WDXS32 PGTW 152100 AMD
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING 
NR 017 AMENDED//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 26.0S 41.0E
   INITIAL INTENSITY: 85 KTS
   GEOGRAPHIC REFERENCE: 232 NM SOUTH OF EUROPA ISLAND
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
TROPICAL CYCLONE (TC) 21S (GEZANI) WITH AN IRREGULARLY SHAPED AND 
NEWLY FORMED CENTRAL DENSE OVERCAST (CDO) OBSCURING THE LOW-LEVEL
CIRCULATION CENTER (LLCC). DRY AIR IS EVIDENT THROUGHOUT THE
ENTIRETY OF THE WESTERN SEMICIRCLE, WITH MINIMAL CONVECTION AND
AMPLIFYING COLD-AIR STRATOCUMULUS FIELDS WRAPPING INTO THE
NORTHERN QUADRANT, ASSOCIATED WITH THE DEEP LONG-WAVE TROUGH CENTERED
DIRECTLY TO THE SOUTH. THE ENVIRONMENT HAS BECOME MARGINAL SINCE
THE PREVIOUS WARNING, WITH INCREASING AND PERSISTENT UNFAVORABLE
VERTICAL WIND SHEAR (VWS) OF OVER 25 KTS AND MODERATE DRY AIR
ENTRAINMENT AROUND THE WESTERN SEMICIRCLE, COMPETING AGAINST
ROBUST POLEWARD OUTFLOW ALOFT AND WARM SEA SURFACE TEMPERATURES (SST)
BETWEEN 27-28 C. THE INITIAL POSITION HAS BEEN PLACED WITH HIGH
CONFIDENCE BASED ON RECENT 151815Z ASCAT DATA. THE INITIAL
INTENSITY HAS BEEN ASSESSED WITH HIGH CONFIDENCE BASED ON EARLIER
151542Z RCM-1 SAR SPD DATA.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT FROM A
DEEP-LAYER SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST AND AN
EASTWARD PASSING LONG-WAVE TROUGH DIRECTLY TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   DEMS: T4.5 - 77 KTS
   FMEE: T4.0 - 65 KTS
   CIMSS ADT: 57 KTS AT 151800Z
   CIMSS AIDT: 52 KTS AT 151700Z
   CIMSS D-MINT: 67 KTS AT 151552Z
   CIMSS D-PRINT: 63 KTS AT 152000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: RECENT RADIAL OUTFLOW HAS BECOME EVIDENT OVER THE
   LAST HOUR

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 FORECASTED TO TRACK NORTHEASTWARD
THROUGHOUT THE NEXT 12-18 HOURS, PUSHED ALONG THE DEEP LONG-WAVE
TROUGH AXIS CENTERED DIRECTLY TO THE SOUTH. AS THE MID-LATITUDE
BAROCLINIC TROUGH CONTINUES EASTWARD, THE CYCLONE WILL BEGIN TO
TRACK SOUTHEASTWARD ALONG THE WESTERN EXTENT OF THE STR POSITIONED
TO THE EAST UNTIL TAU 24. FOLLOWING TAU 24, A MAJOR SHORTWAVE
RIDGE STACKING VERTICALLY INTO THE MID-LATITUDE WESTERLY LONG-WAVE
PATTERN WILL INITIALLY SLOW THE FORECASTED SOUTHWARD TRACK. AS THE
MAJOR SHORTWAVE RIDGE IS DRIVEN EASTWARD ALONG THE EQUATORWARD
EDGE OF THE MID-LATITUDE JET TO THE SOUTH, THE STR TO THE EAST WILL
ONCE AGAIN BECOME THE PRIMARY DRIVING FORCE FOR THE SOUTHWARD 
APPROACH FOR TC 21S THROUGH EXTRATROPICAL TRANSITION. REGARDING 
INTENSITY, A GRADUAL WEAKENING TREND IS FORECASTED THROUGH THE 
ENTIRETY OF THE FORECAST PERIOD. UNTIL TAU 36, SUPPORTIVE POLEWARD 
UPPER-LEVEL OUTFLOW AND WARM SST WILL CONTINUE TO COMPETE AGAINST 
UNFAVORABLE VWS OF GREATER THAN 25 KTS AND AMPLIFYING DRY AIR TO THE 
WEST. FOLLOWING TAU 36, SST WILL DECREASE TO BELOW 26 C NEAR THE SAME
PERIOD TC 21S BECOMES EMBEDDED WITHIN THE EQUATORWARD EXTENT OF
THE MID-LATITUDE WESTERLIES. DURING THIS PERIOD, EXTRATROPICAL
TRANSITION WILL EXPAND THE SURFACE WIND FIELD AS DRY AIR
CONTINUES TO WRAP AROUND THE NORTHERN QUADRANT AND INTO THE
EASTERN SEMICIRCLE. TC 21S IS FORECASTED TO COMPLETE EXTRATROPICAL
TRANSITION BY TAU 72, WHILE MAINTAINING SURFACE INTENSITIES NEAR
55 KTS. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE AND ARTIFICIAL
INTELLIGENCE (AI) MODEL GUIDANCE CONTINUES TO ILLUSTRATE MODERATE
CONFIDENCE IN A GENERALLY EAST-NORTHEASTWARD TRACK OVER THE NEXT
12-18 HOURS, WITH A SOUTHEASTWARD, THEN SOUTHWARD TRACK THEREAFTER.
THE CROSS-TRACK SPREAD AT TAU 72 IS JUST 63 NM, HOWEVER, THE
NUMERICAL MODEL GUIDANCE AND AI MODEL GUIDANCE TREND OVER THE LAST
FEW WARNINGS HAS PROGRESSIVELY PUSHED THE MULTI-MODEL CONSENSUS
TRACK EASTWARD. THE JTWC TRACK FORECAST CONTINUES TO BE ALIGNED 
CLOSELY TO THE MULTI-MODEL CONSENSUS WITH MODERATE CONFIDENCE, NEAR 
RELIABLE DETERMINISTIC AND AI SOLUTIONS. REGARDING INTENSITY, ALL 
JTWC INTENSITY CONSENSUS SOLUTIONS ILLUSTRATE GRADUAL WEAKENING OVER 
THE NEXT 72 HOURS AS THE ENVIRONMENT BECOMES INCREASINGLY NON-
CONDUCIVE FOR TC SUPPORT. THE JTWC INTENSITY FORECAST HAS BEEN 
ASSESSED WITH MEDIUM CONFIDENCE AND ALIGNED CLOSELY TO THE MULTI-
MODEL CONSENSUS INTO TAU 24, THEN SLIGHTLY BELOW THE CONSENSUS 
THEREAFTER DUE TO THE EXPECTED EXTRATROPICAL TRANSITION PERIOD. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM

4. JUSTIFICATION FOR AMENDMENT: RECENT 151815Z ASCAT DATA REQUIRED
A RELOCATION OF THE INITIAL LOW-LEVEL CIRCULATION CENTER
POSITION.//
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