WDPS32 PGTW 071500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.0S 175.9E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 182 NM SOUTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 38 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY REVEALS TROPICAL
CYCLONE (TC) 31P (VAIANU) PROPAGATING SOUTHEASTWARD ALONG THE
SOUTHWESTERN PERIPHERY OF A NEAR-EQUATORIAL RIDGE (NER) CENTERED TO
THE NORTHEAST. THE EYE FEATURE HAS BECOME FULLY OBSCURED AND CLOUD-
FILLED. WESTERLY VERTICAL WIND SHEAR HAS MARGINALLY SUBSIDED
OVER THE LAST 6 HOURS, BUT THE DRY AIR ENTRAINMENT IS STILL
PREVALENT RESULTING IN AN OVERALL MARGINALLY UNFAVORABLE ENVIRONMENT.
OFFSETTING THESE CHARACTERISTICS ARE FAVORABLE AND WARM
(28-29 C) SEA SURFACE TEMPERATURES (SST) AND A WELL-DEFINED
POLEWARD UPPER-LEVEL OUTFLOW. THE INITIAL POSITION IS PLACED WITH
HIGH CONFIDENCE UTILIZING THE ANIMATED INFRARED SATELLITE LOOP,
COMBINED WITH THE EXTRAPOLATION FROM A 070930Z METOP-C ASCAT PASS.
THE INITIAL INTENSITY OF 95 KTS IS ALSO ESTIMATED WITH HIGH
CERTAINTY, BASED ON A CONSENSUS OF AGENCY DVORAK CLASSIFICATIONS
AND OBJECTIVE SATELLITE-BASED INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: A 070701Z RCM-3 SAR PASS

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.5 - 102 KTS
   PHFO: T5.5 - 102 KTS
   NFFN: T5.0 - 90 KTS
   CIMSS ADT: 97 KTS AT 071140Z
   CIMSS AIDT: 95 KTS AT 071140Z
   CIMSS D-PRINT: 88 KTS AT 071240Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 31P IS PROJECTED TO MAINTAIN ITS
SOUTHEASTWARD TRAJECTORY, STEERED BY THE AFOREMENTIONED
NEAR-EQUATORIAL RIDGE, THROUGH TAU 48. SUBSEQUENTLY, THE CYCLONE
WILL MIGRATE TO THE SOUTH OF THE SUBTROPICAL JET AXIS AND WILL ENTER A
REGION OF INCREASING UPPER-LEVEL TROUGHING. A DEEP-LAYER
ANTICYCLONE TO THE SOUTHEAST WILL THEN INDUCE A BRIEF SOUTHWESTWARD
TURN, A TRAJECTORY EXPECTED TO PERSIST UNTIL THE SYSTEM TRANSITS
THE RIDGE AXIS NEAR TAU 96. CONCURRENTLY, THE ONSET OF SUBTROPICAL
TRANSITION IS ANTICIPATED AS THE CYCLONE MOVES OVER MUCH COOLER
WATERS (BELOW 26 C). THIS TRANSFORMATION IS EXPECTED TO BE COMPLETE
JUST PRIOR TO TAU 120, AS THE SYSTEM MAKES LANDFALL OVER THE NORTH
ISLAND OF NEW ZEALAND, AND BECOMES POSITIONED UNDERNEATH THE JET
MAXIMUM. REGARDING INTENSITY, A CONSISTENT DECAY IS FORECAST AS
VERTICAL WIND SHEAR IS PROJECTED TO INCREASE SUBSTANTIALLY,
REACHING 40-45 KTS AS EARLY AS TAU 24 AND PERSISTING THROUGH AT
LEAST TAU 48.  AS THE SHEAR INTENSITY IS EXPECTED TO RELAX AFTER
TAU 48, THE RATE OF WEAKENING SHOULD DECREASE, AND THE SYSTEMS
INTERACTION WITH THE UPPER-LEVEL TROUGH IS EXPECTED TO HELP IT
SUSTAIN AN INTENSITY NEAR 45-55 KTS DEEP INTO ITS SUBTROPICAL
PHASE.

MODEL DISCUSSION: A HIGH DEGREE OF CONSENSUS EXISTS AMONG THE
NUMERICAL WEATHER PREDICTION MODELS REGARDING THE TRACK FORECAST,
LENDING HIGH CONFIDENCE TO THE OFFICIAL JTWC FORECAST TRACK.
SIMILARLY, THE INTENSITY CONSENSUS MEMBERS ARE IN STRONG AGREEMENT
CONCERNING THE OVERALL WEAKENING TREND. THE GUIDANCE SPREAD
INCREASES MARGINALLY TO 20 KTS BEYOND TAU 48, THEREBY REDUCING
CONFIDENCE IN THE LONG-RANGE INTENSITY FORECAST. OVERALL, BOTH THE
OFFICIAL TRACK AND INTENSITY FORECASTS ARE HEAVILY WEIGHTED TOWARDS
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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