WDPS31 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 23P (URMIL) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5S 169.5E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 128 NM SOUTH-SOUTHEAST OF PORT VILA,
VANUATU
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT SYSTEM FEATURING A SYMMETRICAL CENTRAL DENSE OVERCAST (CDO) 
AND SPORADIC OVERSHOOTING TOPS. THE LOW-LEVEL CIRCULATION CENTER 
(LLCC) REMAINS OBSCURED BY THE CIRRUS SHIELD; FURTHERMORE, A LACK OF 
CONTEMPORARY MICROWAVE OR SCATTEROMETER DATA PRECLUDES A DEFINITIVE 
STRUCTURAL ANALYSIS. SURFACE OBSERVATIONS FROM WHITEGRASS AIRPORT 
(TANNA ISLAND, VANUATU) HAVE REPORTED SUSTAINED EASTERLY WINDS FOR 
THE PAST SEVERAL HOURS, WITH GUSTS REACHING 45 KNOTS RECENTLY. THE 
CIRCULATION PERSISTS ON THE EXTREME MARGINS OF THE NEW CALEDONIA 
RADAR, PROVIDING NEGLIGIBLE SUPPORT FOR PINPOINTING THE LLCC. HENCE, 
THE INITIAL POSITION IS ASSESSED WITH VERY LOW CONFIDENCE AND IS 
GENERALLY CONSISTENT WITH THE MULTI-AGENCY DVORAK POSITION ESTIMATES. 
THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, CONSISTENT 
WITH THE SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES OUTLINED BELOW 
AND AN EARLIER SAR PASS WHICH REVEALED MAXIMUM WINDS OF APPROXIMATELY 
65 KNOTS. ENVIRONMENTAL CONDITIONS REMAIN CONDUCIVE, CHARACTERIZED BY 
LOW DEEP-LAYER SHEAR, VIGOROUS OUTFLOW ALOFT, AND OPTIMAL SSTS, 
ALTHOUGH TEMPERED BY MODERATE MID-LEVEL WESTERLY SHEAR SITUATED 
BENEATH THE OUTFLOW LAYER.

INITIAL WIND RADII BASIS:  SAR DATA 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) POSITIONED NEAR FIJI. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   KNES: T3.5 - 55 KTS
   PHFO: T3.5 - 55 KTS
   CIMSS SATCON: 50 KTS AT 270930Z
   CIMSS ADT: 61 KTS AT 271200Z
   CIMSS AIDT: 50 KTS AT 271200Z
   CIMSS D-MINT: 58 KTS AT 271116Z
   CIMSS D-PRINT: 62 KTS AT 271200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: MODERATE MID-LEVEL WESTERLY SHEAR. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 48
HOURS. 

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 23P HAS TRACKED
EAST-SOUTHEASTWARD OVER THE PAST SIX HOURS, CROSSING OVER THE
ISLAND OF TANNA WITHIN THE PAST TWO HOURS, TRAVELING ALONG THE
WESTERN FLANK OF A DEEP-LAYER STR ANCHORED NEAR FIJI. THE RIDGE IS 
EXPECTED TO RETROGRADE EQUATORWARD OVER THE NEXT 12 HOURS, AND TC 23P 
WILL MAINTAIN ITS CURRENT EAST-SOUTHEASTWARD VECTOR. AFTER TAU 12, 
THE STR WILL ELONGATE ALONG A NORTHWEST-SOUTHEAST AXIS, WHILE THE 
ANTICYCLONIC CORE TRANSITS SOUTHEASTWARD IN TANDEM WITH TC 23P, 
INTENSIFYING THE STEERING GRADIENT. BY TAU 24, TC 23P WILL PIVOT TO A 
SOUTHEASTWARD HEADING AND ACCELERATE POLEWARD THROUGH THE END OF THE 
FORECAST PERIOD. REGARDING INTENSITY, HIGH-RESOLUTION MESOSCALE 
MODELS SUGGEST THE VORTEX EXHIBITS A PRONOUNCED EASTWARD TILT WITH 
HEIGHT INDUCED BY MODERATE MID-LEVEL SHEAR. COUPLED WITH OROGRAPHIC 
INTERFERENCE DURING THE TRANSIT OF TANNA ISLAND, THE INTENSIFICATION 
TREND HAS BEEN TEMPORARILY ABATED. WITH THE VORTEX NOW RE-EMERGING 
OVER OPEN WATERS AND SUBSIDING SHEAR, A BURST OF INTENSIFICATION IS 
FORECAST BETWEEN TAU 12 AND TAU 24, REACHING A PEAK INTENSITY OF 80 
KNOTS. POST-PEAK, VERTICAL WIND SHEAR IS FORECAST TO INCREASE 
PRECIPITOUSLY, REACHING 30 KNOTS BY TAU 36 AND APPROXIMATELY 50 KNOTS 
BY TAU 48, AS THE SYSTEM INTERACTS WITH THE SUBTROPICAL JET STREAK. 
SIMULTANEOUS WITH INCREASED SHEAR, A SUBSIDENT DRY AIRMASS WILL 
ENTRAIN INTO THE INNER CORE BY TAU 40, SUPPRESSING FURTHER CONVECTIVE 
REDEVELOPMENT. THE SYSTEM WILL ALSO TRAVERSE SIGNIFICANTLY COOLER 
WATERS, INITIATING SUBTROPICAL TRANSITION (STT) AS EARLY AS TAU 36. 
STT COMPLETION IS ANTICIPATED BETWEEN TAU 48 AND TAU 60.

MODEL DISCUSSION: DETERMINISTIC, ENSEMBLE AND ARTIFICIAL
INTELLIGENCE (AI) TRACK GUIDANCE IS IN VERY STRONG AGREEMENT FOR
THE DURATION OF THE FORECAST PERIOD. CROSS-TRACK SPREAD EXPANDS TO A 
MAXIMUM OF 85NM AT TAU 36, WITH THE ECMWF SOLUTION DEFINING THE 
EASTERN PERIPHERY AND THE GEFS MEAN MARKING THE WESTERN EDGE. THE 
JTWC FORECAST IS ALIGNED WITH THE MULTI-MODEL CONSENSUS AND FGNI 
TRACKERS, ASSESSED WITH HIGH CONFIDENCE. INTENSITY GUIDANCE REMAINS 
DIVERGENT; HWRF, COTC, AND CTCX DEPICT A STAGNANT INTENSITY TREND 
THROUGH TAU 24, FORECASTING ONLY MINOR STRENGTHENING TO 65-70 KNOTS 
BY TAU 36. CONVERSELY, HAFS-A, SHIPS, AND THE CONSENSUS MEAN SUGGEST 
A MORE AGGRESSIVE DEVELOPMENT, PEAKING BETWEEN 75-85 KNOTS. THE JTWC 
FORECAST IS POSITIONED ALONG THE CONSENSUS MEAN BUT ANTICIPATES PEAK 
INTENSITY 12 HOURS PREMATURELY AND FORECASTS A MORE RAPID DECAY PHASE 
THAN THE BULK OF THE GUIDANCE SUITE.

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