WDPS32 PGTW 080300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 22.2S 177.4E
   INITIAL INTENSITY: 80 KTS
   GEOGRAPHIC REFERENCE: 252 NM SOUTH-SOUTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS THE LOWER
LEVELS OF TROPICAL CYCLONE (TC) 31P BECOMING DECOUPLED FROM THE
UPPER-LEVEL CONVECTION, WHICH IS DISPLACED TO THE SOUTHEAST BY
STRONG WESTERLY VERTICAL WIND SHEAR. ENVIRONMENTAL ANALYSIS
INDICATES A MARGINALLY UNFAVORABLE ENVIRONMENT CHARACTERIZED BY
MARGINAL (27-28C) SEA SURFACE TEMPERATURES AND HIGH (25-30 KTS)
WESTERLY VERTICAL WIND SHEAR, AND A DRY SURROUNDING ENVIRONMENT,
OFFSET BY STRONG OUTFLOW ALOFT. THE INITIAL POSITION IS PLACED WITH
LOW CONFIDENCE BASED ON LOW-LEVEL CLOUD BANDING IN ANIMATED MSI,
SUPPORTED BY THE AGENCY POSITION FIXES. THE INITIAL INTENSITY OF 80
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE CIMSS AUTOMATED
INTENSITY AIDS AND AGENCY SUBJECTIVE DVORAK ESTIMATES, SUPPORTED BY 
THE STORM'S WORSENING APPEARANCE ON ANIMATED EIR.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE (NER) CENTERED TO
THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   KNES: T5.5 - 102 KTS
   PHFO: T4.5 - 77 KTS
   NFFN: T4.5 - 77 KTS
   CIMSS ADT: 82 KTS AT 080140Z
   CIMSS AIDT: 75 KTS AT 080140Z
   CIMSS D-PRINT: 70 KTS AT 080140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30+ KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 31P (VAIANU) IS FORECAST TO CONTINUE
TRACKING SOUTHEASTWARD FOR THE NEXT 36 HOURS WHILE IT REMAINS UNDER
THE INFLUENCE OF THE NER TO ITS NORTHEAST. THE RIDGE IS EXPECTED TO
DEVELOP A WEAK SOUTHWARD-ORIENTED EXTENSION, INDUCING A BRIEF
DECELERATION IN VAIANU'S TRANSLATION SPEED BETWEEN TAUS 24-36 AS IT
SETTLES INTO A COL WITHIN THE EXTENSION. THE EXTENSION IS FORECAST
TO BUILD AFTER TAU 36, ALLOWING 31P TO ACCELERATE AND CURVE
SOUTHWESTWARD FOR THE REMAINDER OF THE FORECAST PERIOD. 31P IS
FORECAST TO BEGIN SUBTROPICAL TRANSITION AROUND TAU 72 AS IT BEGINS
TO INTERACT WITH THE UPPER-LEVEL JET, ENTRAIN DRY AIR, AND SUB-26C
SEA SURFACE TEMPERATURES. SUBTROPICAL TRANSITION IS FORECAST TO BE
COMPLETE NO LATER THAN TAU 96. REGARDING INTENSITY, 31P IS EXPECTED
TO SIGNIFICANTLY WEAKEN FOR THE NEXT 24 HOURS AS IT'S BATTERED WITH
HIGH VERTICAL WIND SHEAR. THE CONTINUOUS WEAKENING TREND WILL
CONTINUE FOR THE REMAINDER OF THE FORECAST PERIOD AS 31P
TRANSITIONS AND TRACKS INTO THE INCREASINGLY HOSTILE ENVIRONMENT TO
ITS SOUTH.

MODEL DISCUSSION: DETERMINISTIC TRACK MODEL GUIDANCE IS IN STRONG
AGREEMENT THAT 31P WILL TRACK SOUTHEASTWARD THROUGH TAU 36 WITH A
BRIEF PERIOD OF SLOW TRANSLATION SPEEDS BETWEEN TAU 24-48, FOLLOWED
BY A SOUTHEASTWARD TRACK FOR THE LATTER HALF OF THE FORECAST
PERIOD. THERE IS SLIGHT UNCERTAINTY REGARDING THE TIMING OF THE
SOUTHWESTWARD TURN, LEADING TO A 90NM CROSS-TRACK SPREAD AT TAU 48.
THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE MULTI-MODEL
CONSENSUS WITH OVERALL HIGH CONFIDENCE. RELIABLE INTENSITY AIDS ARE
IN TIGHT AGREEMENT, WITH ALL MESOSCALE AND DETERMINISTIC MODELS
DEPICTING A STEADY WEAKENING TREND THROUGH TAU 34 AND A GRADUAL
WEAKENING TREND FROM TAU 24 THROUGH TAU 96. THE JTWC INTENSITY
FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE OVERALL, WITH NEAR-TERM
UNCERTAINTY SURROUNDING VAINAU'S RECENT DECOUPLING AND THE IMPACT
THAT WILL HAVE ON INTENSITY, AND LONG-TERM INTENSITY FORECAST
UNCERTAINTY REGARDING THE STORM'S INTENSITY WHILE IT UNDERGOES
SUBTROPICAL TRANSITION.

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