WDXS32 PGTW 170900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING 
NR 020//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 29.6S 43.9E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 487 NM SOUTH-SOUTHEAST OF EUROPA ISLAND
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 30 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 21S HAS UNDERGONE RAPID VORTEX DECOUPLING AS IT 
INTERACTED WITH A SUBTROPICAL JET STREAK. ANIMATED MULTISPECTRAL 
SATELLITE IMAGERY (MSI) DEPICTS A FULLY DECOUPLED AND INCREASINGLY 
ELONGATED LLCC ACCELERATING TOWARDS THE SOUTHWEST, INTEGRATING WITH A 
BAROCLINIC CLOUD SHIELD. ANIMATED WATER VAPOR IMAGERY AND GLOBAL 
MODEL FIELDS REVEAL A CUT-OFF UPPER-LEVEL, COLD-CORE LOW JUST 
SOUTHWEST OF THE SYSTEM CENTER. A 170545Z ASCAT-C PASS REVEALED A 
SMALL INNER-CORE OF LIGHT WINDS, SURROUNDED BY A BROAD GALE-FORCE WIND 
REGION, WITH A MAXIMUM WIND SPEED OF APPROXIMATELY 50 KNOTS IN THE 
SOUTHEASTERN QUADRANT. A 170336Z SSMIS 37GHZ IMAGE INDICATES THE 
SYSTEM IS ACQUIRING ASYMMETRIC BAROCLINIC FEATURES, WITH A WEAK 
FRONTAL BAND EXTENDING NORTHEASTWARD AND A FRONTOGENETIC RAIN SHIELD 
TO THE SOUTHWEST, VALIDATING THE TRANSITION TO A HYBRID THERMAL 
STRUCTURE. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE 
BASED ON THE EXPOSED LLCC SUPPORTED BY THE ASCAT PASS NOTED ABOVE. 
THE INITIAL INTENSITY IS ASSESSED WITH HIGH CONFIDENCE, CONFIRMED BY 
THE ASCAT DATA. ENVIRONMENTAL CONDITIONS ARE HOSTILE, CHARACTERIZED 
BY EXCESSIVE WIND SHEAR, MARGINAL THERMODYNAMIC SUPPORT (SSTS 25-
26C), NEGLIGIBLE OCEAN HEAT CONTENT (OHC), AND STRONG POLEWARD 
OUTFLOW ALOFT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   DEMS: T3.0 - 45 KTS
   FMEE: T3.5 - 55 KTS
   CIMSS ADT: 39 KTS AT 170600Z
   CIMSS AIDT: 50 KTS AT 170600Z
   CIMSS D-MINT: 33 KTS AT 170336Z
   CIMSS D-PRINT: 38 KTS AT 170600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 30+ KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DEEP LAYER OF DRY AIR OVERTOPPING THE LOW LEVEL
CIRCULATION CENTER (LLCC). 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 24
HOURS AND REINTRODUCED SUBTROPICAL TRANSITION (STT) AS THE
TRANSITION MECHANISM. 

FORECAST DISCUSSION: TC 21S IS NOW ASSESSED AS A HYBRID TYPE SYSTEM
WITH CHARACTERISTICS OF BOTH A TROPICAL CYCLONE AND A MID-LATITUDE
EXTRATROPICAL LOW. TC 21S WILL CONTINUE TO PROPAGATE SOUTH-
SOUTHWESTWARD ALONG THE STR PERIPHERY THROUGH THE NEXT 24 HOURS, AND 
WILL SHORTLY BECOME VERTICALLY CO-LOCATED WITH THE UPPER-LEVEL COLD-
CORE LOW. WHILE THIS ALIGNMENT REDUCES EFFECTIVE SHEAR, WHICH MIGHT 
TYPICALLY FACILITATE DEEP CONVECTIVE REORGANIZATION, THE ENTRAINMENT 
OF A DEEP LAYER OF SUBSIDENT DRY AIR WILL SUPPRESS SIGNIFICANT 
VERTICAL DEVELOPMENT THROUGH TAU 12. THE RESIDUAL LOW-LEVEL VORTEX 
WILL GRADUALLY DECAY VIA CYCLOLYSIS; HOWEVER, THE FORECAST 
ANTICIPATES TC 21S WILL COMPLETE STT AS A 45 KNOT SYSTEM BY TAU 24.

MODEL DISCUSSION: DETERMINISTIC, ENSEMBLE AND ARTIFICIAL
INTELLIGENCE (AI) MODELS ARE IN STRONG AGREEMENT WITH THE FORECAST,
WITH MINIMAL CROSS-TRACK OR ALONG-TRACK SPREAD DURING THE SHORT
DURATION OF THE FORECAST. INTENSITY GUIDANCE IS ALSO IN GOOD
AGREEMENT, HOWEVER THE HWRF DOES DEPICT THE SYSTEM INTENSIFYING
BRIEFLY THROUGH TAU 12 BEFORE GRADUALLY WEAKENING THEREAFTER. THE
REMAINDER OF THE GUIDANCE PACKAGE DEPICTS THE SYSTEM GRADUALLY
WEAKENING FROM THE BEGINNING OF THE FORECAST PERIOD. 

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