WDPS33 PGTW 260300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21P (SERU) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.6S 170.3E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 161 NM SOUTHEAST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A LARGE SYSTEM
WITH SHORT FRAGMENTED FORMATIVE BANDS LOOSELY FEEDING INTO AN
EXPANSIVE CENTRAL COLD COVER WITH FLARING CONVECTION THAT IS
TOTALLY OBSCURING THE LOW-LEVEL CIRCULATION (LLC). THE INITIAL
POSITION AND INTENSITY ARE EXTRAPOLATED WITH HIGH CONFIDENCE FROM A
252212Z ASCAT BULLSEYE PASS. THE INTENSITY IS ALSO VALIDATED BY THE
MAJORITY OF AGENCY AND AUTOMATED ESTIMATES. ANALYSIS INDICATES A
FAVORABLE ENVIRONMENT WITH WARM SST, LOW VWS, AND MODERATE DUAL
OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA.

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   KNES: T3.5 - 55 KTS
   PHFO: T4.0 - 65 KTS
   NFFN: T3.5 - 55 KTS
   CIMSS ADT: 57 KTS AT 252330Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 21P WILL TRACK SOUTHEASTWARD THEN EASTWARD
UNDER THE STEERING INFLUENCE OF THE STR TO THE EAST. THE FAVORABLE
ENVIRONMENT WILL BECOME MARGINAL AS VWS INCREASES, SUSTAINING THE 
INTENSITY UP TO TAU 24; AFTERWARD, THE ENVIRONMENT WILL BECOME 
UNFAVORABLE WITH A FURTHER INCREASE IN VWS, IN ADDITION TO COOLING 
SSTS, GRADUALLY ERODING THE SYSTEM TO DISSIPATION BY TAU 96.

MODEL DISCUSSION: THE AVAILABLE NUMERICAL MODELS ARE IN TIGHT
AGREEMENT WITH A SPREAD TO 82NM BY TAU 72 THEN MORE ERRATICALLY AND
WIDELY TO 334NM BY TAU 120, LENDING MEDIUM CONFIDENCE TO THE
NEAR- TO MID-TERM PORTION, THEN LOW CONFIDENCE TO THE EXTENDED
PORTION OF THE JTWC TRACK FORECAST. THERE IS ALSO MEDIUM TO LOW
CONFIDENCE IN THE INTENSITY FORECASTS TO THE RESPECTIVE PORTIONS OF
THE JTWC FORECAST TRACK. NOTABLE LEFT-OF-TRACK OUTLIERS INCLUDE
UKMET, ECMF, AND AEMN THAT RECURVE THE VORTEX EQUATORWARD AFTER TAU
72.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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