MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN) WARNING NR 01// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION (TD) 18W (EIGHTEEN), LOCATED APPROXIMATELY 31 NM SOUTH OF HAGATNA, GUAM, HAS TRACKED WEST-NORTHWESTWARD AT 15 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY AND RADAR IMAGERY INDICATE A CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC) WITH IMPROVED CONVECTIVE STRUCTURE. A 111723Z SSMI 85GHZ IMAGE REVEALS MULTIPLE CONVECTIVE BANDS WRAPPING INTO AN ELONGATED, WELL-DEFINED CENTER. THE MOST RECENT SURFACE OBSERVATIONS FROM PGUM SHOW SUSTAINED SURFACE WINDS OF 25 TO 30 KNOTS WITH GUSTS UP TO 35 TO 40 KNOTS. THE CURRENT INTENSITY IS ASSESSED AT 30 KNOTS BASED ON THE IMPROVED CONVECTIVE STRUCTURE AND SURFACE WIND REPORTS FROM GUAM; THE DVORAK INTENSITY ESTIMATES OF T1.5 (25 KNOTS) ARE SLIGHTLY LOWER THAN THE CURRENT INTENSITY ESTIMATE. UPPER-LEVEL ANALYSIS DEPICTS A VERY FAVORABLE ENVIRONMENT WITH AN ENHANCED POLEWARD OUTFLOW CHANNEL ASSOCIATED WITH A BROAD TUTT CELL TO THE NNE AND LOW VERTICAL WIND SHEAR. TD 18W IS TRACKING ALONG THE SOUTHERN PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE (STR) ENTRENCHED TO THE NORTH. 3. FORECAST REASONING. A. THIS IS THE FIRST PROGNOSTIC REASONING MESSAGE FOR 18W AND SETS THE FORECAST PHILOSOPHY. B. TD 18W IS FORECAST TO TRACK WEST-NORTHWESTWARD TO NORTHWESTWARD THROUGH TAU 72. THE STR IS FORECAST TO ERODE WITH THE SLOW APPROACH OF A MAJOR SHORTWAVE TROUGH (CURRENTLY OVER EASTERN CHINA AND IMPACTING THE FORECAST TRACK OF TY 16W). THIS FORECAST SCENARIO IS CONSISTENT WITH THAT OF TY 16W AS BOTH SYSTEMS WILL BE AFFECTED BY THE MAJOR SHORTWAVE TROUGH AS IT PROGRESSES EASTWARD. DYNAMIC GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 72 WITH A 100-NM SPREAD IN SOLUTIONS AT TAU 72. TD 18W IS FORECAST TO INTENSIFY RAPIDLY OVER THE NEXT TWO TO THREE DAYS DUE TO THE ROBUST POLEWARD OUTFLOW ASSOCIATED WITH THE AFOREMENTIONED TUTT CELL, WHICH IS EXPECTED TO TRACK WESTWARD WITH THE SYSTEM. CONSEQUENTLY, PEAK INTENSITY SHOULD BE NEAR 115 KNOTS (OR HIGHER) BY TAU 72. C. AFTER TAU 72, TD 18W IS FORECAST TO TRACK POLEWARD TOWARD OKINAWA ALONG THE WESTERN PERIPHERY OF THE POLEWARD-ORIENTED STR. DYNAMIC GUIDANCE DIVERGES WITH A SPREAD OF 300-NM IN SOLUTIONS AT TAU 120. DYNAMIC GUIDANCE INDICATES INCREASING UNCERTAINTY WITH MODEL SOLUTIONS JUST EAST OF ISHIGAKI-JIMA TO JUST EAST OF OKINAWA; THESE SOLUTIONS ARE ALL VIABLE CONSIDERING THE COMPLEX TIMING OF THE MAJOR SHORTWAVE TROUGH OVER EASTERN CHINA AND THE EAST CHINA SEA. TD 18W SHOULD WEAKEN GRADUALLY AS IT INTERACTS WITH THE TROUGH. OVERALL, THERE IS LOW CONFIDENCE (LARGE MODEL SPREAD) IN THE JTWC FORECAST, PARTICULARLY AFTER TAU 72.// NNNN NNNN