MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 004// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 21W (TWENTYONE), LOCATED APPROXIMATELY 555 NM EAST-NORTHEAST OF MANILA, PHILIPPINES, HAS TRACKED NORTHWESTWARD AT 07 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (AMSI) SHOWS A SMALL CIRRUS SHIELD WITH LIMITED SPIRAL RAIN BANDS WRAPPING INTO THE LOW LEVEL CIRCULATION CENTER (LLCC). A 170458Z AMSR2 36 GHZ MICROWAVE IMAGE SHOWED SHALLOW RAIN BANDS WRAPPING TOWARD A NOTCH FEATURE. THERE IS LOW CONFIDENCE IN THE INITIAL POSITION BASED ON A COMBINATION OF THE AMSI AND MICROWAVE IMAGE. THE INITIAL INTENSITY ALIGNS WITH DVORAK ESTIMATES OF T1.5-T2.0 (25-30 KTS) WITH GOOD CONFIDENCE. MODERATE (15-20 KT) VERTICAL WIND SHEAR (VWS), FAIR POLEWARD OUTFLOW, AND WARM (28-29 CELSIUS) SEA SURFACE TEMPERATURE (SST) MAKE THE ENVIRONMENT MARGINALLY FAVORABLE. TD 21W IS TRACKING NORTHWESTWARD TOWARD A BREAK IN A SUBTROPICAL RIDGE (STR) TO THE NORTH. 3. FORECAST REASONING. A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS PROGNOSTIC REASONING MESSAGE. HOWEVER, THE FORECAST HAS BEEN EXTENDED TO TAU 96 AS THE TRACK SPEED IS SLOWER THAN THE PREVIOUS FORECAST. B. BY TAU 12, THE STR IS EXPECTED TO BUILD AND THE TRACK IS EXPECTED TO BECOME WESTWARD WHERE IT SHOULD REMAIN THROUGH TAU 72. THE MARGINAL ENVIRONMENT WILL CONTINUE AND ALLOW FOR SLIGHT INTENSIFICATION. BY TAU 72, INCREASING VWS SHOULD CAUSE THE SYSTEM TO WEAKEN. UNCERTAINTY IN THE MODEL GUIDANCE IS QUITE LARGE, LENDING TO LOW CONFIDENCE IN THIS PORTION OF THE JTWC TRACK FORECAST. C. BEYOND TAU 72, TD 21W SHOULD CONTINUE WESTWARD AND DISSIPATE (TAU 96) AS THE STR TO THE NORTH IS EXPECTED TO BUILD AND THE ENVIRONMENT IS EXPECTED TO BE UNFAVORABLE OVERALL. HOWEVER, THIS FORECAST IS HIGHLY UNCERTAIN AS SEVERAL MODELS MAINTAIN THE WEAKNESS IN THE STR AND ALLOW A POLEWARD TURN. EVEN IF THE POLEWARD TURN WERE TO OCCUR, TD 21W SHOULD DISSIPATE AROUND TAU 96 DUE TO HIGH VWS. BASED ON THE HIGH UNCERTAINTY IN THE MODEL GUIDANCE, THERE IS LOW CONFIDENCE IN THE EXTENDED JTWC TRACK FORECAST.// NNNN NNNN