MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (FOUR) WARNING NR 03// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 04W (FOUR), LOCATED APPROXIMATELY 417 NM NORTH-NORTHWEST OF KAYANGEL, PALAU HAS TRACKED NORTHWESTWARD AT 06 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION IS BASED ON LOW-LEVEL BANDING EVIDENT IN RECENT ANIMATED MULTI-SPECTRAL SATELLITE IMAGERY AND A 290445Z AMSR 89GHZ MICROWAVE IMAGE. THE INTENSITY OF 35 KNOTS IS HEDGED SLIGHTLY ABOVE RECENT SUBJECTIVE DVORAK ESTIMATES OF T2.0 FROM PGTW AND KNES, IN LINE WITH 290045Z SATCON AND A 290610Z ADT ESTIMATE OF 35 KNOTS. TS 04W HAS TRACKED NORTHWESTWARD ALONG THE PERIPHERY OF A SUBTROPICAL STEERING RIDGE SITUATED TO THE NORTH AND EAST OVER THE PAST SIX HOURS. DEEP CONVECTION CONTINUES TO FLARE OVER THE SOUTHWESTERN PORTION OF THE LOW LEVEL CIRCULATION CENTER (LLCC) AS THE SYSTEM EXPERIENCES PERSISTENT NORTHEASTERLY VERTICAL WIND SHEAR (VWS). THE SEA SURFACE TEMPERATURE IN VICINITY OF THE LLCC IS VERY SUPPORTIVE AT 29 TO 30 DEGREES CELSIUS. 3. FORECAST REASONING. A. EXTENDED RANGE FORECAST INTENSITIES HAVE BEEN SLIGHTLY INCREASED FROM THE PREVIOUS WARNING. HOWEVER, THERE IS NO MAJOR CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS WARNING. B. TS 04W IS EXPECTED TO TRACK NORTHWESTWARD IN THE NEAR TERM UNDER THE CONTINUED STEERING INFLUENCE OF THE SUBTROPICAL RIDGING TO THE NORTH AND EAST. VWS WILL STAY MODERATE THROUGH TAU 36, UNTIL GRADUALLY INCREASING AS THE SYSTEM APPROACHES TAIWAN, LIMITING INTENSIFICATION TO A SLOW RATE DESPITE PASSAGE OVER VERY WARM WATER AND FAVORABLE SOUTHWESTWARD DIFFLUENCE ALOFT. NUMERICAL MODEL TRACK FORECAST GUIDANCE REMAINS SPLIT, WITH A GROUPING CONSISTING OF THE GEFS ENSEMBLE AND HWRF MODELS PREDICTING A TRACK TO THE SOUTH OF TAIWAN AND THE ECMWF, UKMET, NAVGEM AND AFUM MODELS PREDICTING A TRACK FARTHER TO THE NORTH. THE CURRENT FORECAST IS CONSERVATIVELY CONSISTENT WITH THE PREVIOUS JTWC TRACK FORECAST AND CURRENT MULTI-MODEL CONSENSUS THROUGH TAU 72. THE EXACT TRACK WILL DEPEND ON THE ORIENTATION OF THE AFOREMENTIONED SUBTROPICAL STEERING RIDGE DURING THIS PERIOD. C. TS 04W IS EXPECTED TO BEGIN WEAKENING IN THE EXTENDED RANGE IN RESPONSE TO INCREASING VWS AND LAND INTERACTION. NUMERICAL MODEL TRACK FORECAST SPREAD IS PARTICULARLY PRONOUNCED IN THE EXTENDED RANGE. A FEW CONSENSUS MODELS DEPICT TS 04W TURNING WESTWARD AND INTERACTING WITH A DEVELOPING CIRCULATION IN THE NORTHERN SOUTH CHINA SEA, WHILE OTHER MODELS DEPICT TS 04W MAINTAINING A NORTHWESTWARD TRACK OR TURNING POLEWARD ALONG THE STEERING RIDGE PERIPHERY. THE CURRENT TRACK FORECAST IS SET NORTHWESTWARD BASED ON EXPECTATIONS OF SLOW BUT SUFFICIENT NEAR TERM INTENSIFICATION. OVERALL, THERE IS LOW CONFIDENCE IN THE CURRENT JTWC TRACK FORECAST.// NNNN NNNN