WDPN31 PGTW 030900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 24W (TWENTYFOUR) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 9.7N 138.2E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 13 NM NORTH-NORTHEAST OF YAP MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 17 KTS SIGNIFICANT WAVE HEIGHT: 19 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A COMPACT SYSTEM WITH A SMALL BURST OF DEEP CONVECTION FIRING OFF NEAR THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC), PASSING OVER OR VERY NEAR YAP. A 030430Z AMSR2 COLOR ENHANCED 89GHZ MICROWAVE IMAGE REVEALED FRAGMENTED CONVECTIVE BAND WRAPPING INTO A SMALL LLCC JUST SOUTHEAST OF YAP AT THAT TIME. SURFACE OBSERVATIONS FROM YAP SHOW WINDS BACKING FROM NORTHEASTERLY AT 030000Z TO NORTH-NORTHWESTERLY BY 030500Z AND THEN WEST-NORTHWESTERLY AT 030600Z AND FINALLY WESTERLY BY 030700Z. MEAN SEA LEVEL PRESSURE BOTTOMED OUT AT 1003.9MB AT 030600Z AND THE COMBINATION OF WIND SHIFTS AND PRESSURE READINGS STRONGLY SUGGEST THE LLCC PASSED JUST EAST AND THEN NORTH OF YAP AROUND 0600Z. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON AN EXTRAPOLATION OF THE LLCC AS SEEN IN THE AMSR2 IMAGERY AS WELL AS ANALYSIS OF THE SURFACE OBSERVATIONS NOTED ABOVE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, HEDGED HIGHER THAN THE T1.5 DVORAK CURRENT INTENSITY ESTIMATES FROM PGTW AND KNES IN LIGHT OF EARLIER ASCAT AND OTHER SCATTEROMETER DATA SHOWING A LARGE BAND OF SOUTHERLY WINDS IN THE EASTERN HEMISPHERE EXCEEDING 25-30 KNOTS, WHICH ARE STARTING TO WRAP INTO THE NORTHERN PORTION OF THE CIRCULATION. THE IS FAVORABLE FOR INTENSIFICATION, WITH LOW TO MODERATE (15-20 KNOTS) OF SHEAR, WARM SSTS, AND MODERATE POLEWARD OUTFLOW ALOFT. THE SYSTEM IS MOVING NORTHWEST AT A GOOD CLIP OF 17 KNOTS ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE NORTHEAST. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER STR FAR TO THE NORTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.0 - 25 KTS KNES: T1.5 - 25 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 15-20 KTS SST: 30-31 CELSIUS OUTFLOW: MODERATE POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 24W (TWENTYFOUR) WILL CONTINUE TO TRACK NORTHWESTWARD AT A STEADY 16 TO 17 KNOTS, ALONG THE SOUTHWESTERN PERIPHERY OF THE STRONG STR TO THE NORTHEAST, FOR THE NEXT 48 TO 60 HOURS. AROUND TAU 60, THE SYSTEM WILL BEGIN TO SLOW DOWN AS A PASSING MID-LATITUDE TROUGH MOVES BY TO THE NORTH, INDUCING A BREAK IN THE RIDGE TO THE EAST OF TAIWAN. THE SYSTEM IS ANTICIPATED TO BE STRONG ENOUGH BY THAT POINT TO RESIST THE URGE TO PULL TOO FAR NORTH INTO THE FAIRLY WEAK BREAK IN THE STEERING RIDGE, BUT A THE STEERING GRADIENT WEAKENS, TD 24W WILL SLOW DOWN. BY TAU 72, A RELATIVELY SHALLOW (EXTENDING UP TO ABOUT 850-700MB) WEDGE OF NORTHERLY FLOW WILL DIVE SOUTHWARD AS PART OF A COLD SURGE EVENT. THIS NORTHERLY FLOW WILL SERVE TO SLOW THE FORWARD PROGRESS OF TD 24W EVEN MORE, WITH TRACK SPEEDS EXPECT TO BE DOWN TO ABOUT THREE KNOTS AFTER TAU 72. WHILE THE DEEP LAYER STEERING FLOW REMAINS RELATIVELY WEAK AFTER TAU 72, THE LOW- TO MID-LEVEL NORTH-NORTHEASTERLIES WILL HELP TO FORCE TD 24W ONTO A MORE WESTWARD, OR EVEN WEST-SOUTHWESTWARD TRACK AFTER TAU 72. BY TAU 96, THE STEERING GRADIENT WILL TIGHTEN UP ONCE AGAIN AS BUILDING STR OVER HONG KONG EXTENDS EASTWARD OVER TAIWAN, WHICH WILL ACCELERATE TD 24 TO THE WEST THROUGH THE END OF THE FORECAST PERIOD. IN TERMS OF INTENSITY, WHILE THE ENVIRONMENT IS FAVORABLE, IT IS NOT EXCEPTIONALLY SO, AND MID-LEVEL SHEAR COULD BE A PROBLEM THROUGH MOST OF THE FORECAST PERIOD. THUS, SLOW BUT STEADY INTENSIFICATION IS EXPECTED THROUGH TAU 72, REACHING A PEAK OF 75 KNOTS BY THAT POINT. AS THE SYSTEM CRASHES INTO THE COLD SURGE AFTER TAU 72, RELATIVELY COOL AND DRY AIR WILL START TO GET INGESTED INTO THE WESTERN PORTION OF THE SYSTEM, THE ONLY QUESTION IS HOW FAR INTO THE CORE WILL IT PENETRATE, WHICH WILL BE HIGHLY DEPENDENT UPON THE STRENGTH OF THE SYSTEM AT THAT POINT. INTENSITY IS EXPECTED TO PEAK AND THEN SLOWLY DECREASE AFTER TAU 96 AS THE SYSTEM MOVES INTO COOLER WATERS LEFT BEHIND BY STY 23W, WHICH COMBINED WITH THE DRY AIR ENTRAINMENT WILL LEAD A STEADILY WORSENING ENVIRONMENT. MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE SHOWS A MODERATE AMOUNT OF SPREAD THROUGH TAU 72, THOUGH ALL AVAILABLE GUIDANCE DOES AGREE ON A GENERAL NORTHWESTWARD TRACK THROUGH THIS POINT IN THE FORECAST. THE UKMET ENSEMBLE MEAN IS THE FURTHEST WEST OF THE TRACKERS, WHILE NAVGEM IS THE MOST POLEWARD, WITH A 230NM SPREAD BETWEEN THEM. AFTER TAU 72, THINGS GET A BIT SQUIRRELLY, THOUGH ALL MODELS DO AGREE THAT THE SYSTEM WILL SLOW DOWN AT THIS POINT. WHERE THEY TAKE THE SYSTEM AFTER THE SLOWDOWN VARIES WILDLY HOWEVER. THE GFS DRIVES THE SYSTEM WEST-SOUTHWESTWARD AND OVER NORTHERN LUZON BY TAU 120, WHILE THE NAVGEM CONTINUES TRACKING THE SYSTEM NORTHWARD TOWARDS MIYAKOJIMA, LEADING TO A 400NM SPREAD BY THE END OF THE FORECAST PERIOD. THE GEFS AND ECENS MEAN TRACKERS ALONG WITH THE CONSENSUS (NOT INCLUDING NAVGEM) TAKE THE SYSTEM WESTWARD INTO THE EASTERN PART OF THE LUZON STRAIT. THE JTWC FORECAST LIES ON THE NORTHERN EDGE OF GUIDANCE PACKAGE THROUGH TAU 72, THEN MOST CLOSELY FOLLOWS THE GEFS, ECENS AND CONSENSUS MEAN THROUGH TAU 120. INTENSITY GUIDANCE IS IN GOOD AGREEMENT ON A STEADY RATE OF INTENSIFICATION TO TAU 84, FOLLOWED BY A WEAKENING TREND THEREAFTER. THE JTWC FORECAST IS CONSISTENT WITH THE CONSENSUS MEAN THROUGH THE ENTIRE FORECAST. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: HIGH INTENSITY 72-120 HR: MEDIUM// NNNN