WDPS32 PGTW 062100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.1S 174.4E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 236 NM WEST-NORTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 38 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
FLUCTUATING INNER CORE STRUCTURE, OSCILLATING BETWEEN A RAGGED
CONVECTIVE RING SURROUNDING A CENTRAL EYE FEATURE AND MORE OBSCURED 
SCENES WITH ASYMMETRIC CONVECTIVE BURSTS. OVERALL, THE CYCLONE APPEARS 
TO BE GRADUALLY ORGANIZING DESPITE 20-25 KT OF SOUTHWESTERLY VERTICAL 
SHEAR, WHICH HAS CAUSED WEAKNESSES IN THE NORTHERN (DOWNSHEAR-RIGHT) 
SEGMENT OF THE EYEWALL IN RECENT MICROWAVE IMAGERY, SUCH AS THE 
061344Z AMSR-2 PASS. THE INITIAL INTENSITY IS SET AT 95 KT BASED ON A 
BLEND OF AGENCY ESTIMATES AND OBJECTIVE SATELLITE-BASED ESTIMATES.

INITIAL WIND RADII BASIS: AMSR-2 DATA

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE EAST OF FIJI

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   CIMSS SATCON: 101 KTS AT 061336Z
   CIMSS ADT: 79 KTS AT 061420Z
   CIMSS AIDT: 79 KTS AT 061420Z
   CIMSS D-MINT: 96 KTS AT 061344Z
   CIMSS D-PRINT: 90 KTS AT 061720Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE CYCLONE IS NOW EXPECTED TO
UNDERGO SUBTROPICAL TRANSITION BEGINNING AROUND TAU 96, RATHER THAN
EXTRATROPICAL TRANSITION.

FORECAST DISCUSSION: TROPICAL CYCLONE 31P (VAIANU) CONTINUES TO
TRACK SOUTHEASTWARD AROUND THE WESTERN PERIPHERY OF A
NEAR-EQUATORIAL RIDGE TO THE EAST. THE TRACK FORECAST IS RELATIVELY
STRAIGHT FORWARD, WITH NUMERICAL MODELS IN GOOD AGREEMENT THAT VAIANU 
WILL CONTINUE SOUTHEASTWARD FOR ABOUT 48 HOURS BEFORE TURNING 
SOUTHWARD AND THEN SOUTHWESTWARD UNDER THE INFLUENCE OF THE 
SUBTROPICAL RIDGE EAST OF NEW ZEALAND. THE CYCLONE IS NOT EXPECTED TO 
ACCELERATE MUCH DURING THE FORECAST PERIOD DUE TO A SPLIT-JET PATTERN, 
IN WHICH A SOUTHWARD DISPLACEMENT OF THE POLAR JET WILL DELAY 
ACCELERATION INTO THE MID-LATITUDE WESTERLIES. THE CYCLONE WILL, 
HOWEVER, MOVE POLEWARD OF THE SUBTROPICAL JET AFTER 72 HOURS, AND ITS
SUPERPOSITION BENEATH A WEAK UPPER-LEVEL TROUGH, COUPLED WITH
COOLING SEA SURFACE TEMPERATURES, IS EXPECTED TO INITIATE
SUBTROPICAL TRANSITION AROUND 96 HOURS, COMPLETING BY 120 HOURS AS
31P APPROACHES NEW ZEALAND. THE INTENSITY FORECAST IS ALSO STRAIGHT
FORWARD, AS THE ENVIRONMENT IS ONLY MARGINALLY FAVORABLE FOR
FURTHER INTENSIFICATION SHORT-TERM WITH MODERATE MID-LEVEL SHEAR
PRESENT AND ALREADY AFFECTING THE STORM'S INNER CORE STRUCTURE. AT
ABOUT 36 HOURS, THIS SHEAR WILL INCREASE SIGNIFICANTLY, CONCURRENT
WITH A DROP IN SEA SURFACE TEMPERATURES TO AROUND 26 DEGREES
CELSIUS. THIS SHOULD REVERSE THE INTENSITY TREND, CAUSING WEAKENING
THEREAFTER. THIS WEAKENING IS UNLIKELY TO BE RAPID AS THE CYCLONE
MOVES BENEATH THE AFOREMENTIONED UPPER-LEVEL TROUGH, ALLOWING A
SYMMETRIC VORTEX TO PERSIST FOR MULTIPLE DAYS AS IT METHODICALLY
TRACKS POLEWARD OVER GRADUALLY COOLING WATERS. THE CYCLONE MAY
RETAIN SURFACE WINDS OF 50 KT OR HIGHER WELL INTO ITS SUBTROPICAL
PHASE NORTH OF NEW ZEALAND.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS, LEANING A TOUCH FASTER AND EAST OF THE
PHYSICS-BASED MODELS DUE TO AI-BASED GUIDANCE. THE JTWC INTENSITY
FORECAST IS CLOSE TO THE MULTI-MODEL CONSENSUS, INTENSITY SPREAD
LOWER THAN AVERAGE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: HIGH
   INTENSITY 72-120 HR: HIGH//
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