WDXS31 PGTW 220900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING 
NR 013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 10.5S 62.1E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 639 NM NORTH-NORTHEAST OF PORT LOUIS,
MAURITIUS
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 29 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM (TS) 04S (CHENGE) STARTING TO WEAKEN FROM PROLONGED EXPOSURE
TO HIGH VERTICAL WIND SHEAR (20-25 KTS) AND SURROUNDING DRY AIR
WRAPPING INTO THE CORE OF THE VORTEX FROM THE NORTHEAST.
TEMPERATURES WITHIN THE CIRCULATION HAVE STARTED TO WARM AND THE
ASSOCIATED CONVECTIVE ACTIVITY HAS LESS PERSISTENT FLARING OVER THE
PAST 6 HOURS. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON A 220441Z METOP-C ASCAT SCATTEROMETER DATA. THE INITIAL
INTENSITY OF 60 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
AFOREMENTIONED ASCAT PASS AND MULTI-AGENCY SUBJECTIVE DVORAK AND
AUTOMATED INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: 220441Z METOP-C ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   CIMSS SATCON: 57 KTS AT 220201Z
   CIMSS ADT: 57 KTS AT 220300Z
   CIMSS AIDT: 50 KTS AT 220500Z
   CIMSS D-MINT: 59 KTS AT 220200Z
   CIMSS D-PRINT: 58 KTS AT 220500Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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: TS CHENGE WILL CONTINUE TO TRACK WEST UNDER
THE INFLUENCE OF THE SUBTROPICAL RIDGE TO THE SOUTHEAST OVER THE
NEXT 36 HOURS. A BUILDING RIDGE CENTERED TO THE EAST OF MADAGASCAR
WILL THEN TAKE OVER AND CONTINUE DRIVING THE SYSTEM TO THE
WEST-NORTHWEST FOR THE REMAINDER OF THE FORECAST PERIOD.
ENVIRONMENTAL CONDITIONS WILL CONTINUE TO BE MARGINALLY UNFAVORABLE
WITH VERTICAL WIND SHEAR EXPECTED TO REMAIN AT 20-25 KNOT STRENGTH
AND DRY AIR BECOMING ENTRAINED INTO THE CIRCULATION. THE SYSTEM
WILL EXPERIENCE A SLOW WEAKENING TREND WITH SUFFICIENT WARM WATER
(26 C TO 27 C) AND GOOD OUTFLOW BALANCING THE VERTICAL WIND SHEAR
AND DRY AIR. TS CHENGE IS EXPECTED TO CONTINUE TO DISSIPATE AS IT
APPROACHES THE COAST OF TANZANIA. FULL DISSIPATION WILL OCCUR OVER
LAND AROUND TAU 120. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS ASSESSED WITH
MEDIUM CONFIDENCE, AS WITNESSED BY A CROSS-TRACK SPREAD OF UNDER 70
NM THROUGH TAU 48. AFTERWARDS, UNCERTAINTY INCREASES SLIGHTLY IN
REGARD TO THE MAGNITUDE IN WHICH THE RIDGING OVER MADAGASCAR WILL
BE ABLE TO PUSH THE SYSTEM FURTHER NORTHWESTWARD. NAVGEM AND ECMWF
TAKE THE MOST NORTHERN TRACK FORECASTING LANDFALL JUST SOUTH OF
DAR ES SALAAM. DETERMINISTIC GFS KEEPS A MORE SOUTHERN TRACK, WITH
LANDFALL JUST NORTH OF THE BORDER BETWEEN TANZANIA AND MOZAMBIQUE.
THE INTENSITY SPREAD IS AROUND 20 KTS THROUGHOUT THE FORECAST
PERIOD WITH HAFS-A BEING RELUCTANT TO BRING INTENSITY BELOW 40 KTS.
THE JTWC OFFICIAL FORECAST IS PLACED CLOSE TO THE MULTI-MODEL
CONSENSUS AND IS ASSESSED 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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