WDXS31 PGTW 210900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 13S (EMNATI) WARNING NR 011// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 17.9S 53.5E INITIAL INTENSITY: 100 KTS GEOGRAPHIC REFERENCE: 186 NM NORTH-NORTHWEST OF ST DENIS MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 06 KTS SIGNIFICANT WAVE HEIGHT: 32 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) INDICATES TC 13S HAS CONTINUED TRACKING IN A TROCHOIDAL FASHION, PERFORMING SEVERAL CLOCKWISE LOOPS OVER THE PAST 12 HOURS, WITH AN AVERAGE MOTION VECTOR NOW HAVING SHIFTED FROM WEST TO A WEST-SOUTHWESTWARD DIRECTION OVER THE PAST SIX HOURS, WITH ANOTHER LOOP POTENTIALLY BEGINNING JUST IN THE LAST COUPLE OF HOURS. THE CORE CONVECTIVE SIGNATURE CONTINUES TO FLUCTUATE RAPIDLY, AT TIMES SURROUNDING THE SMALL EYE WHILE AT OTHERS THE INNER-CORE CONVECTION WANES AND SIGNIFICANT GAPS IN THE INNER EYEWALL EMERGE. THE EYE HAD BROADENED OUT SOMEWHAT BY ANALYSIS TIME, UP TO APPROXIMATELY 12NM. A 210146Z RADARSAT-2 SAR IMAGE CAPTURED THE EYE AND THE EASTERN PORTION OF THE CIRCULATION AND SHOWED 16NM RMW SURROUNDED BY A MOAT OF WINDS LESS THAN 40 KNOTS, FOLLOWED BY A SECONDARY EYEWALL AT AROUND 40NM. THIS GENERAL SETUP WAS CONFIRMED BY A 210335Z AMSU-B 89GHZ MICROWAVE IMAGE THAT SHOWED THE SYMMETRICAL SECONDARY EYEWALL AROUND 40-45NM FROM THE CENTER. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE 12NM EYE IN THE MSI AND EIR IMAGERY. THE INITIAL INTENSITY IS ASSESSED AT 100 KNOTS WITH MEDIUM CONFIDENCE BASED ON A BLEND OF THE SUBJECTIVE AND OBJECTIVE DVORAK CURRENT INTENSITY ESTIMATES NOTED BELOW. THE RADARSAT-2 PASS INDICATED A MAXIMUM VMAS OF ONLY 63 KNOTS, WHICH BEING NEARLY 40 KNOTS LOWER THAN THE FIX ESTIMATES, HAS BEEN DISCOUNTED. EARLIER SMAP AND SMOS PASSES AROUND THE 210300Z HOUR SHOWED MAXIMUM WINDS OF LESS THAN 80 KNOTS, BUT LIKELY ARE NOT CAPTURING THE ACTUAL PEAK INTENSITY DUE TO THE VERY SMALL NATURE OF THE INNER CORE VERSUS THE RESOLUTION OF THE SENSORS. INITIAL WIND RADII BASIS: SAR AND SMAP DATA. CURRENT STEERING MECHANISM: NORTH-NORTHWESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) COMPLEX TO THE SOUTH-SOUTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T5.5 - 102 KTS FMEE: T5.5 - 102 KTS CIMSS SATCON: 100 KTS AT 210336Z CIMSS ADT: 102 KTS AT 210315Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 5-10 KTS SST: 27-28 CELSIUS OUTFLOW: STRONG POLEWARD AND EQUATORWARD OTHER FACTORS: EYEWALL REPLACEMENT CYCLE, UPWELLING OF COOLER WATERS AND MARGINAL OCEAN HEAT CONTENT. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: OVER THE PAST 12 HOURS, A MESOSCALE SUBTROPICAL RIDGE (STR) HAS MOVED RAPIDLY FROM WEST TO EAST ALONG THE 30S LATITUDE LINE, AND IN RESPONSE TC 13S HAS TRACKED IN A TROCHOIDAL FASHION ALONG A MEAN VECTOR TOWARDS THE WEST, ALONG THE NORTHERN PERIPHERY OF THIS RIDGE. BY ANALYSIS TIME, UPPER-LEVEL ANALYSIS INDICATES THIS STR HAS PASSED THE LONGITUDE OF TC13S AND THE SYSTEM IS NOW TAKING ON A MORE SOUTHWESTWARD MOTION. THE STR IS EXPECTED TO REORIENT AND RECENTER TO A POSITION NEAR 30S 060E IN THE NEXT 12 HOURS, AND TC13S WILL STEADILY TURN ONTO A SOUTHWESTWARD TRACK AFTER TAU 12. THE TROCHOIDAL MOTION IS EXPECTED TO SMOOTH OUT AS THE SYSTEM PICKS UP SPEED AND THE STEERING GRADIENT TIGHTENS. THE ONGOING ERC IS TAKING ITS TIME TO COMPLETE, BUT SHOULD BE WRAPPED UP BY TAU 12. SOME ADDITIONAL NEAR-TERM WEAKENING IS POSSIBLE BUT OVERALL NO SIGNIFICANT CHANGE IN INTENSITY IS EXPECTED THROUGH TAU 12. ONCE THE NEW EYEWALL IS ESTABLISHED, THE STRONG OUTFLOW ALOFT AND LOW SHEAR WILL ALLOW FOR RENEWED INTENSIFICATION, HOWEVER THE VERY LOW OHC (LESS THAN 40 KJ PER CM2) AND LIMITED REMAINING TIME OVER WATER WILL PROVE TO BE TOUGH BARRIERS TO SIGNIFICANT INTENSIFICATION. THE SYSTEM IS EXPECTED TO WEAKEN SLIGHTLY JUST PRIOR TO LANDFALL AS IT IS IMPACTED BY AN INFLUX OF LOW LEVEL OFFSHORE FLOW AFTER LANDFALL NEAR TAU 36, THE SYSTEM WILL RAPIDLY WEAKEN OVER THE MOUNTAINOUS TERRAIN. THE SYSTEM EMERGES BACK OVER WATER AROUND TAU 60 AND TURNS SHARPLY POLEWARD INTO A WEAKNESS IN THE STR PRODUCED BY THE APPROACH OF A STRONG MID-LATITUDE TROUGH. ONCE MORE OVER WATER THE SYSTEM WILL HAVE A BRIEF TIME OVER WATERS WARMER THAN 26C, AND A SMALL AMOUNT OF INTENSIFICATION IS EXPECTED. TC 13S WILL BEGIN SUBTROPICAL TRANSITION (STT) BY TAU 72 AND IT MOVES UNDER AN UPPER-LEVEL TROUGH. STT IS EXPECTED TO BE COMPLETE NO LATER THAN TAU 120 AS THE SYSTEM IS EMBEDDED UNDER THE UPPER-LEVEL TROUGH WITH HIGH (30-40 KNOTS) VWS AND COOL (23-24C) SSTS. MODEL DISCUSSION: THE JTWC FORECAST REMAINS VERY CONSISTENT WITH THE PREVIOUS FORECAST TRACK, SUPPORTED BY A RELATIVELY TIGHT ENVELOPE OF DETERMINISTIC AND ENSEMBLE GUIDANCE. THE GUIDANCE ENVELOPE IS 160NM TAU 72, AND SHOWS ONLY A MARGINAL INCREASE TO 180NM THROUGH TAU 120. BUT WHILE THE GUIDANCE IS IN GOOD AGREEMENT, AT LEAST IN THE NEAR-TERM, THERE IS SOME UNCERTAINTY IN THE TRACK AS THE TROCHOIDAL MOTION INFLUENCES THE ULTIMATE TRACK. THUS THE JTWC FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE THROUGH THE PERIOD. THE INTENSITY FORECAST IS HIGHLY UNCERTAIN, PARTICULARLY THROUGH LANDFALL. THE HWRF AND COAMPS GUIDANCE SHOWS A PEAK OF 105 KNOTS AT TAU 12, THEN HOVERING BETWEEN 100-105 KNOTS THROUGH TAU 24, WHILE THE REMAINDER OF THE GUIDANCE REMAINS BELOW 100 KNOTS THROUGH TAU 24. AFTER THE RAPID WEAKENING OVER LAND, GUIDANCE SHOWS A VERY WIDE SPREAD BETWEEN 20 KNOTS AND 60 KNOTS. THE JTWC FORECAST LIES SLIGHTLY ABOVE THE GUIDANCE AT TAU 24, THEN TRACKS JUST ABOVE THE CONSENSUS MEAN THROUGH THE REMAINDER OF THE FORECAST. CONFIDENCE IS MEDIUM IN THE NEAR-TERM DUE TO UNCERTAINTY ON THE AMOUNT OF POST-ERC INTENSIFICATION AND LOW AFTER TAU 72 DUE TO UNCERTAINTY IN THE AMOUNT OF WEAKENING OVER LAND. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN