WDPS32 PGTW 072100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.1S 177.0E
   INITIAL INTENSITY: 90 KTS
   GEOGRAPHIC REFERENCE: 197 NM SOUTH-SOUTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 35 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM (TC) 31P WITH A FILLED EYE AND DRY AIR ALONG THE WESTERN
FLANK OF THE STORM, SLIGHTLY ENTRAINING INTO THE CORE. A 071610Z
SSMIS REVEALS THE MICROWAVE EYE IS BROAD WITH SLIGHT SOUTHEASTWARD
TILT WITH HEIGHT. 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 HIGH CONFIDENCE BASED ON
THE AFOREMENTIONED 071610Z SSMIS MICROWAVE IMAGE. THE INITIAL
INTENSITY OF 90 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: T5.0 - 90 KTS
   KNES: T5.5 - 102 KTS
   PHFO: T5.0 - 90 KTS
   CIMSS ADT: 87 KTS AT 071930Z
   CIMSS AIDT: 87 KTS AT 071930Z
   CIMSS D-MINT: 92 KTS AT 071609Z
   CIMSS D-PRINT: 83 KTS AT 071930Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 KTS
   SST: 25-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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 EXTENSION, INDUCING A BRIEF DECELERATION IN VAIANU'S
TRANSLATION SPEED BETWEEN TAUS 24-36 AS IT SETTLES INTO A COL. 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 CONTINUOUSLY WEAKEN FOR THE
DURATION OF THE FORECAST PERIOD AS IS 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 36-48 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. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE
MULTI-MODEL CONSENSUS WITH OVERALL HIGH CONFIDENCE. RELIABLE
INTENSITY AIDS ARE ALSO IN STRONG AGREEMENT, WITH ALL AVAILABLE
MODELS DEPICTING A STEADY WEAKENING TREND THROUGH TAU 36 AND A
GRADUAL WEAKENING TREND FROM TAU 36 THROUGH TAU 96. THE JTWC
INTENSITY FORECAST IS ASSESSED WITH HIGH CONFIDENCE FROM TAU 0-72
AND MEDIUM CONFIDENCE FROM TAU 72-96, WITH 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: HIGH
   INTENSITY 72-120 HR: MEDIUM//
NNNN