WDPS32 PGTW 082100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.4S 178.5E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 438 NM SOUTH OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) THAT HAS DECOUPLED FROM
THE MID-UPPER LEVEL CIRCULATION WHICH HAS BEEN SHEARED TO THE SOUTH
AND EAST AS A RESULT OF STRONG VERTICAL WIND SHEAR (VWS). A 081557Z
F18 SSMIS 91 GHZ MICROWAVE IMAGE REVEALS LITTLE REMAINING DEEP
CONVECTION ASSOCIATED WITH THE LLCC IN THE SOUTHERN PERIPHERY. THE
IMAGE ALSO SHOWS WELL-DEFINED LOW-LEVEL CLOUD LINES WRAPPING
TIGHTLY INTO THE LLCC. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE SSMIS IMAGE AND ANIMATED EIR IMAGERY OF THE
EXPOSED LLCC. THE INITIAL INTENSITY OF 60 KTS IS ASSESSED WITH LOW
CONFIDENCE DUE TO THE UNDER-ESTIMATION AND LOW BIAS OF THE DVORAK
TECHNIQUE EXPECTED WITH A SYSTEM THAT LACKS CONVECTION NEAR THE
CENTER. THE ENVIRONMENT IS CHARACTERIZED AS UNFAVORABLE FOR FURTHER
TROPICAL DEVELOPMENT WITH VERY HIGH VWS, DRY AIR ENTRAINMENT FROM
THE NORTH, AND COOL SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: PERSISTENCE FROM EARLIER SAR DATA WITH
ADJUSTMENTS BASED ON THE OBJECTIVE BEST TRACK

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A
NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS SATCON: 49 KTS AT 081610Z
   CIMSS ADT: 35 KTS AT 081800Z
   CIMSS AIDT: 43 KTS AT 081800Z
   CIMSS D-MINT: 50 KTS AT 081556Z
   CIMSS D-PRINT: 40 KTS AT 081800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 35-40 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE NORTH

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: TRUNCATED FORECAST TO 48 HOURS

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 31P (VAIANU) IS FORECAST
TO CONTINUE ON A SOUTHWARD TRAJECTORY THROUGH TAU 36 UNDER THE
STEERING INFLUENCE OF THE NER TO THE NORTHEAST. A
SOUTH-SOUTHWESTWARD TRACK TOWARD THE NORTH ISLAND OF NEW ZEALAND IS
ANTICIPATED AS THE SYSTEM INTERACTS WITH AN EXTENSION OF DEEP-LAYER
RIDGING TO THE EAST. TC 31P IS FORECAST TO TRACK UNDER AN
UPPER-LEVEL JET STREAK AND WILL BE LOCATED POLEWARD OF THE JET
AROUND TAU 24. A GRADUAL WEAKENING TREND IS FORECAST THROUGH THE
ENTIRE FORECAST PERIOD UNDER COMPETING DYNAMICAL FORCING. COOL SEA
SURFACE TEMPERATURES, DRY AIR ENTRAINMENT, AND HIGH VWS THROUGH TAU
24 ARE OFFSET BY EXCELLENT POLEWARD OUTFLOW ALOFT IN THE PRESENCE
OF THE JET STREAK. AFTER TAU 24, THE VWS IS EXPECTED TO ABATE WHILE
A JET STREAK TO THE SOUTH CONTINUES TO SUPPORT ROBUST POLEWARD
OUTFLOW. THE COMBINATION OF SYNOPTIC FORCING WILL ALLOW VAIANU TO
MAINTAIN A BROAD WIND FIELD AND INTENSITIES OF 50 KNOTS THROUGH TAU
48. THIS SYNOPTIC SETUP WILL INITIATE SUBTROPICAL TRANSITION AROUND
TAU 36, WHICH IS EXPECTED TO BE COMPLETE BY TAU 48 AS THE SYSTEM
DEVELOPS BAROCLINICITY AND TRACKS OVER COLD (22-23 C) SEA SURFACE
TEMPERATURES.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT ABOUT
THE SOUTHWARD TRACK AND TIMING OF A SHIFT TOWARD THE
SOUTH-SOUTHWEST, WITH CROSS-TRACK SPREAD OF 80 NM AT TAU 48. THE
JTWC TRACK FORECAST IS PLACED CLOSE TO THE MULTI-MODEL CONSENSUS
WITH HIGH CONFIDENCE. REGARDING INTENSITY, THE GUIDANCE IS IN
GENERALLY GOOD AGREEMENT WITH THE GRADUAL WEAKENING TREND
THROUGHOUT THE ENTIRE FORECAST PERIOD. THE JTWC INTENSITY FORECAST
IS ALSO PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH HIGH
CONFIDENCE. 

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