WDXS32 PGTW 120300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.7S 42.2E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 187 NM NORTHEAST OF EUROPA ISLAND
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT AND RAPIDLY CONSOLIDATING CIRCULATION WITH TIGHTLY CURVED
CONVECTIVE BANDING ALONG THE NORTHERN AND SOUTHERN SEMICIRCLES. AN
112235Z AMSR2 36 GHZ MICROWAVE IMAGE REVEALED THAT A DEFINED 22 NM
MICROWAVE EYE FEATURE HAS FORMED, INDICATIVE OF A QUICKLY
DEVELOPING SYSTEM. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON EXTRAPOLATION FROM THE AMSR2 IMAGE. THE INITIAL
INTENSITY OF 50 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
AGENCY DVORAK FIXES AND CIMSS OBJECTIVE INTENSITY ESTIMATES LISTED
BELOW. ENVIRONMENTAL ANALYSIS INDICATES THAT TC 21S IS IN A HIGHLY
FAVORABLE ENVIRONMENT CHARACTERIZED BY RADIAL OUTFLOW ALOFT, LOW
(5-10 KTS) VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE
TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: THE NORTHWESTERN PERIPHERY OF A
SUBTROPICAL RIDGE (STR) THAT IS CENTERED TO THE SOUTH OF MADAGASCAR

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   FMEE: T3.5 - 55 KTS
   CIMSS SATCON: 51 KTS AT 112216Z
   CIMSS ADT: 37 KTS AT 120000Z
   CIMSS AIDT: 38 KTS AT 120000Z
   CIMSS D-MINT: 44 KTS AT 112235Z
   CIMSS D-PRINT: 38 KTS AT 120000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: INCREASED THE PEAK INTENSITY TO 95
KTS FROM 75 KTS IN THE PREVIOUS FORECAST.

FORECAST DISCUSSION: TC 21S IS FORECAST TO TRACK SOUTHWESTWARD,
ALONG THE NORTHWESTERN PERIPHERY OF THE STR TO THE SOUTH THROUGH
TAU 36. NEAR TAU 36, THE STR IS EXPECTED TO PROPAGATE EASTWARD AND
A MAJOR SHORTWAVE TROUGH WILL FOLLOW BEHIND. 21S IS FORECAST TO
TRACK SOUTHWARD THROUGH TAU AS IT ATTEMPTS TO ROUND THE WESTERN
PERIPHERY OF THE RIDGE. NEAR TAU 72, THE MAJOR SHORTWAVE TROUGH
WILL BEGIN INTERACTING WITH THE VORTEX, PUSHING IT EASTWARD THROUGH
TAU 96. HOWEVER, MODELS GREATLY DIFFER ON WHAT DIRECTION THE SYSTEM
WILL BE STEERED AFTER TAU 72. ECMEF SHOWS A SUBTROPICAL RIDGE THAT
BUILDS OVER SOUTHEASTERN AFRICA, CAUSING A SHARP TURN NORTHWARD,
BACK INTO THE MOZAMBIQUE CHANNEL. GFS, ON THE OTHER HAND, SHOWS THE
SYSTEM TAKING A MORE STRAIGHTFORWARD TRACK TOWARD THE SOUTHEAST AND
INTERACTING WITH THE BAROCLINIC ZONE. THE CURRENT JTWC FORECAST IS
HEDGED CLOSER TO THE GFS PREDICTION AND MORE IN LINE WITH THE
GOOGLE DEEPMIND (GDM) ENSEMBLE MEAN WITH A MUCH SLOWER POLEWARD
TURN DUE TO THE COMPLEX STEERING PATTERN WITH INFLUENCES FROM
MULTIPLE DIRECTIONS. REGARDING INTENSITY, 21S IS FORECAST TO
RAPIDLY INTENSIFY OVER THE NEXT 36 HOURS TO AROUND 90 KTS
(POTENTIALLY HIGHER) IN RESPONSE TO THE HIGHLY FAVORABLE
ENVIRONMENT IN CONJUNCTION WITH THE COMPACT CIRCULATION. THE
INTENSITY IS THEN FORECAST TO PEAK AT AROUND 95 KTS AND STABILIZE
AS IT APPROACHES THE COAST OF MOZAMBIQUE DUE TO SOME DRY
CONTINENTAL AIR INTRUDING INTO THE VORTEX. NEAR TAU 96, SHEAR WILL
INCREASE AS THE SYSTEM INTERACTS WITH THE UPPER-LEVEL TROUGH,
FURTHER AIDING IN WEAKENING THE SYSTEM. ADDITIONALLY, SLOW TRACK
SPEEDS NEAR TAU 96 COULD ALLOW FOR UPWELLING OF COOLER WATERS
WITHIN THE SOUTHERN MOZAMBIQUE CHANNEL DUE TO A TONGUE OF LOW OHC
WATERS WITHIN THE AREA. A COMBINATION OF THESE FACTORS WILL WEAKEN
THE SYSTEM THROUGH TAU 120.  

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT THROUGH TAU 48 WITH A CROSS-TRACK SPREAD OF 110 NM AT
THAT TIME. AFTER TAU 48, MODELS GREATLY DIVERGE WITH SOME
VARIATIONS OF THE MODELS OUTLINED IN THE FORECAST DISCUSSION
SECTION. ALONG-TRACK SPREAD AT TAU 120 IS NEARLY 1000 NM BETWEEN
THE FAST GFS AND THE SLOW ECMWF. THE JTWC TRACK FORECAST IS PLACED
NEAR THE CONSENSUS THROUGH TAU 72 WITH MEDIUM CONFIDENCE AND THEN
CLOSER TO THE GDM THROUGH TAU 120 WITH VERY LOW CONFIDENCE.
INTENSITY GUIDANCE GENERALLY AGREES ON A RAPID RATE OF
INTENSIFICATION THROUGH TAU 36, A STEADY INTENSITY THROUGH TAU 72,
AND WEAKENING THROUGH TAU 120. THE JTWC INTENSITY FORECAST IS
PLACED NEAR THE MULTI-MODEL CONSENSUS WITH MEDIUM CONFIDENCE
THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER DUE TO A COMBINATION
OF SIGNIFICANT TRACK UNCERTAINTY AND INHERENT UNCERTAINTIES
REVOLVING AROUND RAPID INTENSIFICATION.

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