WDPN31 PGTW 171500 MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 06W WARNING NR 04// RMKS/ 1. FOR METEOROLOGISTS. 2. 12 HOUR SUMMARY AND ANALYSIS. A. TROPICAL DEPRESSION (TD) 06W (SIX), LOCATED APPROXIMATELY 430 NM EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED NORTH- WESTWARD AT 13 KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY SHOWS THE DEEP CONVECTION ASSOCIATED WITH THE SYSTEM CONTINUES TO BE SHEARED TO THE WEST OF THE LOW LEVEL CIRCULATION CENTER. THE INITIAL POSITION IS BASED ON A 170921Z SSMIS MICROWAVE IMAGE AND THE INITIAL INTENSITY IS SUPPORTED BY DVORAK ESTIMATES FROM PGTW, KNES AND RJTD. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM IS THE SOUTHWEST OF THE NEAR EQUATORIAL RIDGE (NER) AXIS IN AN AREA OF MODERATE 15-20 KNOT VERTICAL WIND SHEAR (VWS). TD 06W IS CURRENTLY STEERING ALONG THE SOUTHWESTERN PERIPHERY OF THE LOW- TO MID-LEVEL SUBTROPICAL RIDGE (STR) TO THE NORTHEAST. 3. FORECAST REASONING. A. NO SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST PHILOSOPHY. B. THE SYSTEM IS EXPECTED TO CONTINUE TRACKING NORTHWESTWARD ALONG THE ABOVE-MENTIONED STEERING RIDGE. AS IT GAINS LATITUDE, IT WILL MOVE INTO A REGION OF MORE FAVORABLE VWS AS IT GETS CLOSER TO THE NER AXIS. CONCURRENTLY, OUTFLOW IS EXPECTED TO IMPROVE WITH THE APPROACH OF A MIDLATITUDE TROUGH FROM THE NORHTWEST. THESE FACTORS WILL FAVOR A GRADUAL, ALBEIT MARGINAL INTENSIFICATION OF THE TD INTO A TROPICAL STORM. C. BEYOND TAU 72, TD 06W WILL DIAGONALLY CROSS THE LUZON STRAIT JUST SOUTH OF TAIWAN, THEN INTO THE CHINESE COASTLINE. INCREASED VWS ASSOCIATED WITH THE PREVAILING WESTERLIES WILL WEAKEN THE SYSTEM TO A TROPICAL DISTURBANCE BEFORE MAKING LANDFAL. THE AVAILABLE NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT UP TO TAU 48, AFTERWHICH THE MODELS SPREAD OUT WITH ECMWF AND WBAR TO THE LEFT OF AND NOGAPS TO THE RIGHT OF THE ENVELOPE. THIS TRACK FORECAST IS JUST TO THE RIGHT OF CONSENSUS TO OFFSET AN UNLIKELY WESTWARD TRACK.// NNNN