WDPS32 PGTW 051500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING 
NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.9S 172.4E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 290 NM NORTHEAST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
RAPIDLY CONSOLIDATING, COMPACT CENTRAL DENSE OVERCAST (CDO), WITH A
NASCENT EYE BEGINNING TO DEVELOP IN THE MOST RECENT FRAMES IN THE
ANIMATION. A 051210Z GMI 89GHZ IMAGE REVEALED WELL-DEFINED SPIRAL
BANDING FEATURES WRAPPING IN A WIDE ARC INTO A QUICKLY SEPARATING
INNER CORE OF DEEP CONVECTION, WITH A SMALL MICROWAVE EYE FEATURE.
DEEP CONVECTIVE HOT TOWERS ARE DEVELOPING IN THE CENTRAL CORE
REGION OF THE CDO, SURROUNDING THE EYE, AND ACCOMPANIED BY A ROBUST
INNER-CORE LIGHTNING BURST, A POTENTIAL INDICATOR OF NEAR-TERM
RAPID INTENSIFICATION (RI). THE INITIAL INTENSITY IS ASSESSED WITH
HIGH CONFIDENCE BASED ON THE MICROWAVE EYE IN THE GMI IMAGERY. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, BASED ON THE
SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES AVAILABLE AT ANALYSIS
TIME, THOUGH IT IS ALREADY LIKELY TOO LOW BASED ON THE RAPIDLY
DEVELOPING SITUATION. ENVIRONMENTAL CONDITIONS ARE HIGHLY FAVORABLE
FOR RI, WITH LOW VERTICAL WIND SHEAR (VWS), WARM SSTS, HIGH OHC,
AND ROBUST POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   KNES: T3.5 - 55 KTS
   PHFO: T3.5 - 55 KTS
   NFFN: T3.0 - 45 KTS
   CIMSS ADT: 57 KTS AT 051140Z
   CIMSS AIDT: 43 KTS AT 051140Z
   CIMSS D-PRINT: 60 KTS AT 051300Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 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: TROPICAL CYCLONE (TC) 31P (VAIANU) IS FORECAST
TO TRACK POLEWARD ALONG THE WESTERN FLANK OF THE DEEP STR THROUGH
THE MAJORITY OF THE FORECAST PERIOD. ONLY AFTER TAU 96 WILL THE
TRACK DEVIATE TO A MORE SOUTHWARD TRAJECTORY, AS IT IS BOTH PULLED
SOUTHWARD INTO A DEEP-LAYER TROUGH AND PUSHED SOUTH ALONG THE
WESTERN PERIPHERY OF A VERY STRONG RIDGE REACHING ALL THE WAY TO
45S ALONG THE 160W LONGITUDE. REGARDING INTENSITY, TC 31P IS PRIMED
TO RI IN THE NEAR-TERM, UNDER VERY FAVORABLE ENVIRONMENTAL
CONDITIONS. THE WINDOW OF OPPORTUNITY HOWEVER, IS FINITE, AND THE
CONDITIONS WILL ONLY LAST FOR ABOUT 36 HOURS. THEREAFTER, SHEAR
WILL RAPIDLY INCREASE, TO MORE THAN 30 KNOTS BY TAU 48, MORE THAN
50 KNOTS BY TAU 96 AND HIGHER STILL BY THE END OF THE FORECAST
PERIOD AS THE SYSTEM PASSES UNDER A POLAR FRONT JET STREAK AND
BEGINS EXTRA-TROPICAL TRANSITION (ETT). THE RAPIDLY INCREASING
SHEAR WILL ALSO USHER IN EXTREMELY DRY MID-LEVEL AIR, HELPING TO
SQUASH TC 31P. WHILE THE SYSTEM WILL BE DECAPITATED BY TAU 72 AND
SMOTHERED BY DRY AIR AFTER TAU 96, IT WILL STILL TAKE TIME TO SPIN
DOWN IN INTENSITY. BY THE END OF THE FORECAST PERIOD, BAROCLINIC
FORCING WILL FURTHER SLOW THE RATE OF WEAKENING AS THE SYSTEM
BEGINS ETT. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN STRONG AGREEMENT, WITH
NAVGEM THE ONLY OUTLIER AMONGST THE CONSENSUS MEMBERS, DEPICTING A 
TRACK FAR OUT TO THE EAST TOWARDS TONGA. THE REMAINDER OF THE 
CONSENSUS MEMBERS ARE CONFINED TO A GRADUALLY EXPANDING ENVELOPE THAT 
WIDENS TO 105NM BY TAU 72 AND 280NM BY TAU 120. THE LARGER ISSUE IS 
ALONG-TRACK SPREAD, PARTICULARLY BETWEEN THE AI MODELS (EC-AIFS, GDM 
AND THE AI CONSENSUS) AND THE REMAINDER OF THE GUIDANCE. AT TAU 72, 
ALONG-TRACK SPREAD IS ABOUT 80NM, INCREASING TO 250NM BETWEEN THE 
SLOW GFS AND FAST AI MODELS, BY TAU 120. THE JTWC FORECAST IS 
POSITIONED CLOSE TO THE CONSENSUS MEAN (MINUS NAVGEM) THROUGH TAU 72, 
THEN ALONG THE TRACK LINE OF THE MEAN, BUT SLIGHTLY FASTER, HEDGED 
TOWARDS THE AI CONSENSUS. CONFIDENCE IS HIGH THROUGH THE ENTIRE 
FORECAST PERIOD. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT. NEARLY 
EVERY RI AID IS TRIGGERED, INDICATING A PEAK INTENSITY BETWEEN 105-
130 KNOTS. ALL MODEL GUIDANCE SHOWS THE PEAK OCCURRING NEAR TAU 36, 
THEN A MORE OR LESS RAPID WEAKENING THROUGH THE REMAINDER OF THE 
FORECAST PERIOD. THE JTWC FORECAST IS PLACED CONSISTENT WITH THE RI 
AIDS THROUGH TAU 36, THEN CLOSE TO THE CONSENSUS MEAN THEREAFTER, 
WITH MEDIUM CONFIDENCE. 

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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