WDXS32 PGTW 151500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING NR 018// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 17.9S 114.2E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 264 NM NORTH OF LEARMONTH, AUSTRALIA MOVEMENT PAST 6 HOURS: WESTWARD AT 06 KTS SIGNIFICANT WAVE HEIGHT: 14 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL CYCLONE (TC) 18S HAS CONTINUED TO DEPICT SIGNS OF SLOW CONSOLIDATION WITH PERSISTENT DEEP CONVECTION AND ASSOCIATED OVERSHOOTING TOPS JUST WEST OF THE LOW-LEVEL CIRCULATION CENTER (LLCC). THE SURFACE CENTER HAS BECOME INCREASINGLY OBSCURED BETWEEN TWO CONVECTIVE STRUCTURES JUST TO THE EAST AND WEST, WITH OBSERVABLE WARMING TOPS OVER THE LAST FEW HOURS AS REVEALED IN THE ANIMATED ENHANCED INFRARED SATELLITE IMAGERY. WEAK RADIAL OUTFLOW IS EVIDENT OVER THE LLCC, AS UPPER-LEVEL OUTFLOW FROM ASSOCIATED CONVECTION CONTINUES TO FUEL THE ONGOING AND SLOW INTENSIFICATION TREND. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON A 151052Z 37 GHZ SSMIS MICROWAVE IMAGE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE SUBJECTIVE DVORAK ESTIMATES RANGING BETWEEN 30-35 KNOTS. INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE SOUTHEAST LOCATED OVER CENTRAL AUSTRALIA AND EXTENDING WESTWARD INTO THE SOUTHWESTERN INDIAN OCEAN. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS KNES: T2.5 - 35 KTS DEMS: T2.5 - 35 KTS APRF: T2.0 - 30 KTS CIMSS ADT: 33 KTS AT 150820Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 15-20 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TC 18S IS CURRENTLY TRACKING WEST-SOUTHWESTWARD ALONG THE NORTHWESTERN PERIPHERY OF THE STR LOCATED OVER CENTRAL AUSTRALIA AND EXTENDING WESTWARD INTO THE SOUTHWESTERN INDIAN OCEAN (IO). AS THE AFOREMENTIONED DEEP-LAYER STR CONTINUES TO BUILD SLIGHTLY WESTWARD, TC 18S IS FORECASTED TO CONTINUE ITS SLOW, MEANDERING WEST- SOUTHWESTWARD TRACK THROUGH TAU 36. AFTER TAU 36, A DEEP LONGWAVE TROUGH WILL WEAKEN THE STR STEERING FLOW, ALLOWING FOR A SHARP SOUTHWESTWARD TRACK UNTIL TAU 72. AS THE HIGH IN THE SOUTHERN IO REORIENTS EASTWARD, TC 18S WILL BEGIN A WEST-NORTHWESTWARD TRACK ALONG THE NORTHERN PERIPHERY OF THE SOUTHEAST TRADE FLOW THROUGH TAU 120. AS THE CYCLONE PROGRESSES GENERALLY WESTWARD, A MORE CONDUCIVE ENVIRONMENT IS EXPECTED AS VERTICAL WIND SHEAR (VWS) REMAINS LOW (10- 15 KNOTS), THE SYSTEM'S CORE MAINTAINS ITS MOIST STRUCTURE, AND FAVORABLE POLEWARD OUTFLOW INCREASES, RESULTING IN A STEADY INCREASE IN INTENSITY THROUGH TAU 120 OF UP TO 65 KNOTS. MODEL DISCUSSION: THE DETERMINISTIC MODEL TRACK GUIDANCE IS IN POOR AGREEMENT THROUGH TAU 72, AS THE NAVGEM AND GFS TRACKERS HAVE ILLUSTRATED ERRATIC FORECAST SOLUTIONS OVER THE LAST 12 HOURS AWAY FROM THE CONSENSUS. THE 150600Z ECMWF ENSEMBLE (EPS) AND GFS ENSEMBLE (GEFS) ALSO INDICATE CONTINUED UNCERTAINTY AFTER TAU 72 DUE TO COMPETING STEERING MECHANISMS, WITH TWO DISTINCT GROUPINGS OF SOLUTIONS IN A SOUTHWARD AND WESTWARD TRAJECTORY. ROUGHLY 90 PERCENT OF THE EPS AND GEFS SOLUTIONS DEPICT A GENERAL WESTWARD TRACK, WHILE 10 PERCENT OF ENSEMBLE SOLUTIONS HAVE SHOWN A SLOW SOUTHWARD TRACK INTO NORTHWESTERN AUSTRALIA. THE 150600Z COAMPS-TC INTENSITY ENSEMBLE (CTCXEPS) SHOWS A STEADY INTENSIFICATION TREND, WITH DECREASING PEAK PROBABILITIES REACHING MODERATE INTENSIFICATION OF 30 TO 40 PERCENT BETWEEN TAU 60 AND TAU 84. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN