WDPS33 PGTW 242100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21P (TWENTYONE) WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.4S 170.7E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 158 NM EAST-NORTHEAST OF PORT VILA,
VANUATU
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A RAPIDLY
CONSOLIDATING SYSTEM WITH SHORT AND FRAGMENTED FORMATIVE BANDS
LOOSELY FEEDING INTO A DEEPENING AND EXPANDING COLD DENSE OVERCAST
WITH OVERSHOOTING COLD TOPS NEAR THE CENTER. THE INITIAL POSITION
IS EXTRAPOLATED WITH MEDIUM CONFIDENCE FROM A DISCERNIBLE LOW LEVEL
CIRCULATION FEATURE IN THE 241419Z GCOMW IMAGE. THE INITIAL
INTENSITY IS HELD SLIGHTLY HIGHER THAN THE PGTW ESTIMATE TO REFLECT
THE RAPID INTENSIFICATION. ANALYSIS INDICATES A FAVORABLE
ENVIRONMENT WITH WARM SST, LOW VWS, AND MODERATE DUAL OUTFLOW.

INITIAL WIND RADII BASIS: RECENT SCATTEROMETER DATA.  

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO
THE EAST-SOUTHEAST.  

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 21P WILL TRACK SOUTHEASTWARD THEN EASTWARD
UNDER THE STEERING INFLUENCE OF THE STR TO THE EAST. THE FAVORABLE
ENVIRONMENT WILL FUEL A STEADY INTENSIFICATION TO A PEAK OF 65KTS
BY TAU 48. AFTERWARD, INCREASING VWS AND COOLING SST WILL MOSTLY
CAUSE A GRADUAL EROSION OF THE SYSTEM DOWN TO 35KTS BY TAU 120.

MODEL DISCUSSION: THE AVAILABLE NUMERICAL MODELS ARE IN OVERALL
AGREEMENT WITH A SPREAD TO 84NM BY TAU 72 THEN MORE ERRATICALLY AND
WIDE TO 343NM 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 AND LOW CONFIDENCE IN
THE INTENSITY FORECASTS TO THE RESPECTIVE PORTIONS OF THE JTWC
FORECAST TRACK. NOTABLE LEFT-OF-TRACK OUTLIERS INCLUDE UKMET AND
ECMWF THAT BOTH 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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