WDPS31 PGTW 150900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 08P (EIGHT) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.6S 170.5E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 225 NM NORTHEAST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A COMPACT,
BUT SYMMETRICAL CENTRAL DENSE OVERCAST (CDO) FEATURE, WITH THE
CLOUD TOP TEMPERATURE OF THE OVERSHOOTING TOPS EXCEEDING -90C. A
LACK OF RECENT MICROWAVE IMAGERY OR SCATTEROMETER HITS PRECLUDES A
HIGH CONFIDENCE ASSESSMENT OF THE UNDERLYING LOW-LEVEL STRUCTURE
BUT BASED ON THE CIMSS SHEAR ANALYSIS INDICATING WEST-NORTHWEST
SHEAR AT AROUND 17 KNOTS, IT IS ASSUMED THE VORTEX REMAINS HIGHLY
TILTED, WITH THE LLCC ESTIMATED TO BE WEST OF THE CDO. ANIMATED
WATER VAPOR IMAGERY SHOWS GOOD RADIAL OUTFLOW, WITH AN ENHANCED
POLEWARD CHANNEL INTO THE REAR ENTRANCE REGION OF A JET MAX TO THE
SOUTH. THE PRIMARY COMPONENT OF THE MODERATE TO HIGH WESTERLY SHEAR
APPEARS TO BE IN THE SUB-OUTFLOW LAYER, AROUND 400MB, BASED ON A
SOUNDING FROM PORT VILA. THE INITIAL POSITION IS ASSESSED WITH LOW
CONFIDENCE DUE TO A LACK OF MICROWAVE AND SCATTEROMETER DATA, BEING
BASED SOLELY ON THE ANIMATED MSI AND THE AGENCY FIX POSITIONS. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, CONSISTENT
WITH THE OBJECTIVE INTENSITY ESTIMATES AND SLIGHTLY LOWER THAN THE
AGENCY T3.0 CI ESTIMATES. ENVIRONMENTAL CONDITIONS ARE MARGINALLY
FAVORABLE, WITH ROBUST POLEWARD OUTFLOW, MODERATE TO HIGH DEEP
LAYER WESTERLY SHEAR AND WARM SSTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF AN WEST-NORTHWEST
TO EAST-SOUTHEAST ORIENTED DEEP-LAYER NEAR EQUATORIAL RIDGE (NER)
CENTERED NORTH OF WESTERN SAMOA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 41 KTS AT 150226Z
   CIMSS ADT: 51 KTS AT 150540Z
   CIMSS AIDT: 39 KTS AT 150540Z
   CIMSS D-MINT: 40 KTS AT 150226Z
   CIMSS D-PRINT: 38 KTS AT 150540Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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: TROPICAL CYCLONE (TC) 08P (EIGHT) CONTINUES TO
TRACK SLOWLY SOUTH-SOUTHEASTWARD, THOUGH IT IS SHOWING THE INITIAL
SIGNS OF BEGINNING A GRADUAL TURN TO A MORE EASTWARD TRACK AS IT
SLIDES UNDER THE SOUTHERN PERIPHERY OF THE NEAR EQUATORIAL RIDGE
(NER) TO THE NORTHEAST. AS THE RIDGE BUILDS AND EXTENDS WESTWARD
AND BECOMES ALIGNED ON AN EAST-WEST AXIS, ALLOWING THE TRAJECTORY
OF TC 08P TO FLATTEN OUT AFTER TAU 24. BY TAU 72, THE SYSTEM WILL
GRADUALLY TURN MORE SOUTHEASTWARD AS IT WEAKENS AND COMES UNDER THE
INFLUENCE OF THE LOW- TO MID-LEVEL FLOW AND STARTS TO INTERACT WITH
A TROF PASSING BY TO THE SOUTH. IN TERMS OF INTENSITY, TC 08P WILL
HAVE A SHORT WINDOW OF OPPORTUNITY, ABOUT 36 HOURS, IN WHICH TO
INTENSIFY. AS THE TRACK FLATTENS OUT, SHEAR IS FORECAST TO WEAKEN,
WHICH SHOULD ALLOW FOR THE VORTEX TO ALIGN. HOWEVER, BY TAU 36, THE
SYSTEM WILL MOVE CLOSE ENOUGH TO THE JET TO THE SOUTH TO SEE A
FAIRLY DRAMATIC UPTICK IN WESTERLY SHEAR. COMBINED WITH AN
INJECTION OF DRY MID-LEVEL AIR WHICH OCCURS AROUND TAU 48, THE
SYSTEM WILL STEADILY AND RAPIDLY WEAKEN THROUGH THE REMAINDER OF
THE FORECAST PERIOD, WITH DISSIPATION EXPECTED NO LATER THAN TAU
96. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE STILL DEPICTS A
LARGE AMOUNT OF UNCERTAINTY, PARTICULARLY IN THE ALONG-TRACK
DIRECTION. THE NAVGEM, EGRR AND ECMWF CONTINUE TO SHOW A MUCH
WEAKER SYSTEM, WHICH TRACKS MUCH MORE SLOWLY TOWARDS THE EAST, AND
ULTIMATELY LOSE TRACK OF THE VORTEX BETWEEN TAU 72 AND 96.
MEANWHILE, THE GFS AND GEFS DEPICT A MUCH STRONGER STORM, LEADING
TO A FASTER TRACK, WHICH REACHES SOUTHEAST OF PAGO PAGO BY TAU 96,
RESULTING IN AN ALONG-TRACK SPREAD OF 750NM OR MORE BETWEEN THE GFS
AND ECMWF. THE MULTI-MODEL CONSENSUS MEAN LIES IN BETWEEN THE TWO
BUT GENERALLY CLOSER TO THE GEFS MEAN THAN ANYTHING ELSE. A
MULTI-MODEL ENSEMBLE PACKAGE DEPICTS A FAIRLY CONSTRAINED ENVELOPE
AND THE MEAN DEPICTS A TRACK OVER VANUA LEVU ISLAND. THE JTWC
FORECAST TRACK IS POSITIONED CLOSE TO THE GEFS MEAN. CONFIDENCE
REMAINS LOW HOWEVER, DUE TO THE SIGNIFICANT DISAGREEMENT BETWEEN
THE MODELS. INTENSITY GUIDANCE ON THE OTHER HAND IS IN VERY GOOD
AGREEMENT, WITH ALL MODELS INDICATING A PEAK INTENSITY OF 50-55
KNOTS OCCURRING AT TAU 36, FOLLOWED BY STEADY WEAKENING AFTER TAU
48. THE JTWC FORECAST IS PLACED NEAR THE CONSENSUS MEAN THROUGH THE
FORECAST WITH MEDIUM CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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