WDXS32 PGTW 080900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 13S (VINCE) WARNING NR 
014//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.4S 73.4E
   INITIAL INTENSITY: 120 KTS
   GEOGRAPHIC REFERENCE: 800 NM SOUTH OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 52 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THAT TROPICAL
CYCLONE (TC) 13S (VINCE) REMAINS AN IMPRESSIVE SPECIMEN, HOWEVER IT
IS STARTING TO SHOW SIGNS OF WEAKENING. BD-ENHANCED INFRARED
IMAGERY SHOWS A WEAKENING WESTERN EYEWALL BETWEEN 080200Z AND
080700Z BUT AS OF THIS WRITING, THE EYEWALL IS QUICKLY SOLIDIFYING
ONCE MORE. A 080132Z SSMIS 89GHZ MICROWAVE IMAGE PROVIDES A CLUE
THAT THIS EYEWALL WEAKENING MAY HAVE BEEN A RESULT OF AN EYEWALL
MERGE OR EYEWALL REPLACEMENT CYCLE (EWRC), THOUGH A LACK OF MORE
RECENT MICROWAVE IMAGERY PRECLUDES A FULL ANALYSIS. THE MICROWAVE
AT 0132Z HOWEVER SHOWED A STATIONARY BANDING COMPLEX (SBC) TO THE
SOUTH, WITH A DEVELOPING SECONDARY EYEWALL MERGING ON THE WESTERN
SIDE OF THE INNER EYEWALL. MODEL-DERIVED SOUNDINGS ALSO SUGGEST
INFLUENCE OF MODERATE WESTERLY MID-LEVEL SHEAR WHICH COULD HELP
EXPLAIN THE EYEWALL WEAKENING. THE INITIAL POSITION IS ASSESSED
WITH HIGH CONFIDENCE BASED ON A 19NM EYE IN THE EIR. THE INITIAL
INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED ABOVE. THE
ENVIRONMENT REMAINS OVERALL FAVORABLE WITH WARM SSTS, GOOD RADIAL
OUTFLOW AND LOW DEEP-LAYER SHEAR.

INITIAL WIND RADII BASIS: SAR AND SCATTEROMETER DATA.

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.5 - 127 KTS
   KNES: T6.0 - 115 KTS
   DEMS: T6.5 - 127 KTS
   FMEE: T6.5 - 127 KTS
   FIMP: T6.5 - 127 KTS
   CIMSS ADT: 124 KTS AT 080600Z
   CIMSS AIDT: 124 KTS AT 080600Z
   CIMSS D-PRINT: 115 KTS AT 080630Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG RADIAL
   OTHER FACTORS: MODERATE MID-LEVEL WESTERLY SHEAR. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE END-STAGE TRANSITION HAS BEEN
CHANGED TO SUBTROPICAL AND THE FORECAST HAS BEEN TRUNCATED TO 96
HOURS. 

FORECAST DISCUSSION: TC 13S WILL TRACK GENERALLY WEST-SOUTHWESTWARD
FOR THE NEXT 24 HOURS, ALONG THE NORTHWESTERN SIDE OF THE DEEP STR
POSITIONED TO THE SOUTHEAST. THE STR BUILDS NORTHWARD THROUGH THE
FIRST 24 HOURS OF THE FORECAST, AND TC 13S WILL SLOWLY TURN
EVER-MORE SOUTHWARD AS IT DOES SO.  A MORE ABRUPT CHANGE IS
EXPECTED BY TAU 36, AS A RAPIDLY DEEPENING AREA OF LOW HEIGHTS
DEVELOPS NEAR 30S 60E, SWEEPING ASIDE A MIGRATORY RIDGE AND OPENING
UP A CLEAR PATH POLEWARD FROM TC 13S. CAUGHT BETWEEN THE STR
BUILDING NORTH AND MOVING EAST, AND THE TROF TO THE WEST, TC 13S
WILL QUICKLY ROUND THE RIDGE AND ACCELERATE POLEWARD AFTER TAU 48.
AN EWRC AND MODERATE MID-LEVEL WESTERLY SHEAR WILL SLOWLY ERODE THE
INNER-CORE STRUCTURE OF TC 13S IN THE NEAR-TERM, AND THE SYSTEM IS
FORECAST TO SLOWLY WEAKEN FOR THE NEXT 24 HOURS. A LEVELING OFF OF
INTENSITY IS EXPECTED FOR ABOUT 12 HOURS, UP TO TAU 36, AS THE
SYSTEM GETS A BOOST FROM A SLIGHT MOISTENING AND A TAP INTO STRONG
POLEWARD OUTFLOW INTO THE BASE OF A VERY STRONG JET MAX TO THE
SOUTH. THE FUN DOES NOT LAST LONG HOWEVER, AND BEGINNING AROUND TAU
48, MASSIVE INGESTION OF DRY AIR FROM THE NORTHWEST ACCOMPANIED BY
RAPIDLY INCREASING SHEAR, WILL MARK THE BEGINNING OF A RAPID
WEAKENING PHASE. THE SYSTEM PASSES SOUTH OF THE 26C ISOTHERM BY TAU
72, IS OVERWHELMED BY DRY AIR IN THE MID-LEVELS AND BEGINS TO MOVE
UNDER THE SUBTROPICAL WESTERLIES ALOFT, MARKING THE ONSET OF A
PROLONGED SUBTROPICAL TRANSITION (STT) PHASE. THE SYSTEM WILL
CONTINUE TO RAPIDLY WEAKEN DURING STT AND IS EXPECTED TO BECOME
FULLY SUBTROPICAL NO LATER THAN TAU 96. THE SYSTEM WILL EVENTUALLY
TRANSITION TO AN EXTRATROPICAL LOW, BUT THIS TRANSITION WILL BE
PROLONGED AND OCCUR FAR TO THE SOUTH. 

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE ARE IN
TIGHT AGREEMENT ON THE OVERALL TRACK SCENARIO. CROSS- AND
ALONG-TRACK SPREAD IS MINIMAL TO TAU 48, BUT ALONG-TRACK SPREAD IN
PARTICULAR, RAPIDLY INCREASES TO 165NM BY TAU 72 BETWEEN THE SLOW
ECMWF AND THE FAST GFS. BY TAU 96, THE SPREAD BETWEEN THESE
OUTLIERS INCREASES TO 340NM, THOUGH CROSS-TRACK UNCERTAINTY REMAINS
FAIRLY LOW. THE JTWC FORECAST IS SITUATED CLOSE TO THE MULTI-MODEL
CONSENSUS MEAN THROUGHOUT THE FORECAST PERIOD. INTENSITY GUIDANCE
IS IN GOOD AGREEMENT AND ALL AVAILABLE AIDS CLOSELY MATCH THE JTWC
FORECAST. 

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