WDXS32 PGTW 080300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (TWENTYONE) 
WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.0S 56.9E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 128 NM NORTH OF PORT LOUIS, MAURITIUS
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 21S WITH FLARING CONVECTION ALONG THE NORTHERN PERIPHERY OF
A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER. DEEP CONVECTION
HAS BEEN CYCLING OVER THE PAST 24 HOURS, WITH PERIODS OF CURVED
BANDING AND OTHER PERIODS OF SPARSITY. A 071818Z METOP-C ASCAT
IMAGE REVEALED AN ASYMMETRIC WIND FIELD WITH PATCH OF 30-40 KTS IN
THE EASTERN SEMICIRCLE AND WEAKER WINDS (15-20 KTS) WINDS
ELSEWHERE. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED PRIMARILY ON THE ANIMATED EIR IMAGERY. THE INITIAL INTENSITY
OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY
DVORAK CI FIXES OF T2.5. AN EARLY CIMSS DPRINT ESTIMATE OF 37 KTS
ALSO SUPPORTED THE INITIAL INTENSITY. ENVIRONMENTAL ANALYSIS
INDICATES THAT 21S IS IN A MARGINALLY FAVORABLE ENVIRONMENT
CHARACTERIZED BY MODERATE OUTFLOW ALOFT, LOW (10-15 KTS) VERTICAL
WIND SHEAR, AND WARM (28-29 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: A WEAK STEERING ENVIRONMENT WITH
SUBTROPICAL RIDGES TO BOTH THE SOUTHEAST AND SOUTHWEST THAT IS
SEPARATED BY AN UPPER-LEVEL TROUGH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   FMEE: T2.5 - 35 KTS
   CIMSS D-PRINT: 37 KTS AT 080000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: DRY AIR SURROUNDING THE SYSTEM

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 21S IS FORECAST TO TRACK SLOWLY WESTWARD
OVER THE NEXT 12-24 HOURS AS THE SYSTEM REMAINS PLACED WITHIN A
WEAK STEERING ENVIRONMENT. NEAR TAU 24, THE SUBTROPICAL RIDGE (STR)
TO THE SOUTHWEST WILL BUILD AND BECOME THE PRIMARY STEERING
MECHANISM. 21S WILL THEN TRACK WESTWARD, ALONG THE NORTHERN
PERIPHERY OF THE STR, AS IT APPROACHES MADAGASCAR. LANDFALL ALONG
THE EASTERN COAST OF MADAGASCAR IS FORECAST TO OCCUR NEAR TAU 60.
21S IS THEN FORECAST TO CONTINUE ON A WESTWARD TRACK ACROSS CENTRAL
MADAGASCAR AND EMERGE WITHIN THE MOZAMBIQUE CHANNEL NEAR TAU 84.
AFTER TAU 84, THE SYSTEM IS FORECAST TO BEGIN A SOUTHWESTWARD TURN
AS IT ROUNDS THE NORTHWESTERN PERIPHERY OF THE STR. REGRADING
INTENSITY, 21S IS FORECAST TO GRADUALLY INTENSIFY OVER THE NEXT 24
HOURS AS SHEAR REMAINS LOW AND THE SYSTEM FIGHTS AGAINST THE DRY
AIR AROUND IT. NEAR TAU 24, THE ENVIRONMENT IS EXPECTED TO BECOME
MORE FAVORABLE AS MID-LEVEL MOISTURE AND OUTFLOW CHANNELS IMPROVE.
THIS WILL ALLOW FOR 21S TO MORE QUICKLY INTENSIFY FROM TAU 24
THROUGH THE TIME OF LANDFALL. A WEAKENING TREND WILL THEN ENSUE AS
THE SYSTEM TRAVERSES ACROSS MADAGASCAR. ONCE 21S EMERGES WITHIN THE
MOZAMBIQUE CHANNEL, ENVIRONMENTAL CONDITIONS WILL ONCE AGAIN BECOME
MUCH MORE FAVORABLE WITH LOW SHEAR AND WARM SEA SURFACE
TEMPERATURES. AS A RESULT, 21W WILL REINTENSIFY THROUGH THE
REMAINDER OF THE FORECAST PERIOD.

MODEL DISCUSSION: DISCOUNTING NAVGEM, NUMERICAL MODEL TRACK
GUIDANCE IS IN MODERATE AGREEMENT, WITH A CROSS-TRACK SPREAD OF 125
NM AT TAU 60 (NEAR THE TIME OF LANDFALL) WHICH OPENS UP TO 290 NM
AT TAU 120. NAVGEM IS A SIGNIFICANT OUTLIER, IT HAS A MUCH SLOWER
TRACK SPEED AND IS THE ONLY MODEL THAT LOSES THE VORTEX OVER
MADAGASCAR. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE ALTERED
CONSENSUS (REMOVING NAVGEM) WITH OVERALL MEDIUM CONFIDENCE.
INTENSITY GUIDANCE GENERALLY AGREES ON A GRADUAL INTENSIFICATION
TREND THROUGH TAU 24 AND A QUICKER INTENSIFICATION TREND THROUGH
TAU 48. PEAK INTENSITIES RANGE FROM 50 KTS (GFS) TO 85 KTS
(COAMPS-TC) AT TAU 60. MODELS THEN AGREE ON WEAKENING THROUGH TAU
84 AND REINTENSIFICATION THEREAFTER. THE JTWC INTENSITY FORECAST IS
PLACED NEAR THE CONSENSUS 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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