WDXS31 PGTW 290900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27S (COURTNEY) WARNING 
NR 014//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.0S 91.0E
   INITIAL INTENSITY: 120 KTS
   GEOGRAPHIC REFERENCE: 488 NM SOUTHWEST OF THE COCOS ISLANDS
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 42 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) AND ENHANCED
INFRARED (EIR) IMAGERY DEPICT A COMPACT, SYMMETRICAL CDO WITH A
10NM WIDE, VERY WARM EYE. EYE TEMPERATURES AS RECORDED BY BOTH THE
ADT AND THE PGTW SATELLITE ANALYSIS WERE AT OR ABOVE 9C, IMPRESSIVE
BY ANY MEASURE. THE SURROUNDING DEEP CONVECTION HAS CLOUD TOP
TEMPERATURES AROUND -80C AND HAVE COOLED SIGNIFICANTLY OVER THE
PAST SIX HOURS OR SO. QUALITATIVELY, THE SYSTEM LOOKS MORE
IMPRESSIVE SINCE THE LAST FIX CYCLE. A 290429Z GMI 89GHZ MICROWAVE
IMAGE REVEALS A COMPLETE EYEWALL THOUGH THE EASTERN SIDE IS
SIGNIFICANTLY WEAKER AND THINNER THAN THE EYEWALL ON THE WESTERN
SIDE. AS WELL, THE MICROWAVE IMAGE REVEALS A STATIONARY BANDING
FEATURE (SBC) WRAPPING IN FROM THE NORTH ALONG THE EASTERN SIDE AND
CONNECTING WITH THE EYEWALL IN THE SOUTHWESTERN QUADRANT. THE
PRESENCE OF THE SBC IS A SIGN OF AN EYEWALL REPLACEMENT CYCLE (ERC)
KICKING OFF WITHIN THE NEXT 12 TO 24 HOURS. THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON AN EXTRAPOLATION OF THE
MICROWAVE EYE IN THE GMI PASS. THE INITIAL INTENSITY IS ASSESSED
WITH MEDIUM CONFIDENCE, HEDGED SLIGHTLY ABOVE THE OBJECTIVE AND
SUBJECTIVE FIXES BASED ON A QUALITATIVELY BETTER APPEARANCE AND THE
RAW ADT WHICH IS NEAR T6.5. THE ENVIRONMENT REMAINS FAVORABLE WITH
LOW VWS, MODERATELY WARM SSTS AND GOOD OUTFLOW ALOFT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA.

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   KNES: T6.0 - 115 KTS
   APRF: T6.0 - 115 KTS
   CIMSS SATCON: 113 KTS AT 290600Z
   CIMSS ADT: 115 KTS AT 290600Z
   CIMSS AIDT: 111 KTS AT 290600Z
   CIMSS D-MINT: 116 KTS AT 290431Z
   CIMSS D-PRINT: 114 KTS AT 290615Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 0-5 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG RADIAL
   OTHER FACTORS: WEAK POLEWARD OUTFLOW CHANNEL. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 96
HOURS. 

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 27S (COURTNEY) WILL
CONTINUE TO TRACK ALONG THE NORTHWESTERN SIDE OF THE STR TO THE
SOUTHEAST AND TRACE A GRACEFUL LEFT TURN, SUCH THAT BY TAU 48 IT
WILL BE TRACKING DUE SOUTH AS IT REACHES THE RIDGE AXIS. IN THE
NEAR-TERM, WHILE THE ENVIRONMENT REMAINS OVERALL FAVORABLE, THE
PRIMARY DRIVER OF INTENSITY THROUGH TAU 24 WILL BE THE POTENTIAL
ERC. THE CIMSS M-PERC IS CURRENTLY PREDICTING A 50 PERCENT CHANCE
OF AN ERC, AND BASED ON THE SBC SEEN IN THE MICROWAVE IMAGERY, IT
IS LIKELY THAT ERC WILL IN FACT BEGIN WITHIN THE NEXT 12 HOURS OR
SO. INTENSITY WILL DROP RATHER QUICKLY ONCE ERC BEGINS AND DUE TO
AN EXPECTED SHARP INCREASE IN SHEAR BEGINNING AFTER TAU 24, IT IS
UNLIKELY THAT THE ERC WILL BE ABLE TO COMPLETE THE CYCLE AND THE
SYSTEM WILL NOT RECOVER. SHEAR IS FORECAST TO EXCEED 35 KNOTS BY
TAU 36 AND ONLY INCREASE THEREAFTER. EXTREMELY DRY AIR BEGINS TO
INTRUDE INTO THE CORE OF THE SYSTEM AROUND TAU 48 AND WILL
COMPLETELY ENGULF THE SYSTEM BY AROUND TAU 60. THE SYSTEM WILL BE
EFFECTIVELY DECAPITATED AND BY TAU 72, THE REMNANT VORTEX WILL
BE CAPPED ABOVE 700MB BY A DEEP LAYER OF EXTREMELY DRY AIR. AS THE
SYSTEM RAPIDLY WEAKENS AND THE VORTEX SHALLOWS OUT, IT WILL ALSO
SIMULTANEOUSLY BEGIN SUBTROPICAL TRANSITION (STT) AND SO IT WILL
BECOME A RACE TO SEE IF THE SYSTEM DISSIPATES OR COMPLETES STT
FIRST. EITHER WAY, AS THE VORTEX SHALLOWS IT WILL RUN DIRECTLY
INTO A STRONG RIDGE TO THE SOUTH AND SLOW DOWN SHARPLY. AFTER TAU
72 THE REMNANT VORTEX WILL COME UNDER THE STEERING INFLUENCE OF THE
LOW-LEVEL RIDGE TO THE SOUTH AND TURN SHARPLY WESTWARD PRIOR TO
DISSIPATING NO LATER THAN TAU 96. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS ACTUALLY IN
FAIRLY GOOD AGREEMENT THROUGH TAU 72, WITH A 165NM CROSS-TRACK
SPREAD AT THE END OF THAT TIME POINT. THE NAVGEM AND GFS CONTINUE
TO TURN THE SYSTEM SOUTH AND SOUTHEAST AT A FASTER RATE THAN THE
OTHER MODELS AND THUS MARK THE EASTERN EDGE OF THE GUIDANCE
ENVELOPE. THE ECMWF-AIFS TAKES A WIDER TURN AND MARKS THE WESTERN
EDGE. THE REMAINDER OF THE CONSENSUS MEMBERS ARE CONFINED TO A
TIGHT GROUPING CENTERED ON THE CONSENSUS MEAN. THE JTWC FORECAST IS
HEDGED TOWARDS THE WESTERN SIDE OF THE INNER GROUPING OF THE
MODELS, WITH HIGH CONFIDENCE THROUGH TAU 72. AFTER TAU 72, THE
MODELS BEGIN TO DISPERSE, WITH CROSS-TRACK SPREAD OPENING UP TO
OVER 260NM BETWEEN THE GEFS MEAN ON THE WEST SIDE AND THE EC-AIFS
ON THE EAST SIDE. THE JTWC FORECAST MOST CLOSELY FOLLOWS THE GFS
AND CONSENSUS MEAN AFTER TAU 72, WITH LOW CONFIDENCE. INTENSITY
GUIDANCE IS IN GOOD AGREEMENT WITH MODERATE TO RAPID WEAKENING
THROUGH THE FORECAST PERIOD. THE JTWC FORECAST IS SET CLOSE TO THE
HAFS-A AND SLIGHTLY ABOVE THE CONSENSUS MEAN THROUGH THE FORECAST
PERIOD WITH MEDIUM CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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