WDPN32 PGTW 310300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING NR 01// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 20W (TWENTY), LOCATED APPROXIMATELY 199 NM NORTH-NORTHWEST OF YAP, HAS TRACKED WESTWARD AT 07 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS THE SYSTEM HAS BEGUN TO CONSOLIDATE AS FORMATIVE BANDS FEEDING IN FROM THE EASTERN AND SOUTHERN FLANKS WRAPPED TIGHTER INTO THE LOW LEVEL CIRCULATION CENTER THAT IS CURRENTLY OBSCURED BY A CENTRAL DENSE OVERCAST CLOUD FEATURE. THE INITIAL POSITION IS BASED ON THE ABOVE ANIMATION AND EXTRAPOLATED FROM A 302308Z 37V SSMIS MICROWAVE PASS. THE INITIAL INTENSITY IS BASED ON THE DVORAK ESTIMATES FROM PGTW AND RJTD. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM IS APPROXIMATELY 04 DEGREES SOUTH OF THE RIDGE AXIS IN AN AREA WITH LOW (05-10 KNOT) VERTICAL WIND SHEAR. ADDITIONALLY, ANIMATED WATER VAPOR IMAGERY SHOWS A ROBUST POLEWARD OUTFLOW INTO A TROUGH TO THE NORTHEAST THAT IS PROVIDING VENTILATION TO THE ASSOCIATED CONVECTION. THE CYCLONE IS TRACKING ALONG THE SOUTHERN PERIPHERY OF AN EXTENSION OF THE LOW- TO MID-LAYERED SUBTROPICAL RIDGE (STR) TO THE NORTHEAST. 3. FORECAST REASONING. A. THIS IS THE FIRST PROGNOSTIC REASONING BULLETIN ON THIS SYSTEM AND SETS THE FORECAST PHILOSOPHY. B. TD 20W WILL CONTINUE TRACKING GENERALLY WESTWARD OVER THE NEXT 12 HOURS. AFTERWARDS, IT IS EXPECTED TO BEGIN TURNING POLEWARD AS THE STEERING STR RECEDES IN RESPONSE TO AN APPROACHING MID-LATITUDE TROUGH APPROACHING FROM THE NORTHWEST. THE FAVORABLE UPPER LEVEL CONDITIONS, IN ADDITION TO VERY WARM SEA SURFACE TEMPERATURES IN THE PHILIPPINE SEA (GREATER THAN 28 CELSIUS) WILL PROMOTE A HIGHER-THAN NORMAL INTENSIFICATION, PEAKING AT 105 KNOTS BY TAU 72. C. AFTER TAU 72, TD 20W IS EXPECTED TO SLOWLY ROUND THE WESTERN EDGE OF THE STEERING STR AND DRIFT INTO A COL AREA AS A SECONDARY STR TO THE WEST BUILDS. THIS WILL RESULT IN A POSSIBLE QUASI-STATIONARY STATE. AFTER TAU 96, THE PRIMARY STR IS EXPECTED TO RE-BUILD AND NUDGE THE CYCLONE BACK ON A POLEWARD TRACK. INCREASING VERTICAL WIND SHEAR DURING THIS STAGE WILL GRADUALLY WEAKEN THE SYSTEM. THE AVAILABLE NUMERICAL GUIDANCE IS IN FAIRLY GOOD AGREEMENT UP TO TAU 72, LENDING GOOD CONFIDENCE TO THAT PORTION OF THE JTWC TRACK FORECAST THAT IS LAID CLOSE TO THE MODEL CONSENSUS. AFTERWARDS, THERE IS A SIGNIFICANT SPREAD IN THE CONSENSUS MEMBERS, LENDING LOW CONFIDENCE TO THE EXTENDED PORTION OF THE FORECAST.// NNNN