Tropical Cyclone VAIANU (31P)

WTPS32 PGTW 090900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/TROPICAL CYCLONE 31P (VAIANU) WARNING NR 014//
RMKS/
1. TROPICAL CYCLONE 31P (VAIANU) WARNING NR 014    
   02 ACTIVE TROPICAL CYCLONES IN SOUTHPAC
   MAX SUSTAINED WINDS BASED ON ONE-MINUTE AVERAGE
   WIND RADII VALID OVER OPEN WATER ONLY
    ---
   WARNING POSITION:
   090600Z --- NEAR 26.8S 179.0E
     MOVEMENT PAST SIX HOURS - 165 DEGREES AT 06 KTS
     POSITION ACCURATE TO WITHIN 060 NM
     POSITION BASED ON CENTER LOCATED BY SATELLITE
   PRESENT WIND DISTRIBUTION:
   MAX SUSTAINED WINDS - 055 KT, GUSTS 070 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   BECOMING SUBTROPICAL
   RADIUS OF 050 KT WINDS - 090 NM NORTHEAST QUADRANT
                            075 NM SOUTHEAST QUADRANT
                            080 NM SOUTHWEST QUADRANT
                            070 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 170 NM NORTHEAST QUADRANT
                            200 NM SOUTHEAST QUADRANT
                            170 NM SOUTHWEST QUADRANT
                            140 NM NORTHWEST QUADRANT
   REPEAT POSIT: 26.8S 179.0E
    ---
   FORECASTS:
   12 HRS, VALID AT:
   091800Z --- 28.0S 179.1E
   MAX SUSTAINED WINDS - 055 KT, GUSTS 070 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   SUBTROPICAL
   RADIUS OF 050 KT WINDS - 090 NM NORTHEAST QUADRANT
                            070 NM SOUTHEAST QUADRANT
                            050 NM SOUTHWEST QUADRANT
                            040 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 160 NM NORTHEAST QUADRANT
                            190 NM SOUTHEAST QUADRANT
                            160 NM SOUTHWEST QUADRANT
                            130 NM NORTHWEST QUADRANT
   VECTOR TO 24 HR POSIT: 190 DEG/ 07 KTS
    ---
   24 HRS, VALID AT:
   100600Z --- 29.3S 178.8E
   MAX SUSTAINED WINDS - 055 KT, GUSTS 070 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   SUBTROPICAL
   RADIUS OF 050 KT WINDS - 080 NM NORTHEAST QUADRANT
                            060 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            020 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 180 NM NORTHEAST QUADRANT
                            240 NM SOUTHEAST QUADRANT
                            240 NM SOUTHWEST QUADRANT
                            190 NM NORTHWEST QUADRANT
   VECTOR TO 36 HR POSIT: 215 DEG/ 08 KTS
    ---
   36 HRS, VALID AT:
   101800Z --- 30.7S 177.7E
   MAX SUSTAINED WINDS - 050 KT, GUSTS 065 KT
   WIND RADII VALID OVER OPEN WATER ONLY
   SUBTROPICAL
   RADIUS OF 050 KT WINDS - 050 NM NORTHEAST QUADRANT
                            060 NM SOUTHEAST QUADRANT
                            020 NM SOUTHWEST QUADRANT
                            000 NM NORTHWEST QUADRANT
   RADIUS OF 034 KT WINDS - 190 NM NORTHEAST QUADRANT
                            250 NM SOUTHEAST QUADRANT
                            230 NM SOUTHWEST QUADRANT
                            160 NM NORTHWEST QUADRANT
    ---
REMARKS:
090900Z POSITION NEAR 27.1S 179.0E.
09APR26. TROPICAL CYCLONE 31P (VAIANU), LOCATED APPROXIMATELY 523
NM SOUTH OF SUVA, FIJI, HAS TRACKED SOUTH-SOUTHEASTWARD AT 06
KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED IMAGERY DEPICTS A 
PARTIALLY EXPOSED LLCC WITH A BURST OF CONVECTION OVER THE SOUTHERN 
SEMICIRCLE. THE SYSTEM EXHIBITS A HYBRID STRUCTURE CONSISTING OF 
SWIRLING MID-LEVEL CLOUDS AND LITTLE CENTRAL CONVECTION, LOCATED IN AN 
UNFAVORABLE ENVIRONMENT CHARACTERIZED BY 30 KTS OF VWS AND 25 DEGREE 
CELSIUS SST. ENVIRONMENTAL ANALYSIS REVEALS WEAK TEMPERATURE ADVECTION 
AND AN UPPER LEVEL TROUGH DIRECTLY ALOFT. THE POSITION AND INTENSITY 
ARE ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED CENTER AND A 
082330Z OSCAT-3 IMAGE DEPICTING 50 KT WINDS, RESPECTIVELY. THE SYSTEM 
WILL BE STEERED TOWARDS THE SOUTH BY A DEEP LAYER RIDGE TO THE EAST. 
TC 31P WILL TURN TOWARDS THE SOUTHWEST AS A RIDGE APPROACHES NEW 
ZEALAND FROM THE WEST AND INDUCES A MORE WESTWARD MOTION. WEAK 
BAROCLINIC FORCING WILL CAUSE THE SYSTEM TO COMPLETE SUBTROPICAL 
TRANSITION BY TAU 12 AND HELP TO SUSTAIN ITS INTENSITY THROUGH TAU 24, 
FOLLOWED BY GRADUAL WEAKENING. THE FORECAST TRACK AND INTENSITY ARE 
SET WITH HIGH CONFIDENCE BASED ON THE MULTI-MODEL CONSENSUS. THIS IS 
THE FINAL WARNING ON THIS SYSTEM BY THE JOINT TYPHOON WRNCEN PEARL 
HARBOR HI. THE SYSTEM WILL BE CLOSELY MONITORED FOR SIGNS OF 
REGENERATION. MINIMUM CENTRAL PRESSURE AT 090600Z IS 981 MB. MAXIMUM 
SIGNIFICANT WAVE HEIGHT AT 090600Z IS 27 FEET. REFER TO TROPICAL 
CYCLONE 30P (MAILA) WARNINGS (WTPS31 PGTW) FOR SIX-HOURLY UPDATES.//
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WDPS32 PGTW 082100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.4S 178.5E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 438 NM SOUTH OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) THAT HAS DECOUPLED FROM
THE MID-UPPER LEVEL CIRCULATION WHICH HAS BEEN SHEARED TO THE SOUTH
AND EAST AS A RESULT OF STRONG VERTICAL WIND SHEAR (VWS). A 081557Z
F18 SSMIS 91 GHZ MICROWAVE IMAGE REVEALS LITTLE REMAINING DEEP
CONVECTION ASSOCIATED WITH THE LLCC IN THE SOUTHERN PERIPHERY. THE
IMAGE ALSO SHOWS WELL-DEFINED LOW-LEVEL CLOUD LINES WRAPPING
TIGHTLY INTO THE LLCC. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE SSMIS IMAGE AND ANIMATED EIR IMAGERY OF THE
EXPOSED LLCC. THE INITIAL INTENSITY OF 60 KTS IS ASSESSED WITH LOW
CONFIDENCE DUE TO THE UNDER-ESTIMATION AND LOW BIAS OF THE DVORAK
TECHNIQUE EXPECTED WITH A SYSTEM THAT LACKS CONVECTION NEAR THE
CENTER. THE ENVIRONMENT IS CHARACTERIZED AS UNFAVORABLE FOR FURTHER
TROPICAL DEVELOPMENT WITH VERY HIGH VWS, DRY AIR ENTRAINMENT FROM
THE NORTH, AND COOL SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: PERSISTENCE FROM EARLIER SAR DATA WITH
ADJUSTMENTS BASED ON THE OBJECTIVE BEST TRACK

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A
NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS SATCON: 49 KTS AT 081610Z
   CIMSS ADT: 35 KTS AT 081800Z
   CIMSS AIDT: 43 KTS AT 081800Z
   CIMSS D-MINT: 50 KTS AT 081556Z
   CIMSS D-PRINT: 40 KTS AT 081800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 35-40 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE NORTH

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: TRUNCATED FORECAST TO 48 HOURS

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 31P (VAIANU) IS FORECAST
TO CONTINUE ON A SOUTHWARD TRAJECTORY THROUGH TAU 36 UNDER THE
STEERING INFLUENCE OF THE NER TO THE NORTHEAST. A SOUTH-SOUTHWESTWARD 
TRACK TOWARD THE NORTH ISLAND OF NEW ZEALAND IS ANTICIPATED AS THE 
SYSTEM INTERACTS WITH AN EXTENSION OF DEEP-LAYER RIDGING TO THE EAST. 
TC 31P IS FORECAST TO TRACK UNDER AN UPPER-LEVEL JET STREAK AND WILL 
BE LOCATED POLEWARD OF THE JET AROUND TAU 24. A GRADUAL WEAKENING 
TREND IS FORECAST THROUGH THE ENTIRE FORECAST PERIOD UNDER COMPETING 
DYNAMICAL FORCING. COOL SEA SURFACE TEMPERATURES, DRY AIR ENTRAINMENT, 
AND HIGH VWS THROUGH TAU 24 ARE OFFSET BY EXCELLENT POLEWARD OUTFLOW 
ALOFT IN THE PRESENCE OF THE JET STREAK. AFTER TAU 24, THE VWS IS 
EXPECTED TO ABATE WHILE A JET STREAK TO THE SOUTH CONTINUES TO SUPPORT 
ROBUST POLEWARD OUTFLOW. THE COMBINATION OF SYNOPTIC FORCING WILL 
ALLOW TC 31P TO MAINTAIN A BROAD WIND FIELD AND INTENSITIES OF 50 
KNOTS THROUGH TAU 48. THIS SYNOPTIC SETUP WILL INITIATE SUBTROPICAL 
TRANSITION AROUND TAU 36, WHICH IS EXPECTED TO BE COMPLETE BY TAU 48 
AS THE SYSTEM DEVELOPS BAROCLINICITY AND TRACKS OVER COLD (22-23 C) 
SEA SURFACE TEMPERATURES.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT ABOUT
THE SOUTHWARD TRACK AND TIMING OF A SHIFT TOWARD THE SOUTH-SOUTHWEST, 
WITH CROSS-TRACK SPREAD OF 80 NM AT TAU 48. THE JTWC TRACK FORECAST IS 
PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE. 
REGARDING INTENSITY, THE GUIDANCE IS IN GENERALLY GOOD AGREEMENT WITH 
THE GRADUAL WEAKENING TREND THROUGHOUT THE ENTIRE FORECAST PERIOD. THE 
JTWC INTENSITY FORECAST IS ALSO PLACED CLOSE TO THE MULTI-MODEL 
CONSENSUS WITH HIGH CONFIDENCE. 

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