WDPS32 PGTW 060900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING 
NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.1S 173.6E
   INITIAL INTENSITY: 90 KTS
   GEOGRAPHIC REFERENCE: 300 NM WEST-NORTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 32 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A COMPACT
BALL OF SYMMETRICAL, VIGOROUS CONVECTION WRAPPING INTO A TIGHT INNER
CORE. ANIMATED ENHANCED INFRARED (EIR) IMAGERY SUGGESTS THE STORM
IS MAKING ANOTHER RUN AT FORMING AN EYE, BUT AS WITH PREVIOUS
ATTEMPTS, AN EYE HAS YET TO PERSIST FOR MORE THAN AN HOUR. THE
INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A WARM
SPOT IN THE EIR (USING THE BD ENHANCEMENT CURVE), WHICH TURNS OUT
TO BE RATHER TRANSIENT, HAVING ALREADY DISAPPEARED FROM THE
IMAGERY. UNFORTUNATELY, A LACK OF RECENT MICROWAVE IMAGERY
PRECLUDES A HIGHER CONFIDENCE ASSESSMENT. THE INITIAL POSITION IS
ASSESSED WITH MEDIUM CONFIDENCE, BASED ON THE PGTW DVORAK CURRENT
INTENSITY ESTIMATE, WHICH ALIGNS WITH THE CIMSS D-MINT FROM A
COUPLE OF HOURS AGO. THE KNES DVORAK FIX IS LIKELY TOO HIGH AS THEY
WERE USING THE EYE TECHNIQUE ON THE TRANSIENT EYE FEATURE, WHILE
PGTW DID NOT. REGARDLESS, THE SYSTEM IS STEADILY INTENSIFYING,
UNDER FAVORABLE ENVIRONMENTAL CONDITIONS CHARACTERIZED BY LOW VWS,
WARM SSTS, MODERATE OHC AND ROBUST POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.5 - 102 KTS
   CIMSS SATCON: 78 KTS AT 060540Z
   CIMSS ADT: 79 KTS AT 060610Z
   CIMSS AIDT: 79 KTS AT 060610Z
   CIMSS D-MINT: 90 KTS AT 060228Z
   CIMSS D-PRINT: 98 KTS AT 060610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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) WILL TRACK
STRAIGHT SOUTHEASTWARDS THROUGH TAU 96, ALONG THE WESTERN FLANK OF
A LARGE, STRONG AND VERY PERSISTENT STR POSITIONED SOUTH OF SAMOA.
PERSISTENT LOW-LEVEL WARM AIR ADVECTION ALONG THE EASTERN FLANK OF
TC 31P WILL BUILD THE RIDGE EVEN MORE, RESULTING IN A VERY STRONG
RIDGE THAT BUILDS POLEWARD, ULTIMATELY REACHING WELL PAST 40S
LATITUDE. IN THE UPPER-LEVELS (200MB), A REX BLOCK DEVELOPS BETWEEN
NEW ZEALAND AND FIJI AFTER TAU 96. THE COMBINATION OF THESE FACTORS
WILL RESULT IN A GRADUAL TRACK SHIFT AFTER TAU 72, WITH TC 31P
TURNING ONTO A SOUTHWESTWARD TRAJECTORY BY THE END OF THE FORECAST
PERIOD. REGARDING INTENSITY, IN THE NEAR-TERM, TC 31P IS FORECAST
TO CONTINUE INTENSIFYING AT A RATE JUST SHY OF THE RAPID
INTENSIFICATION (RI) THRESHOLD, UNDER FAVORABLE ENVIRONMENTAL
CONDITIONS. SHEAR WILL BEGIN INCREASING AFTER TAU 24, WITH THE
WINDOW OF OPPORTUNITY FOR INTENSIFICATION CLOSING AFTER TAU 36. THE
SYSTEM WEAKENS SLOWLY AT FIRST AS SHEAR BEGINS TO TILT THE VORTEX.
BY TAU 60, TC 31P CROSSES THE 26C ISOTHERM AND MOVES INTO COOLER
WATERS, AND SIMULTANEOUSLY SUCCUMBS TO DRY MID-LEVEL AIR
ENTRAINMENT AND WEAKENS MORE RAPIDLY THROUGH TAU 96. AROUND TAU 72
HOWEVER, THE SYSTEM CROSSES POLEWARD OF THE SUBTROPICAL JET WHICH
MARKS THE EQUATORWARD SIDE OF THE REX BLOCK AND MOVES INTO A REGION
OF MUCH LIGHTER UPPER-LEVEL SHEAR. WHILE WATERS WILL BE VERY COOL
AT THIS POINT, THE SYSTEM BEGINS TRANSITION TO AN EXTRA-TROPICAL
LOW (ETT), BY TAU 72. A MORE APPROPRIATE CHARACTERIZATION OF THE
SYSTEM AFTER TAU 72 WOULD BE A HYBRID TYPE SYSTEM, WITH
CHARACTERISTICS OF BOTH A MID-LATITUDE LOW AND A TROPICAL CYCLONE,
NOT FALLING NEATLY INTO EITHER CATEGORY. THE SYSTEM IS EXPECTED TO
RETAIN ITS WARM CORE AND WHILE IT SHALLOWS OUT AFTER TAU 48, ONCE
IT CROSSES UNDER THE REX BLOCK, THE VORTEX DEEPENS ONCE MORE AND
THE SYSTEM WILL LIKELY MAINTAIN INTENSITY AFTER TAU 96. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN STRONG AGREEMENT CROSS-TRACK
AGREEMENT THROUGH TAU 72, WITH ALL MEMBERS OF THE CONSENSUS
CONFINED TO A 100NM WIDE ENVELOPE AT THAT POINT IN TIME.
ALONG-TRACK SPREAD INCREASES SHARPLY AFTER TAU 48 HOWEVER,
INCREASING TO 175NM AT TAU 72, BETWEEN THE VERY SLOW EGRR AND
GALWEM AND THE FASTER AI MODELS. AS THE SYSTEM TURNS SOUTHWESTWARD
AFTER TAU 96, CROSS-TRACK SPREAD INCREASES SUBSTANTIALLY, UP TO
355NM BY TAU 120. EGRR MARKS THE INSIDE OR WESTERN EDGE OF THE
ENVELOPE, AND THE EC-AIFS MAKS THE EASTERN EDGE. SURPRISINGLY,
ALONG-TRACK SPREAD DECREASES TO LESS THAN 100NM DURING THIS SAME
TIMEFRAME. THE JTWC FORECAST IS POSITIONED NEAR THE CONSENSUS MEAN
THROUGH TAU 72, THEN THE AI MODEL CONSENSUS THROUGH THE END OF THE
FORECAST, HUGGING THE EASTERN SIDE OF THE MODEL ENVELOPE.
CONFIDENCE IS HIGH TO TAU 72, THEN MEDIUM. INTENSITY GUIDANCE IS
GENERALLY IN AGREEMENT THAT TC 31P WILL INTENSIFY FURTHER DURING
THE NEXT 24 HOURS, BUT VARY ON THE AMOUNT. SEVERAL RI AIDS
INCLUDING RIPA AND FRIA ARE TRIGGERED, REACHING A PEAK OF 130KTS IN
SOME CASES, WHILE RIPA AND FRIA PEAK AT 110 KNOTS. HWRF AND HAFS-A
ARE SLIGHTLY LESS AGGRESSIVE, REACHING A PEAK OF 105 KNOTS AND THE
REMAINDER OF THE AI GUIDANCE HOVERING AROUND 100 KNOTS. ALL MODELS
RAPIDLY WEAKEN THE SYSTEM BEYOND TAU 36 AND DEPICT A STEADY OR FLAT
INTENSITY TREND AFTER TAU 96. 

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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