WDXS32 PGTW 140300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING 
NR 013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.1S 35.7E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 273 NM WEST-SOUTHWEST OF EUROPA ISLAND
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
THE STRUCTURE OF TROPICAL CYCLONE (TC) 21S (GEZANI) HAS CHANGED
LITTLE OVER THE PAST 12 HOURS. THE SYSTEM HAS MAINTAINED A TIGHT,
SYMMETRICAL OVERALL STRUCTURE WITH AN OBSERVABLE EYE-FEATURE. OVER
THE LAST HOUR, THE ONCE WELL-DEFINED EYE HAS BECOME INCREASINGLY
RAGGED DUE TO INCREASING DRY AIR ENTRAINMENT THROUGHOUT THE
NORTHERN SEMICIRCLE. CLOUD TOP TEMPERATURES HAVE WARMED SLIGHTLY,
WITH HEAVIER ACTIVITY RELEGATED TO THE SOUTHEASTERN QUADRANT OF THE
SYSTEM UNTIL RECENTLY. THE CURRENT ENVIRONMENT REMAINS FAVORABLE
FOR STEADY SURFACE INTENSITIES WITH STRONG RADIAL OUTFLOW, WARM SEA
SURFACE TEMPERATURES (28-29 C), AND RELATIVELY MOIST CORE, TEMPERED
BY INCREASING DRY AIR ENTRAINMENT. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON ANIMATED EIR IMAGERY OF THE VISIBLE
EYE FEATURE ON ANIMATED EIR. THE INITIAL INTENSITY OF 95 KTS IS
ASSESSED WITH HIGH CONFIDENCE BASED ON A 131600Z RCM-3 SAR PASS.

INITIAL WIND RADII BASIS: 131600Z SAR DATA 

CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR)
POSITIONED TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   FMEE: T5.5 - 102 KTS
   CIMSS SATCON: 97 KTS AT 132230Z
   CIMSS ADT: 99 KTS AT 140000Z
   CIMSS AIDT: 93 KTS AT 132000Z
   CIMSS D-MINT: 89 KTS AT 132230Z
   CIMSS D-PRINT: 86 KTS AT 140000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE RADIAL

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 21S IS CONTINUING TO TRACK SOUTHWARD,
ROUNDING THE RIDGE AXIS OF THE DEEP LAYER SUBTROPICAL RIDGE (STR)
POSITIONED TO THE EAST. AN EASTWARD TURN AWAY FROM THE EASTERN
COAST OF MOZAMBIQUE IS EXPECTED WITHIN THE NEXT 12 HOURS AS THE
SYSTEM BEGINS TO INTERACT WITH A DEEP MID-LATITUDE TROUGH
APPROACHING FROM THE SOUTHWEST. THIS NEW STEERING MECHANISM WILL
DRIVE TC 21S SOUTHEASTWARD UNTIL TAU 24 AND THEN EASTWARD UNTIL
JUST AFTER TAU 48 WHEN THE MID-LATITUDE TROUGH HAS PASSED AND THE
STR CENTERED TO THE EAST DRIVES THE SYSTEM SOUTHWARD OVER COOLER
WATERS, DRIER AIR, AND AN UNFAVORABLE VWS, INITIATING EXTRATROPICAL
TRANSITION. TC 21S IS EXPECTED TO COMPLETE EXTRATROPICAL TRANSITION
BY TAU 96 AS THE CIRCULATION BECOMES ENVELOPED IN THE MID-LATITUDE
WESTERLY FLOW. IN REGARDS TO INTENSITY, TC 21S IS EXPECTED TO
MAINTAIN NEAR 95 KT INTENSITY UNTIL TAU 24. DRY AIR ENTRAINMENT AND
INCREASING VWS WILL GET A HEAD START ON WHITTLING DOWN THE SYSTEMS
SURFACE INTENSITIES BEFORE THE SUBTROPICAL RIDGE RETAKES THE HELM
AND QUICKENS THE PACE OF WEAKENING BY FORCING 21S OVER COOLER
WATERS NEAR LATITUDE 30 S. TC 21S IS FORECASTED TO BE AT 55 KT AS
IT COMPLETES EXTRATROPICAL TRANSITION BY TAU 96 INTO AN ASYMMETRIC
COLD-CORE LOW. THE PREVIOUSLY MENTIONED RECURVATURE SCENARIO IS
STILL BEING ENTERTAINED AS SOME NUMERICAL MODEL GUIDANCE PORTRAYS
AN ELONGATION OF THE SUBTROPICAL RIDGING PATTERN TO THE EAST. 

MODEL DISCUSSION: BIFURCATION IN MODEL GUIDANCE IS STILL PREVALENT
MAKING FORECAST UNCERTAINTY FOR TRACK AND INTENSITY HIGH. CROSS
TRACK SPREAD AMONGST THE GUIDANCE MEMBERS EXCEEDS 1000 NM WITH
ECMWF DEPICTING NORTHWARD RECURVATURE AFTER TAU 24 WHILE NAVGEM AND
GFS MAINTAIN THE EASTWARD TRACK CAUSED BY THE FORECASTED EASTWARD
MAJOR SHORTWAVE TROUGH MOVEMENT. THE AI MODEL GUIDANCE DEPICTS A
SIMILAR SCENARIO, ALBEIT LESS DRAMATIC WITH TIMING. THE JTWC
FORECASTED TRACK IS PLACED NEAR THE AI GUIDANCE WITH MEDIUM TO LOW
CONFIDENCE THROUGH THE FORECAST PERIOD. INTENSITY GUIDANCE IS IN
BETTER AGREEMENT, WITH A SPREAD OF UP TO 35 KTS BETWEEN TAU 48 AND
TAU 60, BUT ALL GUIDANCE POINTS TOWARD A WEAKENING TREND AT A
RELATIVELY MODERATE PACE. THE JTWC FORECASTED INTENSITY REFLECTS
VALUES CLOSE TO THE MULTI-MODEL CONSENSUS, WITH MEDIUM TO LOW
CONFIDENCE ACCOUNTING FOR POTENTIAL CHANGES TO THE FORECASTED
TRACK.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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