WTXS31 PGTW 232100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/TROPICAL CYCLONE 18P (NATHAN) WARNING NR 027// RMKS/ 1. TROPICAL CYCLONE 18P (NATHAN) WARNING NR 027 01 ACTIVE TROPICAL CYCLONE IN SOUTHIO MAX SUSTAINED WINDS BASED ON ONE-MINUTE AVERAGE WIND RADII VALID OVER OPEN WATER ONLY --- WARNING POSITION: 231800Z --- NEAR 11.6S 133.5E MOVEMENT PAST SIX HOURS - 255 DEGREES AT 04 KTS POSITION ACCURATE TO WITHIN 030 NM POSITION BASED ON CENTER LOCATED BY SATELLITE PRESENT WIND DISTRIBUTION: MAX SUSTAINED WINDS - 050 KT, GUSTS 065 KT WIND RADII VALID OVER OPEN WATER ONLY RADIUS OF 034 KT WINDS - 045 NM NORTHEAST QUADRANT 050 NM SOUTHEAST QUADRANT 055 NM SOUTHWEST QUADRANT 045 NM NORTHWEST QUADRANT REPEAT POSIT: 11.6S 133.5E --- FORECASTS: 12 HRS, VALID AT: 240600Z --- 11.9S 132.8E MAX SUSTAINED WINDS - 045 KT, GUSTS 055 KT WIND RADII VALID OVER OPEN WATER ONLY VECTOR TO 24 HR POSIT: 255 DEG/ 05 KTS --- 24 HRS, VALID AT: 241800Z --- 12.2S 131.8E MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT WIND RADII VALID OVER OPEN WATER ONLY RADIUS OF 034 KT WINDS - 030 NM NORTHEAST QUADRANT 035 NM SOUTHEAST QUADRANT 035 NM SOUTHWEST QUADRANT 030 NM NORTHWEST QUADRANT VECTOR TO 36 HR POSIT: 260 DEG/ 06 KTS --- 36 HRS, VALID AT: 250600Z --- 12.4S 130.5E MAX SUSTAINED WINDS - 035 KT, GUSTS 045 KT WIND RADII VALID OVER OPEN WATER ONLY VECTOR TO 48 HR POSIT: 265 DEG/ 07 KTS --- EXTENDED OUTLOOK: 48 HRS, VALID AT: 251800Z --- 12.5S 129.0E MAX SUSTAINED WINDS - 035 KT, GUSTS 045 KT WIND RADII VALID OVER OPEN WATER ONLY VECTOR TO 72 HR POSIT: 265 DEG/ 09 KTS --- 72 HRS, VALID AT: 261800Z --- 12.9S 125.5E MAX SUSTAINED WINDS - 040 KT, GUSTS 050 KT WIND RADII VALID OVER OPEN WATER ONLY RADIUS OF 034 KT WINDS - 045 NM NORTHEAST QUADRANT 055 NM SOUTHEAST QUADRANT 055 NM SOUTHWEST QUADRANT 045 NM NORTHWEST QUADRANT VECTOR TO 96 HR POSIT: 265 DEG/ 09 KTS --- LONG RANGE OUTLOOK: --- 96 HRS, VALID AT: 271800Z --- 13.3S 121.9E MAX SUSTAINED WINDS - 045 KT, GUSTS 055 KT WIND RADII VALID OVER OPEN WATER ONLY VECTOR TO 120 HR POSIT: 250 DEG/ 09 KTS --- 120 HRS, VALID AT: 281800Z --- 14.4S 118.5E MAX SUSTAINED WINDS - 045 KT, GUSTS 055 KT WIND RADII VALID OVER OPEN WATER ONLY --- REMARKS: 232100Z POSITION NEAR 11.7S 133.3E. TROPICAL CYCLONE (TC) 18P (NATHAN), LOCATED APPROXIMATELY 162 NM EAST-NORTHEAST OF DARWIN, AUSTRALIA HAS TRACKED WEST-SOUTHWESTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS WEAKENED CONVECTION DISPLACED TO THE SOUTH OF A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). A 231732Z SSMI MICROWAVE IMAGE SHOWS A TIGHTLY WRAPPED LLCC, DESPITE THE LAND INTERACTION, AND FURTHER DEPICTS AN OVERALL REDUCED CONVECTIVE SIGNATURE. UPPER LEVEL ANALYSIS INDICATES A MARGINALLY FAVORABLE ENVIRONMENT WITH LOW TO MODERATE (10-20 KNOT) NORTH- NORTHWESTERLY VERTICAL WIND SHEAR (VWS) WITH GOOD POLEWARD OUTFLOW AS EVIDENT ON WATER VAPOR IMAGERY. THE INITIAL POSITION IS BASED ON THE AFOREMENTIONED MICROWAVE IMAGE AND EIR LOOP, AS WELL AS, THE WARRUWI RADAR LOOP AND OBSERVATIONS ALONG THE ARNHEM LAND COASTLINE. THE INITIAL INTENSITY OF 50 KNOTS IS BASED ON AN AVERAGE OF CURRENT INTENSITY ESTIMATES FROM PGTW, ADRM, AND KNES. TC 18P CONTINUES TO TRACK ALONG THE NORTHERN PERIPHERY OF A DEEP LAYER SUB-TROPICAL RIDGE (STR). HOWEVER, IT IS CURRENTLY TRACKING WEST-SOUTHWESTWARD AND ITS FORWARD SPEED HAS DECREASED AS THE STR HAS WEAKENED DUE TO A PASSING SHORTWAVE TROUGH TO THE SOUTH. ONCE THE TROUGH PASSES, THE STR IS FORECAST TO BUILD BACK IN, CHANGING THE TRACK BACK TO A WESTWARD TRAJECTORY. TC NATHAN IS EXPECTED TO CONTINUE TO WEAKEN OVER THE NEXT 36 TO 48 HOURS DUE TO LAND INTERACTION ALONG THE NORTHERN COAST OF ARNHEM LAND PRIOR TO EMERGING IN THE TIMOR SEA. BEYOND TAU 48, THE SYSTEM WILL SLOWLY RE-INTENSIFY AS IT IS EXPOSED TO SEA SURFACE TEMPERATURES ABOVE 31 CELSIUS AND CONTINUES TO HAVE A MARGINALLY FAVORABLE UPPER LEVEL ENVIRONMENT. BY THE END OF THE FORECAST PERIOD THE CYCLONE WILL GAIN AN ENHANCED POLEWARD OUTFLOW CHANNEL DUE TO AN UPPER LEVEL HIGH AMPLITUDE TROUGH APPROACHING FROM THE WEST. AT THIS TIME, THE SYSTEM WILL ALSO BEGIN TO ROUND THE NORTHWESTERN PERIPHERY OF THE STR. DYNAMIC MODEL GUIDANCE REMAINS IN TIGHT AGREEMENT; THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 231800Z IS 15 FEET. NEXT WARNINGS AT 240900Z AND 242100Z.//