WDXS31 PGTW 210900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING 
NR 011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.6S 65.2E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 459 NM WEST-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 04S (CHENGE) HAS BEEN ABLE TO MAINTAIN ITS
INTENSITY DESPITE ENCOUNTERING A SIGNIFICANT AMOUNT OF VERTICAL
WIND SHEAR (20-25 KTS). ANIMATED ENHANCED INFRARED (EIR) SATELLITE
IMAGERY DEPICTS PERSISTENT DEEP CONVECTION ESPECIALLY IN THE
NORTHWESTERN QUADRANT OF THE CIRCULATION. THE INITIAL POSITION AND
INTENSITY IS PLACED WITH HIGH CONFIDENCE BASED ON THE 210414Z
METOP-B SCATTEROMETER PASS AND THE AGENCY FIXES LISTED BELOW.
ADDITIONAL ENVIRONMENTAL ANALYSIS REVEALS DRY AIR SURROUNDING THE
CIRCULATION AND ENTERING THE VORTEX FROM THE NORTH AND EAST.

INITIAL WIND RADII BASIS: 210414Z METOP-B SCATTEROMETER DATA AND
OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   DEMS: T3.5 - 55 KTS
   CIMSS SATCON: 56 KTS AT 210458Z
   CIMSS ADT: 74 KTS AT 210600Z
   CIMSS AIDT: 68 KTS AT 210500Z
   CIMSS D-MINT: 52 KTS AT 210213Z
   CIMSS D-PRINT: 57 KTS AT 210630Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   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: TROPICAL CYCLONE (TC) 04S (CHENGE) IS EXPECTED
TO CONTINUE TRACKING GENERALLY WESTWARD OVER THE NEXT 72 HOURS AS IT 
IS
PUSHED BY THE SUBTROPICAL RIDGE TO THE SOUTHEAST. AFTERWARDS A
COMPETING STEERING MECHANISM, RESULTING FROM A BUILDING RIDGE EAST OF
MADAGASCAR, WILL CAUSE THE TRACK TO SHIFT MORE NORTH-NORTHWESTWARD
FOR THE REMAINDER OF THE FORECAST. ALTHOUGH THE SYSTEM IS HOLDING
WELL FOR NOW, STRONG VERTICAL WIND SHEAR AND CONTINUOUS DRY AIR
ENTRAINMENT WILL WHITTLE DOWN THE INTENSITY AND LEAD TO FULL
DISSIPATION PRIOR TO TAU 120.  

MODEL DISCUSSION: MODEL GUIDANCE IS IN TIGHT CONSENSUS UNTIL THE
SHIFT IN STEERING PATTERN OCCURS AROUND TAU 72. THIS CREATES A
SPLIT IN THE GUIDANCE WITH THE SOUTHERNMOST MODELS TRACKING OVER
COMOROS AND THE NORTHERNMOST NEAR 7.0N. THE JTWC TRACK REMAINS
CLOSE TO THE CONSENSUS WHICH FAVORS A SLIGHTLY SOUTHERN TRACK. MODEL
GUIDANCE REMAINS RELUCTANT TO PROJECT DECREASING INTENSITIES AFTER
TAU 48. THE JTWC INTENSITY FORECAST DEPICTS A QUICKER DISSIPATION
AS ENVIRONMENTAL CONDITIONS, NAMELY THE SHEAR, ARE LIKELY NOT BEING
WELL REPRESENTED. THE JTWC INTENSITY FORECAST LIES BETWEEN THE
CONSENSUS AND HAFS-A GUIDANCE FOR THE MAJORITY OF THE FORECAST
PERIOD.

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