PEAK APPEARS TO HAVE OCCURRED AROUND 031030Z WHEN CLOUD TOPS COOLED TO NEAR -93C, ACCOMPANIED BY A MASSIVE BURST OF LIGHTNING ACTIVITY. SINCE THEN, THE CDO HAS BECOME A BIT MORE IRREGULAR AND THE COLDEST CLOUD TOPS OF WARMED A BIT BUT THE CDO REMAINS VERY SYMMETRICAL. THE MOST RECENT MICROWAVE IMAGE WAS A 030818Z GMI PASS, WHICH SHOWED AN ILL-DEFINED MASS OF CONVECTION WITH SOME WEAK AND FRAGMENTED SPIRAL BANDS WRAPPING IN TOWARDS THE CENTER. THE LACK OF MORE RECENT MICROWAVE IMAGERY OR SCATTEROMETER DATA TO SEE UNDER THE CDO RESULTED IN A LOW CONFIDENCE INITIAL POSITION. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, HEDGED HIGHER THAN THE AGENCY SUBJECTIVE DVORAK ESTIMATES IN LIGHT OF THE IMPROVED STRUCTURE AND CONSISTENT WITH THE 36 KNOT CIMSS DPRINT ESTIMATE. THE SYSTEM CONTINUES TO ENJOY GENERALLY FAVORABLE ENVIRONMENTAL CONDITIONS, CHARACTERIZED BY VERY WARM SSTS, AND GOOD POLEWARD OUTFLOW AND LOW SOUTHEASTERLY VERTICAL WIND SHEAR, THOUGH MID-LEVEL SHEAR DOES SEEM TO BE SLIGHTLY HIGHER. INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE NORTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS RJTD: T1.0 - 25 KTS KNES: T2.0 - 30 KTS CIMSS ADT: 31 KTS AT 031140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 30-31 CELSIUS OUTFLOW: MODERATE POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: LOW INITIAL INTENSITY: MEDIUM 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) 24W (TWENTYFOUR) WILL CONTINUE TO TRACK NORTHWESTWARD AT A STEADY 17 KNOTS FOR THE NEXT 36 HOURS, ALONG THE SOUTHWESTERN PERIPHERY OF THE STEERING RIDGE FAR TO THE NORTHEAST. AROUND TAU 48, A FAIRLY DEEP UPPER-LEVEL TROF WILL PASS THROUGH THE KOREAN PENINSULA, INDUCING A BREAK IN THE STEERING RIDGE EAST OF TAIWAN AND NORTH OF TS 24W. AS THE STEERING GRADIENT WEAKENS AND REORIENTS, TS 24W WILL SLOW DOWN, WHILE CONTINUING TO TRACK NORTHWESTWARD. AT ABOUT THE SAME TIME, A COLD-SURGE EVENT BEGINS TO PUSH NEAR-GALE FORCE NORTHEASTERLY FLOW DOWN THE EAST COAST OF TAIWAN AND INTO THE SOUTH CHINA SEA. TS 24W WILL RUN HEAD-ON INTO THIS COLD SURGE BY TAU 72, WHICH WILL BRING ITS FORWARD (NORTHWESTWARD) ADVANCE TO A RATHER RAPID HALT. DEEP-LAYER STEERING REMAINS RELATIVELY WEAK AT THIS POINT, AT LEAST IN THE GFS FIELDS, AND THE PRIMARY INFLUENCE ON STEERING WILL BE THE LOW- TO MID-LEVEL NORTHEASTERLIES, WHICH WILL PUSH TS 24W ONTO A MORE WEST-SOUTHWESTWARD TRACK AFTER TAU 72. BY TAU 96 THE SYSTEM BEGINS TO PICK UP SPEED AS A STR SOUTHWEST OF HONG KONG RAPIDLY BUILDS AND EXTENDS NORTHEASTWARD OVER TAIWAN, WITH TS 24W TRACKING TO A POSITION NEAR BABUYAN ISLAND BY TAU 120. AN ALTERNATE SCENARIO, AS SHOWN IN THE ECMWF FIELDS, HAS A STRONGER RIDGE TO THE WEST, FURTHER EAST IN THE SOUTH CHINA SEA, WHICH EFFECTIVELY BLOCKS MUCH WESTWARD MOVEMENT AFTER TAU 72, WITH THE ECMWF TRACKER STAGNATING NEAR THE TAU 72 FORECAST POSITION. IN TERMS OF INTENSITY, THE SYSTEM WILL INTENSIFY GRADUALLY FOR THE FIRST 24 HOURS OF THE FORECAST DUE TO THE MARGINAL MID-LEVEL SOUTHEASTERLY SHEAR AND RAPID TRANSLATION SPEED. THE PACE OF INTENSIFICATION PICKS UP A NOTCH AFTER TAU 36 AS THE UPPER-LEVEL FLOW IMPROVES AND BEGINS TO TAP INTO A POLEWARD OUTFLOW CHANNEL INTO THE STRONG MID-LATITUDE WESTERLIES. THE PEAK OF 80 KNOTS IS FORECAST BY TAU 72, AND THEREAFTER THE INFLUX OF RELATIVELY DRY AND COOL AIR AND A TRACK OVER THE COLD WAKE LEFT BEHIND BY STY 23W WILL LEAD TO A SLOW BUT STEADY WEAKENING TREND THROUGH THE REMAINDER OF THE FORECAST. MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE REMAINS IN GOOD AGREEMENT THROUGH TAU 72, WITH ALL OF THE CONSENSUS MEMBERS SHOWING A NORTHWESTWARD TRACK, WITH MINIMAL SPREAD UNTIL TAU 48, WHEN THE VARIOUS MEMBERS BEGIN TO SPREAD OUT TO A 190NM WIDE ENVELOPE BY TAU 72. THE GALWEM AND UKMET ENSEMBLE MARK THE WESTERN SIDE OF THE ENVELOPE, WHILE THE NAVGEM AND ECMWF MARK THE EASTERN SIDE. AFTER TAU 72, THE TRACKERS START TO SHOW MORE SIGNIFICANT SEPARATION. THE ECMWF TURNS SHARPLY NORTHWARD TO TAU 96 THEN TURNS BACK WEST BUT AT A VERY SLOW PACE, BASICALLY COMING TO A FULL STOP NEAR 127E. THE GFS, GEFS AND JGSM TURN THE SYSTEM SHARPLY WEST-SOUTHWESTWARD AFTER TAU 72, WITH THE GFS ACCELERATING RAPIDLY, TO PLACE THE CENTER OFF THE NORTHWESTERN TIP OF LUZON BY TAU 120, INCREASING THE SPREAD TO OVER 400NM BY TAU 120. ENSEMBLE GUIDANCE IS SIMILARLY MIXED, WITH THE BULK OF THE GEFS MEMBERS SUPPORTING A WESTWARD TURN AFTER TAU 72, THOUGH ABOUT 9 MEMBERS SUPPORT A RECURVE. THE ECENS SHOWS ABOUT A 50-50 SPLIT BETWEEN WEST TURNING MEMBERS AND RECURVING MEMBERS. THE JTWC FORECAST REMAINS CONSISTENT WITH THE PREVIOUS FORECAST AND THE CONSENSUS MEAN BUT CONFIDENCE REMAINS LOW AFTER TAU 72 DUE TO THE ROUGHLY 50-50 SPLIT IN CONSENSUS AND ENSEMBLE MEMBERS. IN TERMS OF INTENSITY, GUIDANCE IS IN GOOD AGREEMENT ON THE SHAPE OF THE CURVE, BUT THE JTWC FORECAST IS ON THE LOWER EDGE OF THE ENVELOPE AND CLOSE TO THE CONSENSUS MEAN WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN