MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 01// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 21W (TWENTYONE), LOCATED APPROXIMATELY 214 NM NORTHWEST OF NAVSTA GUAM, HAS TRACKED WESTWARD AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED IR SATELLITE SHOWS PERSISTENT SYMMETRICAL DEEP CONVECTION AS WELL AS A LOW LEVER SPINNER TO THE EAST. DUE TO THE MULTIPLE VORTICES PRESENT, THERE IS LOW CONFIDENCE IN THE INITIAL POSITIONING OF THE SYSTEM WHICH IS PLACED AT THE CENROID OF THE VORTICES. HOWEVER, DUE TO THE CONVECTIVE STRUCTURE AND AN OLDER 211246Z RSCAT IMAGE, THERE IS GOOD CONFIDENCE IN THE INITIAL INTENSITY OF 25 KNOTS. UPPER LEVEL ANALYSIS SHOWS THAT THERE IS 10 TO 20 KNOTS OF VWS OFFSET BY GOOD DIVERGENCE ALOFT. THE SYSTEM IS CURRENTLY TRACKING ON THE SOUTHERN SIDE OF A STR TO THE NORTH. 3. FORECAST REASONING. A. THIS IS THE INITIAL PROGNOSTIC REASONING AND SETS THE FORECAST PHILOSOPHY. B. TD 21W WILL CONTINUE TO TRACK TO THE WEST UNDER THE INFLUENCE OF THE STR THROUGH TAU 36. BEYOND THAT, A SHORTWAVE THROUGH WILL CAUSE A SMALL BREAK IN THE STR AND ALLOW TD 21W TO TAKE A MORE POLEWARD TRACK. EXPECT MARGINAL INTENSIFICATION THROUGH TAU 48 AS THE SYSTEM CONTINUES TO CONSOLIDATE. HOWEVER THE SYSTEM IS EXPECTED TO EXCEED TYPHOON STRENGTH BY TAU 72. C. IN THE EXTENDED FORECAST, TD 21W WILL CONTINUE TO INTENSIFY AS POLEWARD OUTFLOW IMPROVES. THE SYSTEM WILL CONTINUE TO TRACK TO THE NORTHWEST THROUGH A BREAK IN THE RIDGE. DYNAMIC MODEL GUIDANCE IS VARIED WITH REGARD TO BOTH THE SPEED OF TRACK AND THE POINT AT WHICH THE BREAK IN THE RIDGE DEVELOPS. AS SUCH, THERE IS LIMITED CONFIDENCE IN THE JTWC FORECAST TRACK AT THIS TIME.// NNNN NNNN