WDPS32 PGTW 080900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 23.4S 177.8E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 1079 NM NORTH OF WELLINGTON, NEW ZEALAND
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE (TC) 31P (VAIANU) CONTINUING ITS SOUTHEASTWARD TRANSIT
ALONG THE SOUTHWESTERN FLANK OF A NEAR-EQUATORIAL RIDGE (NER). THE
INTENSITY OF THE WESTERLY VERTICAL WIND SHEAR HAS INCREASED
SIGNIFICANTLY AND IS NOW WELL EXCEEDING 30 KTS, RESULTING IN 
DECOUPLING OF THE DEEP CONVECTION POLEWARD AND EASTWARD. THE LOW-LEVEL 
CIRCULATION CENTER IS STILL OBSCURED, BUT THE NORTHWESTERN QUADRANT IS 
BECOMING INCREASINGLY EXPOSED. THE INITIAL POSITION IS ASSESSED WITH 
HIGH CONFIDENCE BASED ON THE 080442Z F18 SSMIS 37 AND 89 GHZ MICROWAVE 
IMAGES. THE INITIAL INTENSITY OF 75 KTS IS ASSESSED WITH MEDIUM 
CERTAINTY BASED ON AGENCY DVORAK ESTIMATES AND THE OBJECTIVE INTENSITY 
ALGORITHMS LISTED BELOW. ADDITIONAL ENVIRONMENTAL ANALYSIS REVEALS 
CONTINUED ENTRAINMENT OF DRY, MID-TROPOSPHERIC AIR FROM THE NORTH AND 
COOLING SEA SURFACE TEMPERATURES, CULMINATING IN A MARGINALLY 
UNFAVORABLE ENVIRONMENT FOR SIGNIFICANT REORGANIZATION. PARTIALLY 
MITIGATING THESE NEGATIVE FACTORS ARE STILL RELATIVELY WARM (26-27 C) 
SEA SURFACE TEMPERATURES AND ROBUST POLEWARD-DIRECTED OUTFLOW CHANNEL 
ALOFT.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE (NER) CENTERED TO
THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   KNES: T4.5 - 77 KTS
   CIMSS SATCON: 93 KTS AT 080148Z
   CIMSS ADT: 69 KTS AT 080540Z
   CIMSS AIDT: 63 KTS AT 080540Z
   CIMSS D-MINT: 78 KTS AT 080441Z
   CIMSS D-PRINT: 56 KTS AT 080640Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30+ KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE NORTH

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 CONTINUE ON ITS PRESENT
SOUTHEASTWARD HEADING THROUGH TAU 36, UNDER THE PERSISTENT STEERING
INFLUENCE OF THE AFOREMENTIONED NEAR-EQUATORIAL RIDGE. BEYOND THAT
TIMEFRAME, THE CYCLONE IS SLATED TO CROSS TO THE POLEWARD SIDE OF
THE SUBTROPICAL JET AXIS, ENTERING AN AREA OF INCREASING
SYNOPTIC-SCALE FORCING ASSOCIATED WITH AN AMPLIFYING UPPER-LEVEL
TROUGH. DURING THIS TIMEFRAME, VERTICAL WIND SHEAR IS EXPECTED TO
SIGNIFICANTLY ABATE. WHILE MODEL FIELDS SHOW A SIGNIFICANT AMOUNT OF
DRY AIR SURROUNDING THE SYSTEM, TC 31P IS EXPECTED TO MAINTAIN A
BROAD WIND FIELD AND A FAIRLY SYMMETRICAL AND MOIST STRUCTURE,
RESULTING IN A BRIEF PERIOD OF PRESERVED INTENSITY. AROUND TAU 48
AN EXTENSION OF A DEEP-LAYER RIDGING LOCATED TO THE SOUTHEAST WILL
PROMPT A TRANSIENT DEFLECTION AND ACCELERATION OF THE SYSTEM TOWARD
THE SOUTHWEST UNTIL TC VAIANU ROUNDS THE AXIS OF THE RIDGE EXTENSION 
SHORTLY AFTER TAU 72. CONCURRENTLY, THE INITIATION OF A SUBTROPICAL
TRANSITION IS IMMINENT AS THE CYCLONE TRAVERSES A SIGNIFICANTLY
COOLER SEA SURFACE TEMPERATURE GRADIENT (BELOW 26C). THE SUBTROPICAL 
TRANSITION (STT) PROCESS IS FORECAST TO CONCLUDE JUST PRIOR TO TAU 96 
AS THE REMNANT CIRCULATION CROSSES THE COASTLINE OF NORTH ISLAND, NEW 
ZEALAND AND BECOMES EMBEDDED WITHIN THE PRIMARY JET STREAK FLOW. WITH 
RESPECT TO INTENSITY, A STEADY ATTRITION IS PROJECTED AS VERTICAL WIND 
SHEAR IS EXPECTED TO FURTHER INCREASE TO 35-40 KTS BY TAU 24. AS THE 
SHEAR VECTOR IS FORECAST TO MODERATE AROUND TAU 48, THE RATE OF DECAY 
SHOULD DIMINISH, WITH THE DYNAMIC INTERACTION WITH THE UPPER-LEVEL 
TROUGH EXPECTED TO HELP MAINTAIN A VORTEX INTENSITY OF 40-45 KTS DEEP 
INTO ITS SUBTROPICAL PHASE. WHILE THE SYSTEM MAKES LANDFALL OVER NEW 
ZEALAND, FRICTIONAL EFFECTS OF LAND INTERACTION WILL FURTHER 
ACCELERATE THE WEAKENING PHASE AND POTENTIALLY COMPLETELY DISSIPATE 
THE VORTEX CONCURRENTLY TO COMPLETION OF THE STT.

MODEL DISCUSSION: THERE IS A ROBUST AGREEMENT AMONG THE NUMERICAL
MODEL GUIDANCE CONCERNING THE FORECAST TRACK, WHICH PROVIDES HIGH
CONFIDENCE IN THE OFFICIAL JTWC TRAJECTORY. SIMILARLY, THE
INTENSITY MODEL CONSENSUS DEMONSTRATES A STRONG CONFLUENCE
REGARDING THE OVERALL DECAY SCENARIO. THE MULTI-MODEL INTENSITY
SPREAD WIDENS IN THE EXTENDED FORECAST, AS SOME MODELS INDICATE
VORTEX DISSIPATION SOON AFTER TAU 72, THEREBY SLIGHTLY REDUCING
LONG-RANGE FORECAST CONFIDENCE. BOTH THE OFFICIAL TRACK AND THE
INTENSITY FORECASTS ARE ANCHORED FIRMLY TO THE MULTI-MODEL
CONSENSUS MEAN.

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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