WDXS31 PGTW 070900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (MITCHELL) WARNING NR 006// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 19.4S 117.6E INITIAL INTENSITY: 65 KTS GEOGRAPHIC REFERENCE: 78 NM NORTHWEST OF PORT HEDLAND, AUSTRALIA MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 09 KTS SIGNIFICANT WAVE HEIGHT: 28 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A WELL-ORGANIZED SYSTEM, WITH STRONG AND PERSISTENT DEEP CONVECTION, PARTICULARLY ON THE NORTHERN PERIPHERY, AND A TRANSIENT EYE FEATURE. A 070532Z AMSR2 89GHZ MICROWAVE IMAGE DEPICTS A WELL-DEFINED MICROWAVE EYE FEATURE, APPROXIMATELY 31NM IN DIAMETER, WITH STRONG CONVECTION CREATING AN EYEWALL AROUND APPROXIMATELY THREE QUARTERS OF THE LOW-LEVEL CIRCULATION (LLC), WITH THE WEAKEST PORTION OF THE EYEWALL IN THE SOUTHEASTERN QUADRANT. THE 36GHZ CHANNEL DEPICTS A COMPACT MICROWAVE EYE FEATURE DISPLACED ABOUT 12NM EAST OF THE UPPER-LEVEL EYE, CONSISTENT WITH MODERATE 15-20 KNOT EASTERLY VERTICAL WIND SHEAR (VWS) AS ESTIMATED BY CIMSS. THE MICROWAVE DEPICTION, PARTICULARLY IN THE 36GHZ BAND, CORROBORATES THE ABOM RADAR IMAGERY OUT OF PORT HEDLAND, WHICH SHOWS A WELL-DEFINED RADAR EYE, STRONGEST ON THE NORTHWEST PERIPHERY. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AMSR2 36GHZ CENTER AND THE RADAR EYE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE AND MAY BE CONSERVATIVE AS THE SYSTEM IS CURRENTLY UNDERGOING RAPID INTENSIFICATION (RI) AND OBJECTIVE INTENSITY ESTIMATES ARE TRENDING UPWARD. THE CURRENT INTENSITY AS OF 0730Z IS ESTIMATED AT 70-75 KNOTS. ENVIRONMENTAL CONDITIONS ARE HIGHLY FAVORABLE, WITH LOW TO MODERATE EASTERLY SHEAR BEING OFFSET BY ROBUST POLEWARD AND EQUATORWARD OUTFLOW ALOFT AND WARM SSTS WITH HIGH OCEAN HEAT CONTENT (OHC). INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: NORTHWESTERN FLANK OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) OVER EASTERN AUSTRALIA. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T4.0 - 65 KTS DEMS: T4.0 - 65 KTS APRF: T4.0 - 65 KTS CIMSS SATCON: 78 KTS AT 070536Z CIMSS ADT: 72 KTS AT 070630Z CIMSS AIDT: 66 KTS AT 070630Z CIMSS D-MINT: 71 KTS AT 070532Z CIMSS D-PRINT: 70 KTS AT 070600Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE VWS: 10-15 KTS SST: 29-30 CELSIUS OUTFLOW: STRONG DUAL-CHANNEL ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH 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: TROPICAL CYCLONE (TC) 20S (MITCHELL) IS RAPIDLY INTENSIFYING AS IT TRACKS SLOWLY SOUTHWESTWARD, PARALLEL TO THE NORTHWESTERN COAST OF AUSTRALIA. TC 20S IS FORECAST TO CONTINUE TRACKING SOUTHWESTWARD ALONG THE NORTHWEST SIDE OF A DEEP-LAYER STR OVER EASTERN AUSTRALIA THROUGH THE NEXT 24 HOURS. THE SYSTEM WILL REMAIN OFFSHORE UNTIL LANDFALL AT TAU 24 NEAR ONSLOW, AUSTRALIA. BY TAU 36, THE SYSTEM IS PROJECTED TO TRAVERSE THE EASTERN COAST OF THE EXMOUTH GULF, OR POTENTIALLY OVER THE GULF ITSELF, AS IT APPROACHES THE RIDGE AXIS AND DECELERATES. BY TAU 48, TC 20S WILL HAVE ROUNDED THE RIDGE AND WILL BEGIN ACCELERATING SOUTHEASTWARD FURTHER INLAND THROUGH THE REMAINDER OF THE FORECAST PERIOD. REGARDING INTENSITY, THE APPROACH OF AN UPPER-LEVEL TROUGH FROM THE SOUTHWEST WILL ENHANCE UPPER-LEVEL POLEWARD OUTFLOW IN THE NEXT 24 HOURS, WHILE SHEAR IS EXPECTED TO WEAKEN SLIGHTLY. THESE FACTORS, SUPPORTED BY WARM SSTS AND HIGH OHC, WILL RESULT IN FURTHER RAPID INTENSIFICATION (RI) OVER THE NEXT 24 HOURS, WITH A PEAK INTENSITY OF 90-95 KNOTS EXPECTED PRIOR TO LANDFALL. THE CURRENT FORECAST DEPICTS TC 20S TRANSITING THE EASTERN COAST OF THE EXMOUTH GULF WHILE STEADILY WEAKENING. HOWEVER, DIVERGENT MODEL GUIDANCE SUPPORTS A TRACK DOWN THE CENTER OF THE GULF. IF THE SYSTEM REMAINS OVER THE HIGH-OHC WATERS OF THE GULF, IT WILL LIKELY MAINTAIN A HIGH INTENSITY UNTIL COMPLETING LANDFALL. ONCE INLAND, THE SYSTEM WILL RAPIDLY WEAKEN DUE TO FRICTIONAL EFFECTS AND TERRAIN INTERACTION, FOLLOWED BY MODERATE TO HIGH VWS AFTER TAU 48. THE SYSTEM IS EXPECTED TO DISSIPATE OVER WESTERN AUSTRALIA BY TAU 72. MODEL DISCUSSION: ALL MEMBERS OF THE TRACK CONSENSUS ARE IN GOOD AGREEMENT, WITH THE EXCEPTION OF NAVGEM WHICH IS A SIGNIFICANT OUTLIER TRACKING THE SYSTEM OFFSHORE THROUGH TAU 120, WHERE IT APPROACHES GERALDTON. THE MULTI-MODEL ENSEMBLE MEAN, AS WELL AS THE AI CONSENSUS, SUPPORT THE DETERMINISTIC PACKAGE, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK, WHICH IS PLACED ON THE WESTERN SIDE OF THE TRACK ENVELOPE WITH HIGH CONFIDENCE. AS DISCUSSED ABOVE, SLIGHT ADJUSTMENTS IN THE POSITIONING OF THE STR TO THE EAST COULD RESULT IN A TRACK THAT REMAINS OVER THE EXMOUTH GULF, AND THE SYSTEM MAKING LANDFALL OVER THE SOUTHERN END OF THE GULF. INTENSITY GUIDANCE EXHIBITS A FAIRLY LARGE AMOUNT OF UNCERTAINTY. ON THE LOWER-END OF THE GUIDANCE, THE HAFS-A AND COAMPS-TC INDICATE A PEAK INTENSITY OF BETWEEN 65-70 KNOTS, DRIVEN PRIMARILY BY A PROJECTED EARLIER LANDFALL. THE HWRF AND SHIPS-GFS INDICATE A PEAK INTENSITY BETWEEN 80-85 KNOTS. FINALLY, THE RIPA, RIDE, RICN AND CHR4 RI ALGORITHMS HAVE ALL BEEN TRIPPED, SUPPORTING A PEAK INTENSITY BETWEEN 90-105 KNOTS. BASED ON THE FAVORABLE ENVIRONMENT, THE JTWC FORECAST IS HEDGED TOWARD THE RIPA AND CHR4 RI GUIDANCE THROUGH LANDFALL, AND THE CONSENSUS MEAN POST-LANDFALL, WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM INTENSITY 00-72 HR: MEDIUM// NNNN