WDPS32 PGTW 061500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.5S 173.8E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 280 NM WEST-NORTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 32 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT SYSTEM, WITH A SYMMETRICAL BALL OF DEEP CONVECTION ARRAYED
AROUND A NASCENT PINHOLE EYE. THE STRONGEST OF THE CONVECTION IS
ARRAYED ACROSS THE SOUTHERN SEMICIRCLE OF THE CIRCULATION,
CONSISTENT WITH THE 10-15 KNOTS OF SHEAR COMING DOWN ON THE SYSTEM
FROM THE NORTH. VORTICAL HOT TOWERS (VHTS) CAN BE SEEN DEVELOPING
ALONG THE SOUTHERN FLANK OF THE NASCENT EYE FEATURE, WHICH HAS BEEN
A PERSISTENT FEATURE FOR THE PAST SIX HOURS, BUT DUE TO THE
PERSISTENT NORTHERLY SHEAR, THEY ARE FAILING TO WRAP UPSHEAR AND
THUS THE EYE HAS BEEN UNSUCCESSFUL IN FULLY EMERGING. A 061125Z GMI
COLOR-ENHANCED 37GHZ IMAGE REVEALED A WELL-FORMED, HIGHLY
SYMMETRICAL MICROWAVE EYE, WITH STRONG CONVECTION ON THE
SOUTHEASTERN AND SOUTHERN FLANKS, WHILE ONLY WEAK CONVECTION IS
PRESENT IN THE NORTHWESTERN QUADRANT. THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE USING THE MICROWAVE EYE FEATURE IN
THE GMI PASS. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM
CONFIDENCE USING A BLEND OF THE AGENCY INTENSITY ESTIMATES, ALONG
WITH PERSISTENCE WITH AN EARLIER SAR PASS. ENVIRONMENTAL CONDITIONS
REMAIN FAVORABLE IN GENERAL, WITH LOW DEEP-LAYER SHEAR, WARM SSTS
AND VIGOROUS POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER AND SAR DATA.  

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED NEAR SAMOA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.5 - 102 KTS
   PHFO: T5.5 - 102 KTS
   CIMSS ADT: 79 KTS AT 061200Z
   CIMSS AIDT: 79 KTS AT 061200Z
   CIMSS D-PRINT: 88 KTS AT 061200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: MODERATE MID-LEVEL NORTHERLY SHEAR. 

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 31P (VAIANU) APPEARS TO
HAVE FINALLY ACHIEVED VORTEX ALIGNMENT, AND AFTER A PERIOD OF
RELATIVELY SLOW FORWARD SPEED, HAS BEGUN TO ACCELERATE
SOUTHEASTWARD. A STRAIGHT SOUTHEASTWARD TRACK, ALONG THE WESTERN
FLANK OF THE DEEP STR NEAR SAMOA, IS FORECAST FOR THE NEXT 72
HOURS. BY TAU 72, TC 31P PASSES UNDER THE AXIS OF A STRENGTHENING
SUBTROPICAL JET MAX AND BEGINS TO BE BOTH PULLED IN TOWARDS A 200MB
TROUGH, WHILE SIMULTANEOUSLY BEING PUSHED SOUTHWESTWARD ALONG THE
WESTERN SIDE OF A RAPIDLY BUILDING RIDGE TO THE EAST WHICH BUILDS
POLEWARD TO 40S. BY THE END OF THE FORECAST, TC 31P WILL BE
TRACKING SOUTHWESTWARD TOWARDS THE NORTH ISLAND OF NEW ZEALAND.
REGARDING INTENSITY, THE SYSTEM HAS ANOTHER 24 HOURS WORTH OF
FAVORABLE CONDITIONS TO INTENSIFY BEFORE WIND SHEAR INCREASES
SHARPLY. ANTICIPATING AN EYE TO DEVELOP IN THE NEAR-TERM, THE
FORECAST CALLS FOR THE SYSTEM TO INTENSIFY TO A PEAK OF 105 KNOTS
WITHIN THE NEXT 12 TO 24 HOURS, THEN BEGIN TO RAPIDLY WEAKEN UNDER
THE INFLUENCE OF HIGH NORTHWESTERLY SHEAR. THE SYSTEM CROSSES THE
26C ISOTHERM AROUND TAU 60. ONCE THE SYSTEM CROSSES THE AXIS OF THE
SUBTROPICAL JET, IT MOVES INTO THE EQUATORWARD SIDE (UNDER AN
UPPER-LEVEL TROUGH OR LOW) OF A LARGE REX BLOCK THAT DEVELOPS
BETWEEN FIJI AND NEW ZEALAND. AT THIS POINT, SSTS WILL BE IN THE
22-24C RANGE. HOWEVER, ALL GLOBAL MODELS INDICATE THE SYSTEM WILL
RETAIN ITS WARM CORE AND REMAIN RELATIVELY MOIST THROUGH A DEEP
LAYER OF THE ATMOSPHERE, THOUGH UNDER A COLD 200MB LOW, THE
FORECAST CALLS FOR EXTRATROPICAL TRANSITION (ETT) BEGINNING AT TAU
72, COMPLETING BY THE END OF THE FORECAST. HOWEVER, A MORE ACCURATE
DESCRIPTION WOULD BE TO CALL THE REMNANTS OF TC 31P A HYBRID WARM
CORE CYCLONE. UNDER FAVORABLE, LOW SHEAR CONDITIONS AND WITH A WARM
LOW- TO MID-LEVEL VORTEX UNDER A COLD LOW ALOFT, THE SYSTEM COULD
IN FACT REINTENSIFY OR AT LEAST MAINTAIN INTENSITY AS IT UNDERGOES
TRANSITION TO A HYBRID CYCLONE.  

MODEL DISCUSSION: TRACK GUIDANCE IS IN STRONG AGREEMENT THROUGH TAU
72, WITH MINIMAL (LESS THAN 75NM) CROSS-TRACK SPREAD THROUGH THIS
POINT IN THE FORECAST. ALONG-TRACK SPREAD REMAINS A PERSISTENT
ISSUE EVEN AS EARLY AS TAU 48, WITH THE FASTEST AI MODELS OPENING
UP A LEAD OVER THE EGRR AND GALWEM OF 200NM BY TAU 72. AS THE
SYSTEM TURNS SOUTHWESTWARD, CROSS-TRACK SPREAD INCREASES TO 315NM,
BETWEEN THE EGRR-GALWEM PAIR ON THE WEST EDGE AND THE AI MODEL
GROUPING ON THE EASTERN EDGE OF THE ENVELOPE, WHILE ALONG-TRACK
SPREAD DECREASES TO BASICALLY ZERO. THE JTWC FORECAST IS PLACED
NEAR THE CONSENSUS MEAN THROUGH THE ENTIRE FORECAST PERIOD, WITH
HIGH CONFIDENCE THROUGH TAU 72 AND MEDIUM CONFIDENCE THROUGH TAU
120. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, WITH ALL MODELS
EXCEPT THE HWRF INDICATING ABOUT 5-15 KNOTS OF INTENSIFICATION
THROUGH TAU 24 THEN SHARP WEAKENING THROUGH TAU 72. MOST OF THE
GUIDANCE DEPICTS EITHER SLOW WEAKENING OR STEADY INTENSITY FROM
TAU 84 TO 120. THE JTWC FORECAST IS PLACED NEAR THE HAFS-A GUIDANCE
WITH MEDIUM CONFIDENCE. 

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