WDPS31 PGTW 141500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 08P (EIGHT) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.9S 168.9E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 290 NM NORTH OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT AND ASYMMETRIC CENTRAL DENSE OVERCAST (CDO) FEATURE. THE
CDO HAS WARMED SIGNIFICANTLY OVER THE PAST COUPLE OF HOURS, AFTER
HAVING REACHED ITS ZENITH ABOUT FOUR HOURS AGO. ANIMATED SHORTWAVE
INFRARED (SWIR) SHOWS SOME HINTS OF THE OUTER LOW-LEVEL BANDS TO
THE NORTH OF VANUATU, WHICH ALIGNS REASONABLY WELL WITH A 141028Z
MHS 89GHZ MICROWAVE IMAGE WHICH SHOWED THE LOW LEVEL CIRCULATION
CENTER (LLCC) TO BE DISPLACED WEST OF THE CDO FEATURE. THE LAST
SOUNDING FROM PORT VILA, VANUATU (0000Z) INDICATED STRONG WINDS AT
THE 300MB LEVEL PUSHING IN FROM THE WEST, WHICH IS AT ODDS WITH THE
MODEL DERIVED SOUNDINGS, BUT WOULD MAKE SENSE COMPARED TO THE
SATELLITE DEPICTION. THE INITIAL POSITION IS ASSESSED WITH LOW
CONFIDENCE DUE TO THE LACK OF A TRACKABLE LLCC IN THE EIR AND
LOW-RESOLUTION IMAGERY FROM THE MHS SENSOR. THE INITIAL INTENSITY
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE TWO T3.0 (45 KNOTS)
DVORAK CURRENT INTENSITY ESTIMATES FROM PGTW AND KNES, AND A
SIMILAR ESTIMATE FROM THE CIMSS D-PRINT. ENVIRONMENTAL CONDITIONS
ARE GENERALLY FAVORABLE, WITH LOW DEEP-LAYER SHEAR, WARM SSTS AND
STRONG POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: OTHER 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER NEAR
EQUATORIAL RIDGE (NER) CENTERED NEAR WESTERN SAMOA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   CIMSS D-PRINT: 44 KTS AT 141200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: MID-LEVEL WESTERLY SHEAR

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 08P (EIGHT) HAS TRACKED
SOUTHEASTWARD ALONG THE WESTERN SIDE OF A DEEP-LAYER, EAST-WEST
ORIENTED NER, CENTERED NEAR WESTERN SAMOA, FOR THE PAST DAY OR SO.
THIS GENERAL MOTION IS EXPECTED TO CONTINUE FOR THE NEXT 12-18
HOURS BEFORE THE SYSTEM GRADUALLY TURNS ONTO A MORE EASTWARD
TRAJECTORY AS THE RIDGE MOVES NORTH AND ELONGATES TOWARDS THE WEST.
TC 08P WILL TRACK EAST ALONG THE SOUTHERN SIDE OF THIS RIDGE
THROUGH TAU 96, AND THEN AS IT WEAKENS AND COMES UNDER THE
INFLUENCE OF THE LOWER-LEVEL STEERING FLOW, TURNS SOUTHEASTWARD BY
THE END OF THE FORECAST PERIOD. IN TERMS OF INTENSITY, THE MAIN
HINDRANCE IN THE NEAR-TERM WILL BE THE ASYMMETRIC NATURE OF THE
VORTEX, BROUGHT ON THE MODERATE TO HIGH MID-LEVEL WESTERLY SHEAR.
THE MODELS ARE NOT HANDLING THIS SYSTEM WELL AT ALL, AND THUS THE
FORECAST OF THE SHEAR AND THE RESULTING TIMING OF VORTEX ALIGNMENT
IS HIGHLY UNCERTAIN. ASSUMING THE BEST-CASE SCENARIO AS DEPICTED BY
THE GFS, DEEP-LAYER SHEAR SHOULD REMAIN LOW, WHILE MID-LEVEL SHEAR
DECREASES SHARPLY, ALLOWING THE VORTEX TO SYMMETRIZE IN SHORT
ORDER. IF THIS SCENARIO PLAYS OUT, THE SYSTEM SHOULD STEADILY
INTENSIFY OVER THE NEXT 36 TO 48 HOURS. AFTER TAU 48, THE SYSTEM
MOVES CLOSE ENOUGH TO THE JET STREAM TO THE SOUTH TO FEEL AN
INCREASE IN SHEAR, WHICH WILL OFFSET THE ROBUST OUTFLOW INTO THE
JET MAX AND WILL BEGIN TO RAPIDLY WEAKEN THROUGH THE REMAINDER OF
THE FORECAST PERIOD. TC 08P IS FORECAST TO DISSIPATE BY TAU 120,
SOUTHWEST OF AMERICAN SAMOA. 

MODEL DISCUSSION: DETERMINISTIC MODELS ARE IN POOR AGREEMENT, WITH
THE PRIMARY GLOBAL MODELS (ECMWF AND GFS) HANDLING THE SYSTEM IN
DRAMATICALLY DIFFERENT WAYS. THE GFS DEVELOPS A STRONG TC AND THUS
STRENGTHENING THE STEERING GRADIENT, IS ALSO THE FASTEST MOVER,
TAKING THE SYSTEM WELL TO THE SOUTHEAST OF AMERICAN SAMOA BY TAU
120. MEANWHILE, THE ECMWF AND NAVGEM DEPICT A VERY WEAK CIRCULATION
AND ARE MORE INCLINED TO FOLLOW THE LOW-LEVEL FLOW WHICH TAKES THE
SYSTEM VERY SLOWLY SOUTHEASTWARD THEN TURNS THE WEAK VORTEX
NORTHWARD AFTER TAU 48, NEVER PUSHING EAST OF THE 174E LONGITUDE
LINE. THE RESULTING ALONG-TRACK SPREAD EXCEEDS 1200NM. THE
CONSENSUS TRACKER MEANWHILE, BEING PULLED BACK BY THE EXCEEDINGLY
SLOW ECMWF AND NAVGEM, IS ROUGHLY POSITIONED ABOUT MIDWAY BETWEEN
THE TWO OUTLIERS. DISCARDING THE TWO LAGGARDS RESULTS IN A
CONSENSUS MEAN POSITIONED NEAR THE EGRR, GEFS MEAN, AND THE THREE
MESOSCALE MODEL TRACKS (CTCX, HAFS-A, HWRF). ENSEMBLE GUIDANCE IS
IN FAIRLY GOOD AGREEMENT SURPRISINGLY, WITH A MULTI-MODEL ENSEMBLE
MEAN TRACK POSITIONED JUST NORTH OF FIJI AND WELL TO THE SOUTHWEST
OF FIJI. THE JTWC TRACK IS PLACED NEAR THE MODIFIED CONSENSUS AND
THE GEFS MEAN, WITH MEDIUM CONFIDENCE IN THE LONG-RANGE FORECAST.
INTENSITY GUIDANCE IS IN RELATIVELY GOOD AGREEMENT, WITH THE BULK
OF THE MEMBERS CONFINED TO AN INTENSITY ENVELOPE BETWEEN 45 AND 55
KNOTS. THE SHIPS-GFS IS THE ONLY HIGH-SIDE OUTLIER, PEAKING AT
ABOUT 65 KNOTS. THE JTWC FORECAST IS POSITIONED JUST SLIGHTLY UNDER
THE CONSENSUS MEAN THROUGH THE FORECAST PERIOD, WITH MEDIUM
CONFIDENCE. 

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