WDPS32 PGTW 060300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (VAIANU) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.7S 173.1E
   INITIAL INTENSITY: 80 KTS
   GEOGRAPHIC REFERENCE: 337 NM WEST-NORTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 26 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A NEARLY
SYMMETRIC VORTEX WITH SPIRAL BANDS WRAPPING TIGHTLY INTO A COMPACT
CORE. DEEP CONVECTION IS PRESENT AROUND THE CORE AND WITHIN THE
BANDS ENTERING THE CORE IN THE PRIMARILY IN THE EASTERN SEMICIRCLE,
WHILE SOME DRIER AIR ENCROACHING FROM THE WEST IS EVIDENT IN THE
NORTHWEST QUADRANT. AN EYE FEATURE HAS BEEN INTERMITTENTLY VISIBLE
IN ENHANCED INFRARED IMAGERY (EIR) AND MSI OVER THE PAST 12 HOURS,
FURTHER INDICATING THE IMPROVING SYMMETRY OF THE SYSTEM. ANIMATED
WATER VAPOR IMAGERY CONTINUES TO SHOW ROBUST POLEWARD OUTFLOW A
042121Z METOP-C ASCAT PASS REVEALED A COMPACT INNER CORE WITH
IMPROVING WIND FIELD SYMMETRY AROUND THE CORE. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED ASCAT
DATA AND ANIMATED MSI. THE SYSTEM HAS RAPIDLY INTENSIFIED FROM 40
KTS TO 80 KTS OVER THE PAST 24 HOURS AND THE INITIAL INTENSITY IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON A COMPROMISE BETWEEN THE
AGENCY DVORAK ESTIMATES AND CIMSS OBJECTIVE INTENSITY ESTIMATES.
THE CURRENT ENVIRONMENT IS CHARACTERIZED AS FAVORABLE WITH LOW
VERTICAL WIND SHEAR (VWS), HIGH (29-30 C) SEA SURFACE TEMPERATURES,
AND EXCELLENT POLEWARD OUTFLOW. THE SUPPORT FOR CONTINUED
INTENSIFICATION IS ONLY SLIGHTLY UNDERCUT BY THE DRY AIR BEGINNING
TO APPROACH FROM THE WEST.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA AND OBJECTIVE BEST
TRACK 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.0 - 90 KTS
   PHFO: T4.5 - 77 KTS
   CIMSS ADT: 77 KTS AT 060000Z
   CIMSS AIDT: 76 KTS AT 060000Z
   CIMSS D-PRINT: 81 KTS AT 060000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE WEST

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

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) HAS TRACKED
SLOWLY SOUTHEASTWARD OVER THE PAST 12 HOURS AND IS FORECAST TO
CONTINUE ALONG THE SAME TRAJECTORY THROUGH TAU 72 UNDER THE
STEERING INFLUENCE OF THE STR TO THE EAST. AFTER TAU 72, THE RIDGE
TO THE EAST WILL BUILD DEEPER INTO THE MID-LATITUDES AND BEGIN TO
STEER THE SYSTEM SOUTHWARD TOWARD A DEEP-LAYER TROUGH APPROACHING
FROM THE WEST. THE COMBINATION OF PUSHING FROM THE RIDGE AND
PULLING FROM THE TROUGH WILL CAUSE THE TRACK TO SHIFT TOWARD THE
SOUTHWEST AROUND TAU 96 AND ACCELERATE THE SPEED OF ADVANCE. TC
31P IS FORECAST TO STEADILY INTENSIFY IN A FAVORABLE ENVIRONMENT
TO A PEAK OF 105 KTS AROUND TAU 24 AND MAINTAIN THAT INTENSITY
THROUGH TAU 36. AFTER TAU 36 SHEAR WILL RAPIDLY INCREASE TO MORE
THAN 30 KTS BY TAU 60 AND MORE THAN 50 KTS BY TAU 96 AS THE SYSTEM
PASSES UNDER A POLAR FRONT JET STREAK, CAUSING TC 31P TO QUICKLY
WEAKEN THROUGH THE END OF THE FORECAST PERIOD. WEAKENING WILL BE
FURTHER AIDED BY A LARGE INJECTION OF DRY AIR. EXTRATROPICAL
TRANSITION IN THE VICINITY OF THE JET STREAK IS EXPECTED TO BEGIN
AROUND TAU 96 AND TC 31P WILL BE POLEWARD OF THE JET AS A FULLY 
EXTRATROPICAL CYCLONE BY TAU 120.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT ON THE
SOUTHEASTWARD TRACK AND AN EVENTUAL TURN TOWARD THE SOUTHWEST WITH
A FEW EXCEPTIONS: NAVGEM AND COAMPS-TC (NAVGEM BASED) MAKE UP THE
FAR EASTERN EDGE OF THE ENVELOPE, WHILE THE UKMET MAKES AN EARLY
TURN TO THE SOUTHWEST AS THE ONLY WESTERN OUTLIER. EXCLUDING THOSE
SOLUTIONS, THE CROSS-TRACK SPREAD AT TAU 36 IS JUST 40 NM.
VARIABILITY IN THE SPEED OF ADVANCE ACROSS THE MAIN GROUPING IS
AROUND 190 NM AT TAU 120. THE JTWC TRACK FORECAST IS PLACED NEAR
THE TES1 CONSENSUS THROUGHOUT THE ENTIRE FORECAST PERIOD WITH HIGH
CONFIDENCE THROUGH TAU 72 AND MEDIUM CONFIDENCE THEREAFTER.
INTENSITY GUIDANCE IS SPLIT BETWEEN RAPID INTENSIFICATION (RI) AIDS
AND NON-RI AIDS. THE RI AIDS SUGGEST FURTHER INTENSIFICATION TO A
PEAK OF AROUND 115-125 KTS AT TAU 36 WHILE THE NON-RI AIDS ARE
GROUPED NEAR 90-105 KTS. AFTER TAU 36, MODELS AGREE ON STEADY
WEAKENING. THE JTWC INTENSITY FORECAST IS PLACED SLIGHTLY HIGHER
THAN THE MULTI-MODEL CONSENSUS THROUGH TAU 36 AND THEN FOLLOWS THE
CONSENSUS THEREAFTER WITH MEDIUM CONFIDENCE. 

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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