WDPS32 PGTW 070300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING 
NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.1S 174.8E
   INITIAL INTENSITY: 100 KTS
   GEOGRAPHIC REFERENCE: 205 NM WEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 38 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A SLIGHTLY
BETTER DEFINED AND WARMER EYE THAN SIX HOURS AGO. THE EYEWALL AND
CENTRAL CONVECTIVE DISK REMAIN RAGGED, WITH AN EARLIER 061935Z
SSMIS MICROWAVE PASS CONTINUING TO SHOW AN INCOMPLETE EYEWALL OPEN
TOWARDS THE WEST. THIS IS LIKELY DUE TO 25-30 KT OF VERTICAL SHEAR
IN THE 850-400 MB LAYER FROM THE SOUTHWEST, RESULTING IN PERIODIC
EROSION OF INNER CORE CONVECTION ON THE LEFT-OF-SHEAR SIDE (WEST
AND NORTH). THE INITIAL INTENSITY IS BUMPED TO 100 KT BASED ON A
SLIGHT INCREASE IN OBJECTIVE SATELLITE-BASED INTENSITY ESTIMATES
AND AN EARLIER 061803Z RCM-2 SAR PASS. THE GALE-FORCE WIND FIELD ON
THE EASTERN SIDE APPEARS TO ENCOMPASS WESTERN FIJI BASED ON LOCAL
SURFACE OBSERVATIONS GUSTING AS HIGH AS 54 KTS AT NADI
INTERNATIONAL AIRPORT IN RECENT HOURS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   PHFO: T5.5 - 102 KTS
   CIMSS ADT: 87 KTS AT 062030Z
   CIMSS AIDT: 89 KTS AT 062030Z
   CIMSS D-MINT: 87 KTS AT 062114Z
   CIMSS D-PRINT: 95 KTS AT 062300Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 25-30 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

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

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 REMAINS
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 WILL MAINTAIN A
STEADY SPEED OF ADVANCE 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 ALSO
REMAINS STRAIGHT FORWARD. 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. IN 24 TO 36 HOURS, THIS SHEAR WILL INCREASE
SIGNIFICANTLY, CONCURRENT WITH A DROP IN SEA SURFACE TEMPERATURES
TO AROUND 26 DEGREES CELSIUS. THIS SHOULD CAUSE WEAKENING AFTER
ABOUT 24 HOURS. THIS WEAKENING IS UNLIKELY TO BE RAPID AS THE
CYCLONE MOVES BENEATH THE AFOREMENTIONED UPPER-LEVEL TROUGH,
ALLOWING A LARGE, SYMMETRIC VORTEX TO PERSIST FOR MULTIPLE DAYS AS
IT METHODICALLY TRACKS POLEWARD OVER GRADUALLY COOLING WATERS.
NUMERICAL MODELS EXPECT THE CIRCULATION TO 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 AND PLACED WITH HIGH CONFIDENCE, WITH MODEL
SPREAD DECREASING RELATIVE TO THE PREVIOUS FORECAST. THE JTWC
INTENSITY FORECAST IS ALSO CLOSE TO THE MULTI-MODEL CONSENSUS AND
PLACED WITH HIGH CONFIDENCE DUE TO TIGHT MODEL AGREEMENT.

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