WDPS33 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21P (SERU) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.9S 171.0E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 273 NM EAST OF NOUMEA, NEW CALEDONIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
DEGRADING TROPICAL CYCLONE (TC) WITH DEEP CONVECTION SHEARED TO THE
SOUTHEAST. DISTINCT DRY AIR IS IMPINGING ON THE CIRCULATION FROM
THE WESTERN PERIPHERY. THE INITIAL POSITION IS PLACED WITH LOW
CONFIDENCE BASED ON THE AFOREMENTIONED EIR IMAGERY AND DEGRADATION
TO HIMAWARI-9 DATA. THE INITIAL INTENSITY OF 45 KTS IS ASSESSED
WITH MEDIUM CONFIDENCE BASED ON A 270936Z METOP-B ASCAT IMAGERY
REVEALING AN ELONGATED CIRCULATION WITH A SWATH OF 45KT WINDS TO
THE SOUTH AND SOUTHWEST.

INITIAL WIND RADII BASIS: 270936Z METOP-B ASCAT IMAGERY

CURRENT STEERING MECHANISM: A COMPETING STEERING ENVIRONMENT
BETWEEN TWO WEAK RIDGES TO THE EAST AND WEST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 21P WILL SLOWLY CURVE
SOUTHEASTWARD FOR THE REMAINDER OF THE FORECAST PERIOD. THE
COMPETING ENVIRONMENT WILL LEAD TO SLOW TRACK MOVEMENT, AND
POTENTIAL QUASI-STATIONARY PERIODS. THE SYSTEM IS EXPECTED TO
CONTINUALLY WEAKEN IN AN ENVIRONMENT OF HIGH VERTICAL WIND SHEAR,
COOL SEA SURFACE TEMPERATURES, AND UPWELLING FROM SLOW TRACK
MOVEMENT. BY THE END OF THE FORECAST PERIOD, THE SYSTEM WILL HAVE
WEAKENED BELOW THE 35KT WARNING THRESHOLD AND HAVE REACHED
DISSIPATION.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE HAS THE RESEMBLANCE
OF WHAT WE CALL A 'SQUASHED SPIDER' - SPREAD IN MULTIPLE DIRECTIONS
AND THEREBY IN POOR AGREEMENT. THIS IS DUE TO THE UNCERTAINTY OF
THE STEERING INFLUENCE BETWEEN THE TWO RIDGES AS THE SYSTEM WINDS
DOWN. THE INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT TC 21S WILL
DISSIPATE IN 36 HOURS.

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