WDPS31 PGTW 152100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 08P (EIGHT) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.2S 171.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 251 NM NORTHEAST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) BECOMING APPARENT POST-
SUNRISE TO THE SOUTHWEST OF THE DEEP CONVECTIVE MASS, WHICH HAS WARMED 
AND BECOME MORE RAGGED OVER THE PAST SIX HOURS. THE SHEARED NATURE OF 
THE SYSTEM WAS CONFIRMED BY A 151343Z AMSR-2 MICROWAVE PASS, WHICH 
SHOWED LOW-LEVEL CLOUD BANDS WRAPPING AROUND THE LLCC TO THE SOUTHWEST 
OF THE DEEP CONVECTIVE CLUMPS. AN EARLIER 150930Z ASCAT SCATTEROMETER 
PASS SHOWED 30-35 KT WINDS CONFINED TO THE EASTERN SEMICIRCLE, WITH A
MUCH WEAKER WESTERN SEMICIRCLE. THIS SATELLITE PRESENTATION AND
WIND FIELD STRUCTURE INDICATES THAT THE SYSTEM MAY BE BEGINNING TO
SUCCUMB TO THE 20-25 KT OF WESTERLY VERTICAL SHEAR CURRENTLY PRESENT, 
WITH A VORTEX DECOUPLING PROCESS POSSIBLY UNDERWAY. UPPER-LEVEL 
OUTFLOW IS MODERATE BOTH EQUATORWARD AND POLEWARD. THE INITIAL 
INTENSITY IS SET AT 35 KT, WEIGHTED TOWARDS EARLIER SCATTEROMETER DATA 
AND RUNNING ON THE LOWER END OF THE SATELLITE-BASED INTENSITY 
ESTIMATES, WHICH HAVE AVERAGED TOO STRONG DURING THE PAST DAY OR SO.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS SATCON: 45 KTS AT 151334Z
   CIMSS ADT: 51 KTS AT 151730Z
   CIMSS D-MINT: 33 KTS AT 151343Z
   CIMSS D-PRINT: 36 KTS AT 151730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN MADE
SIGNIFICANTLY SLOWER AND WEAKER THAN THE PREVIOUS FORECAST DUE TO
CHANGES IN OBSERVED STORM ORGANIZATION AND FORECAST AIDS.

FORECAST DISCUSSION: TROPICAL CYCLONE 08P CONTINUES TO TRACK
EASTWARD WITHIN MID-LEVEL WESTERLY FLOW OVERLAYING THE SOUTH
PACIFIC CONVERGENCE ZONE (SPCZ). THE EXPECTED FUTURE MOTION IS
TIGHTLY COUPLED TO THE INTENSITY OF THE CYCLONE, AS A STRONGER,
DEEPER VORTEX WOULD CONTINUE FOLLOWING THE MID-LEVEL FLOW BRISKLY
EASTWARD, WHILE A SHALLOWER, DECOUPLED VORTEX WOULD SLOW DOWN AND
BECOME MAROONED IN THE WEAKER SURFACE STEERING FLOW WITHIN THE
SPCZ. RECENT RUNS OF THE GFS AND GFS-BASED DYNAMICAL MODELS SUCH AS
HAFS-A AND HWRF HAVE INITIALIZED 08P SIGNIFICANTLY STRONGER THAN
OBSERVED DATA INDICATE, RESULTING IN QUICK EASTWARD TRACKS AND A
LONGER-LIVED CYCLONE. DUE TO THE WEAKER AND DECOUPLED OBSERVED
STORM STRUCTURE, A MUCH SLOWER MOTION IS NOW ANTICIPATED, MORE IN
LINE WITH THE ECMWF, AIFS, UKMET, AND THE WEAKER MEMBERS OF THE
ECMWF AND GFS ENSEMBLES. THIS IS A DISTINCT CHANGE FROM PRIOR JTWC
FORECASTS. THE JTWC FORECAST IS ALSO WEAKER THAN THE PREVIOUS ONE,
NO LONGER SHOWING A RE-INTENSIFICATION PERIOD, INSTEAD FLAT-LINING
FOR ABOUT 36 HOURS PRIOR TO DISSIPATING BELOW WARNING THRESHOLD AT
TAU 48. THERE REMAINS UNCERTAINTY IN HOW LONG 08P WILL REMAIN AT
WARNING THRESHOLD INTENSITY, GIVEN THAT PERIODIC BURSTS OF
CONVECTION COULD INTERMITTENTLY SUSTAIN GALES DURING THE NEXT DAY
OR TWO. QUICKER WEAKENING BELOW 35 KT THAN FORECAST IS ALSO A
POSSIBILITY.

MODEL DISCUSSION: THE JTWC TRACK FORECAST LEANS TOWARD A BLEND OF
ECMWF, AIFS, ECMWF ENSEMBLE MEAN, AND UKMET, LARGELY DISCARDING THE
FASTER GROUPING OF GFS, HAFS, HWRF, AND COAMPS-TC. THE JTWC
INTENSITY FORECAST IS ON THE LOWER END OF THE RANGE OF DYNAMICAL
MODEL FORECASTS.

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