WDPS32 PGTW 070900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.2S 175.3E
   INITIAL INTENSITY: 100 KTS
   GEOGRAPHIC REFERENCE: 188 NM WEST-SOUTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 40 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE (TC) 31P (VAIANU) TRACKING SOUTHEASTWARD UNDER THE
INFLUENCE OF A NEAR-EQUATORIAL (NER) RIDGE CENTERED TO THE EAST.
OVER THE LAST 6 HOURS, THE EYE FEATURE HAS FILLED NEARLY COMPLETELY, 
WHILE STRONG, MID-LEVEL WESTERLY SHEAR CAN BE OBSERVED, LIMITING THE 
CONVECTIVE ACTIVITY OVER THE WESTERN PORTION OF THE CIRCULATION. 
ADDITIONAL ENVIRONMENTAL ANALYSIS REVEALS DRY AIR ENTERING THE CORE OF 
THE VORTEX, ORIGINATING FROM THE AIR MASS TO THE WEST AND NORTHWEST. 
BALANCING THOSE UNFAVORABLE CONDITIONS ARE WARM (29-29 C) SEA SURFACE 
TEMPERATURES AND ROBUST POLEWARD OUTFLOW CHANNEL. THE INITIAL POSITION 
IS PLACED WITH HIGH CONFIDENCE BASED ON THE ANIMATED VISIBLE 
SATELLITE LOOP AND EXTRAPOLATION FROM A 070455Z F17 SSMIS MICROWAVE 
IMAGE SUITE. THE INITIAL INTENSITY OF 100 KTS IS ASSESSED WITH MEDIUM 
CONFIDENCE BASED ON THE AGENCY DVORAK AND OBJECTIVE ESTIMATES LISTED 
BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE CENTERED TO THE
EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   KNES: T4.5 - 77 KTS
   PHFO: T5.5 - 102 KTS
   NFFN: T5.0 - 90 KTS
   CIMSS ADT: 104 KTS AT 070530Z
   CIMSS AIDT: 97 KTS AT 070530Z
   CIMSS D-MINT: 106 KTS AT 070457Z
   CIMSS D-PRINT: 99 KTS AT 070530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 28-29 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: TC 31P IS FORECAST TO TRACK SOUTHEASTWARD
UNDER THE INFLUENCE OF THE NER CENTERED TO THE EAST THROUGH TAU 48.
AFTER THAT, THE SYSTEM WILL BE LOCATED SOUTH OF THE SUBTROPICAL JET
AND UNDER AN UPPER-LEVEL TROUGH. THE SOUTHERN EXTENSION OF A
DEEP-LAYER RIDGE TO THE SOUTHEAST WILL STEER TC VAIANU SOUTHWESTWARD,
UNTIL THE SYSTEM ROUNDS THE AXIS OF THE RIDGE AROUND TAU 96. 
SIMULTANEOUSLY, WHILE THE SEA SURFACE TEMPERATURES ARE COOLING, THE
SUBTROPICAL TRANSITION (STT) IS EXPECTED TO INITIATE. STT IS
EXPECTED TO CONCLUDE NEAR TAU 120, AS THE SYSTEM APPROACHES NEW
ZEALAND AND POSITIONS ITSELF UNDERNEATH THE JET MAXIMUM. IN REGARD
TO INTENSITY, ALL AVAILABLE GUIDANCE INDICATES STEADY AND CONSISTENT 
WEAKENING AS THE VERTICAL WIND SHEAR (VWS) IS EXPECTED TO FURTHER 
INCREASE, REACHING 40-45 KTS, AROUND TAU 48. CONCURRENTLY, SEA SURFACE 
TEMPERATURES ARE EXPECTED TO DROP TO AROUND 26 DEGREES CELSIUS. AS THE 
VWS BACKS OFF NEAR TAU 48, TC 31P IS EXPECTED TO WEAKEN AT A SLOWER 
RATE, AND CONSIDERING THE PROXIMITY OF THE UPPER-LEVEL TROUGH, THE 
CIRCULATION IS EXPECTED TO MAINTAIN INTENSITY AROUND 50-55 KTS WELL 
INTO ITS SUBTROPICAL PHASE NORTH OF NEW ZEALAND.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT THROUGHOUT THE FORECAST PERIOD AND THE JTWC FORECAST IS
ASSESSED WITH HIGH CONFIDENCE. SIMILARLY, THE MEMBERS OF THE
MULTI-MODEL CONSENSUS AGREE ON THE WEAKENING PATTERN. THE MAXIMUM
WIND SPEED GUIDANCE SPREAD INCREASES EVER SO SLIGHTLY TO 20 KTS
AFTER TAU 48, LOWERING THE LONG TERM INTENSITY FORECAST CONFIDENCE.
OVERALL, BOTH TRACK AND INTENSITY FORECASTS ARE PLACED CLOSE TO THE
GUIDANCE OF THE MULTI-MODEL CONSENSUS.

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