WDXS32 PGTW 162100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING NR 
019//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.3S 43.6E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 264 NM SOUTHEAST OF EUROPA ISLAND
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 21S (GEZANI) TUCKING BACK UNDERNEATH THE DEEP CONVECTION
AFTER BECOMING AT EXPOSED AROUND 161200Z. A PARTIAL 161550Z RCM-1
SAR IMAGE SHOWED A SWATH OF 65-75 KTS IN THE WESTERN SEMICIRCLE AND
A SMALL PATCH OF 85-90 KTS THAT IS ANALYZED TO BE ERRONEOUS. A
161754Z METOP-B ASCAT PASS REVEALED A MAX OF 45-50 KTS WITHIN THE
SAME AREA IN THE WESTERN SEMICIRCLE. THE INITIAL INTENSITY OF 60
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A BLEND OF ALL
AVAILABLE WIND SPEED DATA WITH A LARGE RANGE (45 KTS-90 KTS) FROM
THE LOWER CIMSS ESTIMATES AND HIGHER SAR MEASUREMENTS. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED
METOP-B ASCAT IMAGE. ENVIRONMENTAL ANALYSIS INDICATES THAT 21S IS
IN A MARGINALLY UNFAVORABLE ENVIRONMENT CHARACTERIZED BY STRONG
POLEWARD OUTFLOW ALOFT AND WARM (27-28 C) SEA SURFACE TEMPERATURES
(SST) GREATLY OFFSET BY HIGH (30-35 KTS) NORTHWESTERLY VERTICAL
WIND SHEAR AND DRY AIR ENTRAINMENT.

INITIAL WIND RADII BASIS: A COMBINATION OF 161550Z RCM-1 SAR DATA
AND 161754Z METOP-B ASCAT DATA

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) POSITIONED TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   DEMS: T3.5 - 55 KTS
   FMEE: T4.0 - 65 KTS
   CIMSS ADT: 53 KTS AT 161800Z
   CIMSS AIDT: 52 KTS AT 161800Z
   CIMSS D-MINT: 47 KTS AT 161532Z
   CIMSS D-PRINT: 45 KTS AT 161800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30-35 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TC 21S IS FORECAST TO TRACK SOUTHWARD, ALONG
THE WESTERN PERIPHERY OF THE STR POSITIONED TO THE EAST THROUGH TAU
48. 21S IS FORECAST TO BEGIN EXTRATROPICAL TRANSITION (ETT) NEAR
TAU 36 AS IT ENTERS COLD (LESS THAN 24 C) SST. ETT IS FORECAST TO
COMPLETE NEAR TAU 48 AS THE SYSTEM TRANSITIONS TO A COLD-CORE LOW
WITH SST DROPPING BELOW 22 C. THE COMBINATION OF COOLING SEA
SURFACE TEMPERATURES, WESTERLY SHEAR, AND DRY AIR ENTRAINMENT WILL
CREATE A HOSTILE ENVIRONMENT FOR 21S. AS A RESULT, 21S IS FORECAST
TO GRADUALLY WEAKEN THROUGHOUT THE FORECAST PERIOD WITH AN
INTENSITY OF AROUND 45 KTS AS IT COMPLETES ETT. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE REMAINS IN GOOD
AGREEMENT WITH A 55 NM CROSS-TRACK SPREAD AT TAU 48, WHICH LENDS
HIGH CONFIDENCE TO THE JTWC TRACK FORECAST. INTENSITY GUIDANCE
GENERALLY AGREES ON GRADUAL WEAKENING THROUGHOUT THE FORECAST
PERIOD. HAFS-A IS THE FASTEST TO WEAKEN THE SYSTEM, SUGGESTING AN
INTENSITY BELOW 35 KTS AT TAU 48. THE REMAINDER OF GUIDANCE IS
BUNCHED CLOSE TO AN INTENSITY OF 45 KTS AT TAU 48. THE JTWC
INTENSITY FORECAST IS PLACED CLOSE TO THE MULTI-MODEL CONSENSUS
WITH HIGH CONFIDENCE. 

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