WDPN31 PGTW 070300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W (THIRTEEN) WARNING NR 10// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 13W (THIRTEEN), LOCATED APPROXIMATELY 640 NM NORTH- NORTHWEST OF WAKE ISLAND, HAS TRACKED NORTH-NORTHWESTWARD AT 07 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A TIGHTLY-WRAPPED, LOW-LEVEL CIRCULATION CENTER (LLCC) WITH LIMITED, WEAK CONVECTION OVER THE SOUTHERN SEMI-CIRCLE. A 062214Z SSMIS IMAGE DEPICTS DEFINED SHALLOW CONVECTIVE BANDING WRAPPING INTO A SLIGHTLY ELONGATED LLCC. ANIMATED WATER VAPOR IMAGERY SHOWS A TUTT CELL PERSISTING OVER THE SYSTEM, WHICH IS HINDERING THE FORMATION OF CENTRAL CONVECTION, AND INCREASING DRY AIR WRAPPING INTO THE CORE. DESPITE SOME SUBTROPICAL CHARACTERISTICS, TS 13W APPEARS TO BE PRIMARILY A TROPICAL, WARM- CORE SYSTEM AS SUPPORTED BY THE FOLLOWING EVIDENCE: THE MOST RECENT (061800Z) AMSU CROSS-SECTION INDICATES A +2C WARM-CORE TEMPERATURE ANOMALY; A 062326Z OCEANSAT IMAGE SHOWS A SYMMETRIC WIND FIELD WITH 40-45 KNOT WINDS AND A RADIUS OF MAXIMUM WINDS OF APPROXIMATELY 70- 80 NM, TYPICAL OF A TROPICAL SYSTEM; THE TOTAL PRECIPITABLE WATER IMAGERY INDICATES A SMALL YET DEEP MOISTURE ENVELOPE OVER THE LLCC WITH DRY AIR WRAPPING INTO THE SYSTEM; AND, AS PREVIOUSLY DISCUSSED, WEAK CONVECTIVE BANDING HAS PERSISTED ON THE EQUATORWARD SIDE OF THE LLCC. THE CURRENT INTENSITY IS ASSESSED AT 45 KNOTS BASED ON THE SATELLITE SIGNATURE AND RECENT SCATTEROMETER DATA. THERE IS GOOD CONFIDENCE IN THE INITIAL POSITION BASED ON THE EXPOSED CENTER. UPPER-LEVEL ANALYSIS INDICATES THAT TS 13W IS LOCATED SOUTH OF THE SUBTROPICAL RIDGE AND IS UNDER WEAK VERTICAL WIND SHEAR WITH GOOD POLEWARD OUTFLOW, ENHANCED BY A STRONG COLD-CORE LOW NEAR 25N 175W. 3. FORECAST REASONING. A. BASED ON THE DYNAMIC MODELS AND INTENSITY GUIDANCE, THE SYSTEM IS NOW EXPECTED TO TRANSITION INTO A SUBTROPICAL LOW BY TAU 72 AND IS ALSO FORECAST TO MAINTAIN GALE-FORCE WINDS. B. TS 13W IS EXPECTED TO TRACK POLEWARD UNDER THE STEERING INFLUENCE OF THE SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST. THE SYSTEM IS FORECAST TO TRACK OVER INCREASINGLY COOLER SST (< 25C) NEAR TAU 48 AND WILL ALSO BEGIN TO INTERACT WITH THE EASTERN PERIPHERY OF A SHORTWAVE TROUGH EXPECTED TO DEEPEN EAST OF JAPAN. THIS SHORTWAVE TROUGH IS ASSOCIATED WITH THE SUBTROPICAL JET AND HAS WEAK BAROCLINICITY, THEREFORE, THE SYSTEM IS NOT EXPECTED TO UNDERGO EXTRA-TROPICAL TRANSITION AND SUBTROPICAL TRANSITION IS THE MOST LIKELY RESULT. SLIGHT WEAKENING IS FORECAST BUT TS 13W IS EXPECTED TO MAINTAIN 35-45 KNOT INTENSITY THROUGH TAU 72. BY TAU 72, TS 13W IS FORECAST TO COMPLETE TRANSITION INTO A SUBTROPICAL, COLD-CORE LOW AND WILL WEAKEN SLIGHTLY OVER COLD SST (< 20C) WHILE MAINTAINING GALE-FORCE WINDS. THERE IS HIGH CONFIDENCE IN THE FORECAST TRACK DUE TO TIGHTLY GROUPED OBJECTIVE AIDS.// NNNN