MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 13W (THIRTEEN) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION 13W (THIRTEEN), LOCATED APPROXIMATELY 165 NM SOUTHWEST OF MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 14 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DISORGANIZED, DEEP CONVECTION OBSCURING A LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE POSITION OF THE LLCC IN THE EIR LOOP AS WELL AS IN A 151349Z ASCAT-B PASS. THE INITIAL INTENSITY OF 25 KTS IS ALSO PLACED WITH HIGH CONFIDENCE AND IS HEDGED SLIGHTLY HIGHER THAN MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF T1.0 (PGTW, 25 KTS) AND T1.5 (RJTD, 25 KTS) BASED ON THE 30 KTS WINDS PRESENT IN THE AFOREMENTIONED SCATTEROMETRY DATA. TD 13W IS CURRENTLY TRACKING ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) THAT IS POSITIONED TO THE NORTHEAST AND THROUGH AN ENVIRONMENT THAT IS FAVORABLE FOR FURTHER DEVELOPMENT WITH VERY WARM (30-32 CELSIUS) SEA SURFACE TEMPERATURES (SST) AND MODERATE EQUATORWARD OUTFLOW ALOFT OFFSET SLIGHTLY BY MODERATE (15-20 KTS) VERTICAL WINDSHEAR (VWS) ALONG THE WESTERN PERIPHERY OF THE SYSTEM. 3. FORECAST REASONING. A. THIS IS THE INITIAL PROGNOSTIC REASONING MESSAGE FOR THIS SYSTEM AND SETS THE FORECAST PHILOSOPHY. B. IN THE NEAR-TERM, TD 13W WILL TRACK GENERALLY WEST-NORTHWESTWARD OVER THE WARM WATERS OF THE SOUTH CHINA SEA TOWARDS TO COAST OF VIETNAM UNDER THE STEERING INFLUENCE OF THE STR POSITIONED TO THE NORTHEAST. THE SYSTEM WILL CONTINUE TO INTENSIFY TO 40 KTS BY TAU 12 AS A RESULT OF CONTINUED VERY WARM SSTS, SUFFICIENT UPPER LEVEL OUTFLOW AND LOW TO MODERATE (10-15 KTS) VERTICAL WIND SHEAR. AFTER TAU 12, THE SYSTEM WILL BEGIN A MORE MODERATED INTENSIFICATION DUE TO MODERATE (15-20 KTS) VWS AND CONVERGENT FLOW DUE TO THE DEEP EASTERLY FLOW ALOFT ALONG THE NORTHERN PERIPHERY OF THE SYSTEM, LEADING TO A PEAK INTENSITY OF 70 KTS BY TAU 48. THEREAFTER, THE SYSTEM WILL WEAKEN SLIGHTLY DUE TO CONTINUED MODERATE VWS AND CONVERGENT FLOW ALONG THE NORTHERN PERIPHERY TO AN INTENSITY OF 65 KTS BY TAU 72. NUMERICAL MODEL SOLUTIONS ARE IN FAIR AGREEMENT BOTH ALONG AND ACROSS TRACK WITH A MAXIMUM SPREAD OF 130 NM AT TAU 48 THAT DECREASES TO 105 NM BY TAU 72. THIS SPREAD IS LIKELY THE RESULT OF THE MODELS RESOLVING AN INCOMING SHORTWAVE TROUGH PROPAGATING FROM THE WEST THAT WILL CAUSE THE STEERING RIDGE TO RECEDE TO THE EAST BETWEEN TAU 48 AND TAU 72. THIS FAIR AGREEMENT IN MODEL SOLUTIONS LENDS OVERALL FAIR CONFIDENCE IN THIS INITIAL PORTION OF THE JTWC FORECAST TRACK. C. IN THE EXTENDED PERIOD, THE SYSTEM WILL CONTINUE TO TRACK GENERALLY WEST-NORTHWESTWARD AND REACH LANDFALL ALONG THE VIETNAM COAST JUST AFTER TAU 72. THEREAFTER THE SYSTEM WILL BEGIN TO WEAKEN RAPIDLY DUE TO CONTINUED MODERATE VWS AND INTERACTION WITH THE UNDERLYING TERRAIN. BY TAU 96 THE SYSTEM INTENSITY WILL WEAKEN TO 30 KTS AND BEGIN TO DISSIPATE OVER LAND. BY TAU 120 CONTINUED TERRAIN INTERACTIONS WILL LEAD TO DISSIPATION OVER LAND. NUMERICAL MODEL SOLUTIONS DIVERGE TO A GREATER EXTENT IN THIS EXTENDED PORTION OF THE FORECAST WITH A MAXIMUM ACROSS TRACK SPREAD OF 260 NM BY TAU 96. THIS HIGHER SPREAD IN TRACK GUIDANCE LENDS ONLY FAIR CONFIDENCE TO THE OVERALL JTWC FORECAST TRACK WHICH IS PLACED ALONG THE MULTI-MODEL CONSENSUS.// NNNN NNNN