WDPN31 PGTW 141500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 04W (FOUR) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 16.8N 118.9E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 179 NM NORTHWEST OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS SIGNIFICANT WAVE HEIGHT: 15 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A BROAD LOW LEVEL CIRCULATION (LLC), WHICH IS SLOWLY CONSOLIDATING BUT RETAINS SOME CHARACTERISTICS OF A MONSOON DEPRESSION SUCH AS A THE HIGHEST WINDS AND THE DEEPEST CONVECTION DISPLACED OUTWARD FROM THE CIRCULATION CENTER. HOWEVER, BASED ON THE MOST RECENT MICROWAVE DATA FROM 140920Z, THE BANDS OF CONVECTION HAVE SLOWLY BEGUN TO DEVELOP WITHIN THE MAGICAL 100NM RADIUS, SUGGESTING THAT THE SYSTEM HAS TRANSITIONED FROM A MONSOON DEPRESSION TO A TROPICAL DEPRESSION. THE MOST RECENT SCATTEROMETER DATA FROM 140720Z SHOWED 25-30 KNOT WINDS ALL ALONG THE SOUTHERN PERIPHERY AS WELL AS TO THE NORTH, ASSOCIATED WITH A SMALL VORTICITY MAXIMUM RACING SOUTHWARD ALONG THE NORTHWEST COAST OF LUZON. THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE DUE TO THE LACK OF RECENT MICROWAVE IMAGERY AND THE BROAD NATURE OF THE LLC WHICH PRECLUDES DEFINING A DEFINITIVE CENTER USING INFRARED IMAGERY. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE T1.5 (25 KNOTS) DVORAK CURRENT INTENSITY ESTIMATES FROM BOTH PGTW AND RJTD. ANALYSIS REVEALS A MICROSCALE POINT SOURCE DEVELOPING OVER TOP OF TD 04W, PUSHING OUTFLOW PREDOMINANTLY TOWARDS THE SOUTHWEST. A SMALL TUTT-CELL IS LOCATED TO THE NORTHWEST, TRAILING AN INVERTED TROUGH TO THE SOUTH, WHICH HAS JUST PASSED WEST OF TD 04W. THE PRESENCE OF THE TUTT-CELL WILL PROVIDE SOME ENHANCED POLEWARD OUTFLOW IN THE SHORT TERM AS IT CONTINUES RACING WESTWARD. OVERALL, THE ENVIRONMENT IS FAVORABLE FOR DEVELOPMENT WITH VERY WARM (31C) SSTS, LOW VERTICAL WIND SHEAR (VWS) AND MODERATE EQUATORWARD OUTFLOW AND A DEVELOPING BURST OF POLEWARD OUTFLOW. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: SOUTHERN SIDE OF A DEEP-LAYER, EAST-WEST ORIENTED SUBTROPICAL RIDGE (STR) COMPLEX LYING ALONG THE 28TH LATITUDE. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.5 - 25 KTS KNES: T1.0 - 25 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 5-10 KTS SST: 30-31 CELSIUS OUTFLOW: MODERATE EQUATORWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM 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: WHILE CURRENTLY STILL QUITE BROAD, THE LLC OF TD 04W IS EXPECTED TO CONTINUE TO CONTRACT OVER THE NEXT 24 HOURS. AS THE SYSTEM REMAINS RELATIVELY WEAK, THE STEERING LEVEL WILL REMAIN BELOW THE 500MB LEVEL, WHERE A COL-REGION IN THE LOW TO MID-LEVELS RESIDES TO THE NORTH, IMPARTING A WEAK STEERING INFLUENCE TO THE SYSTEM. THUS, TD 04W IS EXPECTED TO MEANDER SLOWLY TOWARDS THE NORTHWEST THROUGH THE FIRST 12 TO 18 HOURS OF THE FORECAST, WHILE IT SLOWLY INTENSIFIES. BY TAU 24, THE SYSTEM WILL HAVE INTENSIFIED TO TROPICAL STORM STRENGTH, AS IT TAKES ADVANTAGE OF THE ENHANCED BURST OF POLEWARD AND WESTWARD OUTFLOW INTO THE TUTT-CELL THAT MOVES OFF TO THE WEST. HOWEVER, BY AROUND TAU 30 THE SYSTEM WILL FEEL THE EFFECTS OF SOME HIGHER NORTHEASTERLY SHEAR WHICH WILL SLOW THE RATE OF INTENSIFICATION FOR A SHORT PERIOD. BY TAU 24, A STRONG TUTT-CELL SOUTH OF OKINAWA AND SLIDE WESTWARD TOWARDS TAIWAN THROUGH TAU 48, WHICH WILL SERVE TO BLOCK THE NORTHEASTERLY FLOW ALOFT OVER TD 04W, AND ALLOW IT TO ESTABLISH A STRONG UPPER-LEVEL ANTICYCLONE, WHICH WILL PROVIDE ROBUST RADIAL OUTFLOW. AT THIS POINT, AROUND TAU 48, THE SYSTEM WILL COMMENCE RAPID INTENSIFICATION TO A PEAK OF AT LEAST 90 KNOTS BY TAU 72 AS IT TRACKS WEST TOWARDS THE LUICHOW PENINSULA. THE SYSTEM IS EXPECTED TO MAKE LANDFALL BETWEEN TAUS 72 AND 96, THEN TRACK INTO THE GULF OF TONKIN. AFTER PEAKING AT TAU 72, THE SYSTEM WILL WEAKEN SIGNIFICANTLY AS THE LOW-LEVEL FLOW IS DISRUPTED BY INTERACTION WITH THE TERRAIN, UPPER-LEVEL CONVERGENT FLOW AND INCREASED VWS. AFTER MAKING A SECOND LANDFALL NEAR HAIPHONG, VIETNAM AFTER TAU 96, THE SYSTEM WILL TRACK INLAND AND RAPIDLY WEAKEN OVER NORTHERN VIETNAM THROUGH THE END OF THE FORECAST PERIOD. MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN MODEST AGREEMENT, AT LEAST IN THE FIRST 72 HOURS WITH THE BULK OF THE CONSENSUS GUIDANCE CONFINED TO A GRADUALLY EXPANDING ENVELOPE THAT REACHES 110NM AT TAU 72. ALONG-TRACK SPREAD BEGINS TO APPEAR AFTER TAU 48, WITH THE ECMWF AND ECENS IN PARTICULAR STARTING TO RACE OUT AHEAD OF THE REMAINDER OF THE CONSENSUS MEMBERS. BY TAU 120 THE CROSS-TRACK SPREAD INCREASES TO 330NM BETWEEN THE ECMWF ON THE SOUTH EDGE AND THE GFS ON THE NORTHERN SIDE. OF NOTE, WHILE NOT MEMBERS OF THE TRACK CONSENSUS, THE MESOSCALE MODELS INCLUDING HWRF, AND BOTH THE GFS AND NAVGEM VERSION OF COAMPS-TC, BREAK FROM THE OTHER CONSENSUS MEMBERS AFTER TAU 48, AND TAKE A TRACK NORTH OF THE CONSENSUS ENVELOPE, WITH LANDFALL NEAR WUCHUAN AND DO NOT TRACK OVER THE GULF OF TONKIN. TRACK CONFIDENCE IS MEDIUM THROUGH THE ENTIRE FORECAST PERIOD DUE TO THE MODERATELY HIGH SPREAD IN THE EXTENDED FORECAST. INTENSITY GUIDANCE IS MIXED, WITH THE SHIPS GUIDANCE BEING PARTICULARLY UNEXCITED ABOUT TD 04W, REACHING A PEAK OF JUST 55 KNOTS. MEANWHILE THE MESOSCALE MODELS INCLUDING HWRF, COAMPS-TC (GFS) AND THE CTR1 RAPID INTENSIFICATION AID, INDICATE STEADY, THEN RAPID INTENSIFICATION (RI) TO A PEAK OF 90-95 KNOTS BETWEEN TAUS 60 AND 72, WITH THE CONSENSUS ROUGHLY IN THE MIDDLE OF THE TWO EXTREMES. THE COAMPS-TC ENSEMBLE RI GUIDANCE SHOWS A 50 TO 60 PERCENT PROBABILITY OF RI BETWEEN TAU 48 AND TAU 72, PROVIDING ADDITIONAL CONFIDENCE TO THE PEAK INTENSITY IN THE MESOSCALE GUIDANCE. CONFIDENCE IN THE INTENSITY FORECAST REMAINS MEDIUM HOWEVER, DUE TO THE VERY LARGE SPREAD IN THE GUIDANCE PACKAGE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN