MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 15W (FIFTEEN) WARNING NR 03// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION 15W (NAMTHEUN), LOCATED APPROXIMATELY 162 NM SOUTH OF KADENA AB, HAS TRACKED NORTHEASTWARD AT 13 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY AND A 312220Z 91 GHZ SSMIS IMAGE SHOWS A BAND OF DEEP CONVECTION WRAPPING INTO AN AREA OF FLARING CENTRAL CONVECTION FORMING A DISTINCT COMMA SHAPE, AND GIVING HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY IS ASSESSED AT 30 KNOTS AND IS BASED ON THE IMPROVING CONVECTIVE STRUCTURE AND DVORAK ESTIMATES FROM PGTW AND RJTD REPORTING T2.0 (30 KNOTS). UPPER-LEVEL ANALYSIS INDICATES AN IMPROVING ENVIRONMENT WITH A NEWLY FORMED POLEWARD OUTFLOW CHANNEL AND LOW VERTICAL WIND SHEAR. SOUTHWESTERLY FLOW IS CREATING SOME SUBSIDENCE ON THE WESTERN PERIPHERY AND HINDERING THE EQUATORWARD OUTFLOW. SEA SURFACE TEMPERATURES ARE VERY FAVORABLE, NEAR 30 CELSIUS. CURRENTLY TD 15W IS TRACKING NORTHEAST ALONG THE PERIPHERY OF A MID TO LOW LEVEL NEAR-EQUATORIAL RIDGE (NER) TO THE SOUTH. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. IN THE NEAR-TERM TD 15W WILL CONTINUE TO TRACK NORTHWEST UNDER THE INFLUENCE OF THE STEERING RIDGE FOR THE FIRST 24 HOURS. FROM TAU 24 TO 48 TD 15W WILL ENTER A WEAK STEERING ENVIRONMENT AS THE NER MOVES OFF TO THE SOUTHWEST AND A SUBTROPICAL RIDGE BUILDS IN FROM THE NORTHEAST. INTENSIFICATION IS EXPECTED OVER THE NEXT 24 TO 36 HOURS AS UPPER-LEVEL CONDITIONS IMPROVE OVER FAVORABLE WATERS REACHING A PEAK INTENSITY OF 50 KNOTS. THIS PERIOD OF INTENSIFICATION WILL END ABRUPTLY WHEN THE SUBTROPICAL RIDGE BUILDS SUFFICIENTLY TO STEER TD 15W NORTH-NORTHWESTWARD INTO A DEEPENING TROUGH POSITIONED OVER THE EAST CHINA SEA. HIGH VERTICAL WIND SHEAR WILL WEAKEN TD 15W TO A WEAK TROPICAL STORM BY TAU 72. C. IN THE EXTENDED FORECAST BEYOND TAU 72, TD 15W WILL FURTHER WEAKEN UNDER THE INFLUENCE OF THE UPPER-LEVEL TROUGH BELOW TROPICAL STORM STRENGTH. DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 24 AND WITH OVERALL FORECAST PHILOSOPHY. HOWEVER, DUE TO THE WEAK STEERING ENVIRONMENT AFTER TAU 24, THE TRACKS DIVERGE SOMEWHAT DUE TO UNCERTAINTY IN THE SUBTROPICAL RIDGE BUILDING SUFFICIENTLY TO DOMINATE STEERING. DUE TO THE UNCERTAINTY IN THE STEERING ENVIRONMENT THERE IS LOW CONFIDENCE IN THE FORECAST TRACK.// NNNN NNNN