WDXS31 PGTW 170300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 11S (FREDDY) WARNING NR 029// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 15.4S 76.6E INITIAL INTENSITY: 120 KTS GEOGRAPHIC REFERENCE: 553 NM SOUTH-SOUTHEAST OF DIEGO GARCIA MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 14 KTS SIGNIFICANT WAVE HEIGHT: 43 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A WELL-DEFINED INNER CORE WITH A WARM EYE AND A COLD SURROUNDING RING OF DEEP CONVECTION THAT HAS PULSED IN INTENSITY MULTIPLE TIMES OVER THE PAST 12 HOURS. AFTER COOLING SIGNIFICANTLY BETWEEN 161200Z AND 161800Z, THE CENTRAL CONVECTION HAS WARMED A BIT OVER THE PAST SIX HOURS, AND THE EYEWALL HAS BECOME PERIODICALLY BROKEN IN THE SOUTHERN SEMICIRCLE. INFRARED IMAGERY, WATER VAPOR IMAGERY, AND A 162209Z SSMIS MICROWAVE PASS HINT AT OUTER CORE BANDING FORMING OUTSIDE OF THE PRIMARY EYEWALL, AND THE EYE HAS SHOWN SIGNS OF CONTRACTING BETWEEN 170000Z AND 170200Z. THESE STRUCTURAL CHANGES COULD BE THE FIRST HINTS OF AN IMMINENT EYEWALL REPLACEMENT CYCLE. THE ENVIRONMENT REMAINS GENERALLY FAVORABLE FOR SUSTAINING THE CYCLONE WITH LOW VERTICAL SHEAR AND WARM SEA SURFACE TEMPERATURES. UPPER-LEVEL OUTFLOW IS EXPANDING RADIALLY IN NEARLY SYMMETRIC FASHION, THOUGH SLIGHTLY RESTRICTED ON THE EASTERN SIDE. WATER VAPOR SATELLITE IMAGERY REVEALS A BAND OF LOW-LEVEL DRY AIR WRAPPING AROUND THE WESTERN AND NORTHERN SEMICIRCLE OF THE STORM'S CIRCULATION WITHIN 200 NM OF THE EYE. THE INITIAL INTENSITY IS SET AT 120 KT AS A BLEND OF SUBJECTIVE DVORAK ESTIMATES OF T6.0 (115 KT) TO T6.5 (127 KT) FROM ALL AGENCIES AND OBJECTIVE DVORAK ESTIMATES OF 122 TO 125 KT. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE SOUTH AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T6.5 - 127 KTS KNES: T6.0 - 115 KTS DEMS: T6.5 - 127 KTS FMEE: T6.0 - 115 KTS CIMSS SATCON: 128 KTS AT 162012Z CIMSS ADT: 124 KTS AT 170000Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE RADIAL OTHER FACTORS: LOW-LEVEL DRY AIR WRAPPING AROUND THE CYCLONE WITHIN A 200 NM RADIUS 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: TROPICAL CYCLONE 11S (FREDDY) CONTINUES TO TRACK STEADILY WESTWARD, GUIDED BY THE SUBTROPICAL RIDGE TO ITS SOUTH. THE TRACK FORECAST IS STRAIGHT FORWARD, WITH A WEST OR WEST-SOUTHWEST MOTION EXPECTED FOR THE NEXT 5 DAYS BY ALL MODEL GUIDANCE. THE JTWC FORECAST TAKES FREDDY NORTH OF MAURITIUS AND LA REUNION BY A MARGIN OF 100 TO 120 NM IN 84 TO 96 HOURS. AROUND THIS TIME, A LONGWAVE TROUGH IS EXPECTED TO PASS SOUTH OF MADAGASCAR POLEWARD OF LATITUDE 35S, SHIFTING THE STEERING RIDGE TO THE SOUTHEAST OF FREDDY, POTENTIALLY NUDGING ITS HEADING TO THE LEFT. HOWEVER, MODEL GUIDANCE CURRENTLY EXPECTS THIS TROUGH TO MOVE AWAY QUICKLY BEFORE BEING ABLE TO TURN FREDDY SOUTHWARD. THE JTWC FORECAST TRACK FOR FREDDY THUS CONTINUES WEST-SOUTHWESTWARD TO A LANDFALL IN CENTRAL MADAGASCAR BY 120 HOURS. THE INTENSITY FORECAST IS MORE DIFFICULT, AS FREDDY HAS BEEN UNDERGOING QUICK-PACED FLUCTUATIONS IN INTENSITY DURING THE PAST 24 HOURS. WHILE LOW VERTICAL WIND SHEAR AND WARM OCEAN TEMPERATURES ARE FAVORABLE FOR THE CYCLONE AND EXPECTED TO PERSIST THROUGHOUT THE FORECAST, DRY AIR IS ABUNDANT IN THE LOWER TROPOSPHERE AROUND THE OUTSKIRTS OF THE CIRCULATION. IT IS LIKELY THAT PERIODIC INGESTION OF THIS DRY LAYER HAS BEEN OCCURRING AND WILL CONTINUE OVER THE COMING DAYS, CAUSING DISRUPTIONS TO THE INNER CORE CONVECTIVE STRUCTURE. THIS IS SUPPORTED BY THE GFS AND ECMWF MODELS DEPICTING LOWER TROPOSPHERIC DRY AIR ENCIRCLING FREDDY AND CUTTING IT OFF FROM THE DEEP EQUATORIAL MOISTURE RESERVOIR FOR AT LEAST THE NEXT 72 HOURS. COMBINED WITH DRY AIR INTRUSIONS, INTERNAL VORTEX DYNAMICS WILL LIKELY BE THE PRIMARY GOVERNOR OF FREDDY'S INTENSITY. EYEWALL REPLACEMENT CYCLES, COMMON FOR CYCLONES OF THIS STRENGTH AND DIFFICULT TO PREDICT IN ADVANCE, MAY OCCUR AND LEAD TO FLUCTUATIONS IN FREDDY'S INTENSITY. THE JTWC FORECAST SMOOTHS THROUGH THESE ANTICIPATED FLUCTUATIONS AND MAINTAINS AN INTENSITY IN THE 110 TO 120 KT THROUGH 96 HOURS, WITH A SLIGHT OVERALL DECREASE IN ACCORDANCE WITH THE MODEL CONSENSUS. THIS PREDICTED WEAKENING MAY BE DUE TO THE MODEL CONSENSUS FOR AN EXPANDING INNER CORE SIZE BEYOND 48 HOURS, LEADING TO A LESS INTENSE EYEWALL. BY 120 HOURS, LANDFALL IN MADAGASCAR IS EXPECTED TO INITIATE RAPID WEAKENING. MODEL DISCUSSION: THE JTWC TRACK FORECAST CLOSELY FOLLOWS A BLEND OF THE ECMWF, ECMWF ENSEMBLE, GFS, GFS ENSEMBLE, AND UKMET MODELS, AND IS CLOSE TO THE PREVIOUS FORECAST. THE JTWC INTENSITY FORECAST IS SLIGHTLY ABOVE THE MULTI-MODEL CONSENSUS DUE TO THE GFS BEING DRAMATICALLY WEAKER THAN THE OTHER MODELS AND CONSIDERED AN OUTLIER. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: HIGH INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN