MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 20W (TWENTY), LOCATED APPROXIMATELY 946 NM EAST OF ANDERSEN AFB, GUAM, HAS TRACKED WESTWARD AT 15 KNOTS KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A FAST DEVELOPING SYSTEM WITH FORMATIVE BANDS, MOSTLY FROM THE SOUTH, WRAPPING INTO AN OBSCURED LOW LEVEL CIRCULATION (LLC). OVERSHOOTING CONVECTIVE TOPS ARE ALSO PROMINENT NEAR THE CENTER. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE BASED ON THE MSI LOOP AND ON A FAINT BUT DISCERNIBLE LLC FEATURE IN THE 050553Z SSMIS 37GHZ COLORIZED IMAGE. THE INITIAL INTENSITY OF 25KTS IS BASED ON THE CONGRUENT DVORAK ESTIMATES OF T1.5/25KTS FROM PGTW, KNES, AND RJTD AND CONSISTENT WITH THE ENHANCED INFRARED SATELLITE SIGNATURE. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM IS IN AN AREA OF LIGHT (05-10KT) NORTHEASTERLY VERTICAL WIND SHEAR AND EXCELLENT RADIAL OUTFLOW WITH A ROBUST EQUATORWARD BIAS. ADDITIONALLY, ALONG-TRACK SSTS ARE HIGHLY CONDUCIVE AT 30C. THE CYCLONE IS TRACKING ALONG THE SOUTHERN PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO THE NORTH. 3. FORECAST REASONING. A. THIS IS THE INITIAL PROGNOSTIC REASONING MESSAGE AND SETS THE FORECAST PHILOSOPHY ON THIS SYSTEM. B. STEERED BY THE STR THROUGHOUT THE FORECAST, TD 20W WILL CONTINUE TRACKING WESTWARD THEN WEST-NORTHWESTWARD AFTER TAU 48. THE AFOREMENTIONED FAVORABLE CONDITIONS WILL PERSIST AND PROMOTE STEADY INTENSIFICATION, REACHING 95KTS BY TAU 72. B. AFTER TAU 72, TD 20W WILL TRACK MORE NORTHWESTWARD AS THE STR WEAKENS IN RESPONSE TO A MIDLATITUDE TROUGH APPROACHING FROM MONGOLIA. THE INCREASED POLEWARD OUTFLOW AHEAD OF THE TROUGH, IN ADDITION TO THE SUSTAINED FAVORABLE ENVIRONMENTAL CONDITIONS, WILL INITIATE RAPID INTENSIFICATION POTENTIALLY REACHING SUPER TYPHOON INTENSITY OF 130KTS BY TAU 120. THE INITIAL SET OF NUMERICAL MODELS IS IN UNUSUALLY GOOD AGREEMENT FOR A DEVELOPING SYSTEM WITH A MINIMAL SPREAD TO 200NM BY END OF FORECAST. HOWEVER, IN VIEW OF THE FORMATIVE NATURE OF THE SYSTEM AND THE UNCERTAINTY IN THE INITIAL STORM TRACK, THERE IS OVERALL LOW CONFIDENCE IN THE JTWC TRACK FORECAST.// NNNN NNNN