MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 002// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 21W (TWENTYONE), LOCATED APPROXIMATELY 532 NM EAST-NORTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT 06 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS FLARING CONVECTION LOCATED OVER A DEVELOPING LOW LEVEL CIRCULATION (LLC). THE MAJORITY OF THE ASSOCIATED CONVECTION IS LOCATED IN THE NORTHEAST QUADRANT OF THE SYSTEM WITH SOME CONVECTIVE BANDING, SUPPORTED BY 161838Z SSMI 37GHZ MICROWAVE IMAGE. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE BASED ON EIR AND THE AFOREMENTIONED MICROWAVE IMAGE. THE INITIAL INTENSITY IS SET AT 25 KTS BASED ON THE WIND FIELD OBSERVED IN THE 161204Z ASCAT-C IMAGE AND DVORAK CURRENT INTENSITY (CI) ESTIMATES OF T1.5/25 KTS FROM PGTW AND RJTD. TD 21W IS TRANSITING THROUGH AN AREA OF LOW (10-15 KTS) VERTICAL WIND SHEAR (VWS) WITH GOOD POLEWARD AND EQUATORWARD OUTFLOW. SEA SURFACE TEMPERATURES (SST) ARE FAVORABLE AT 28-29 CELSIUS. TD 21W IS TRACKING ALONG THE SOUTHERN PERIPHERY OF THE LOW-LEVEL SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTH. 3. FORECAST REASONING. A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. UNDER THE STEERING INFLUENCE OF THE AFOREMENTIONED STR, TD 21W WILL TRACK WEST-NORTHWESTWARD THEN WESTWARD THROUGH TAU 36. AFTERWARDS, INTERACTION WITH A NORTHEAST SURGE EVENT IS EXPECTED TO TURN THE SYSTEM WEST-SOUTHWESTWARD. UPPER LEVEL DIVERGENCE IS SLIGHTLY OFFSET TO THE NORTHEAST AND WILL SLOWLY DECREASE OVER THE NEXT 36-48 HOURS. THUS, TD 21W IS FORECAST TO HAVE A BRIEF PERIOD FOR INTENSIFICATION TO 30 KNOTS PRIOR TO GRADUALLY DISSIPATING AS IT INTERACTS WITH THE NORTHEAST SURGE AND TERRAIN ON LUZON. MODEL GUIDANCE IS LIMITED TO AEMN, ECMWF AND NVGM DUE TO THE WEAK NATURE OF THIS SYSTEM. MODELS ARE IN GOOD AGREEMENT WITH NVGM AS THE SOUTHERNMOST OUTLIER, LENDING TO HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN