WDXS32 PGTW 090300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING 
NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9S 55.6E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 157 NM NORTH OF ST DENIS
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 21S (GEZANI) WITH A COMPACT LOW-LEVEL CIRCULATION CENTER
(LLCC) AND TIGHTLY CURVED CONVECTIVE BANDING ALONG THE SOUTHERN
SEMICIRCLE. A 082204Z AMSR2 36 GHZ MICROWAVE IMAGE SHOWS A PARTIAL
MICROWAVE EYE FEATURE WITH A BREAK IN THE NORTHWESTERN QUADRANT.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
ANIMATED EIR IMAGERY CORROBORATED BY EXTRAPOLATION FROM THE AMSR2.
THE INITIAL INTENSITY OF 45 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE AGENCY DVORAK FIXES AND OBJECTIVE INTENSITY ESTIMATES
LISTED BELOW. ENVIRONMENTAL ANALYSIS INDICATES THAT 21S IS IN A
FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE POLEWARD AND
EQUATORWARD OUTFLOW ALOFT, LOW (5-10 KTS) VERTICAL WIND SHEAR, AND
WARM (28-29 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: NORTHEASTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) CENTERED FAR TO THE SOUTHWEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   FMEE: T3.0 - 45 KTS
   CIMSS SATCON: 45 KTS AT 082330Z
   CIMSS ADT: 45 KTS AT 090000Z
   CIMSS AIDT: 38 KTS AT 090000Z
   CIMSS D-MINT: 40 KTS AT 082205Z
   CIMSS D-PRINT: 40 KTS AT 090030Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: COMPACT WIND FIELD

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

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 WESTWARD, ALONG
THE NORTHERN PERIPHERY OF THE STR THAT IS CURRENTLY TO THE
SOUTHWEST, THROUGH TAU 60. LANDFALL IS EXPECTED TO OCCUR ALONG THE
EASTERN COAST OF MADAGASCAR, NEAR TOAMASINA, JUST AFTER TAU 36. 21S
WILL THEN CONTINUE TO TRACK WESTWARD, ACROSS CENTRAL MADAGASCAR
THROUGH TAU 60, WHERE IT WILL EMERGE WITHIN THE MOZAMBIQUE CHANNEL.
NEAR TAU 60, THE STR IS EXPECTED TO BEGIN PROPAGATING EASTWARD DUE
TO AN INCOMING SHORTWAVE TROUGH, CAUSING 21S TO TAKE A
SOUTHWESTWARD TURN. 21S IS FORECAST TO TRACK MORE SOUTHWARD THROUGH
TAU 120 AS IT TRACKS ALONG THE WESTERN PERIPHERY OF THE STR.
REGARDING INTENSITY, 21S IS FORECAST TO RAPIDLY INTENSIFY OVER THE
NEXT 24 HOURS, FACILITATED BY AN IMPROVING POLEWARD OUTFLOW CHANNEL
ALONG WITH VERY LOW SHEAR. THE COMPACT WIND FIELD OF 21S WILL
FURTHER ENABLE A RAPID RATE OF INTENSIFICATION AS THE SYSTEM
APPROACHES MADAGASCAR. 21S WILL THEN RAPIDLY WEAKEN AS IT CROSSES
THE RUGGED TERRAIN OF CENTRAL MADAGASCAR. ONCE IT EMERGES WITHIN
THE MOZAMBIQUE CHANNEL, ENVIRONMENTAL CONDITIONS WILL GREATLY
IMPROVE WITH LOW SHEAR AND WARM SEA SURFACE TEMPERATURES. THIS WILL
ALLOW FOR 21S TO REINTENSIFY THROUGH THE REMAINDER OF THE FORECAST
PERIOD. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN MODERATE
AGREEMENT THROUGH TAU 72 WITH A 140 NM CROSS TRACK SPREAD AT TAU 72,
WHICH OPENS UP TO 290 NM AT TAU 120. MODELS HAVE STARTED TO TREND
MORE TOWARD A TRACK THAT STAYS EAST OF MOZAMBIQUE, BUT THERE IS
STILL SOME UNCERTAINTY REVOLVING AROUND THE POSSIBILITY OF A SECOND
LANDFALL. THE JTWC TRACK FORECAST IS PLACED NEAR THE MULTI-MODEL
CONSENSUS WITH OVERALL MEDIUM CONFIDENCE. INTENSITY GUIDANCE
GENERALLY AGREES ON A RAPID OR NEAR-RAPID RATE OF INTENSIFICATION
THROUGH TAU 36 AND THEN WEAKENING THROUGH TAU 60. MODELS THEN ALSO
AGREE ON REINTENSIFICATION WITHIN THE MOZAMBIQUE CHANNEL. THE JTWC
INTENSITY FORECAST IS PLACED ON THE HIGHER END OF GUIDANCE THROUGH
TAU 36, THEN FOLLOWS CLOSELY TO THE CONSENSUS THROUGH TAU 120 WITH
MEDIUM CONFIDENCE.  

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