WDXS31 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.1S 67.0E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 348 NM WEST-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TC 04S HAS CONTINUED TO DEVELOP OVER THE PAST SIX HOURS DESPITE
PERSISTENT EASTERLY VERTICAL WIND SHEAR. ANIMATED ENHANCED INFRARED
(EIR) SATELLITE IMAGERY DEPICTS PERSISTENT DEEP CONVECTION OVER THE
WESTERN PORTION OF THE CIRCULATION. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON LOW-LEVEL CENTER FEATURES EVIDENT IN
201354Z RCM-2 SAR, 201406Z SSMIS 37 GHZ, AND 201710Z METOP-B ASCAT
IMAGES. THE INITIAL INTENSITY OF 60 KTS IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON THE INTENSIFICATION TREND EVIDENT IN SUBJECTIVE
DVORAK AND AUTOMATED INTENSITY ESTIMATES AS WELL AS WIND SPEEDS
EXCEEDING 55 KTS EVIDENT IN THE RCM-2 SAR DATA.

INITIAL WIND RADII BASIS: 201354Z RCM-2 SAR PASS AND PERSISTENCE 

CURRENT STEERING MECHANISM: DEEP LAYER SUBTROPICAL RIDGE TO THE
SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   DEMS: T3.5 - 55 KTS
   FMEE: T3.5 - 55 KTS
   FIMP: T3.0 - 45 KTS
   CIMSS SATCON: 55 KTS AT 201408Z
   CIMSS ADT: 57 KTS AT 201800Z
   CIMSS AIDT: 58 KTS AT 201700Z
   CIMSS D-MINT: 50 KTS AT 201906Z
   CIMSS D-PRINT: 55 KTS AT 201900Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 04S WILL CONTINUE TO TRACK
WEST-SOUTHWESTWARD TO WESTWARD OVER THE NEXT 72 HOURS UNDER THE
PERSISTENT INFLUENCE OF SUBTROPICAL RIDGING TO THE SOUTHEAST.
THEREAFTER, A BUILDING RIDGE TO THE EAST OF MADAGASCAR WILL TAKE
OVER AS THE PRIMARY STEERING MECHANISM AND LIKELY NUDGE THE SYSTEM
ALONG A WEST-NORTHWESTWARD TRAJECTORY UNTIL THE END OF THE FORECAST
PERIOD. IN THE NEAR-TERM, CONDITIONS INCLUDING MODERATE OUTFLOW
ALOFT AND PASSAGE OVER WARM WATER ARE SUFFICIENT TO SUPPORT THE
INTENSITY. HOWEVER, VERY WIND SHEAR DRIVEN BY STRENGTHENING
EASTERLY FLOW ALOFT IS EXPECTED TO INCREASE MARKEDLY OVER TIME,
PARTICULARLY AFTER TAU 48, DRIVING A CONTINUOUS WEAKENING TREND
UNTIL DISSIPATION OCCURS AROUND OR AFTER TAU 96.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
REGARDING THE OVERALL FORECAST SCENARIO BUT DIVERGES SIGNIFICANTLY
ACROSS A RANGE OF POTENTIAL TRACKS. THE ECMWF-BASED AND NAVGEM
SOLUTIONS BRACKET THE SOUTHERN END OF THE ENVELOPE WHILE THE
GFS-BASED AND UKMET SOLUTIONS BRACKET THE NORTHERN END, AS OBSERVED
IN PREVIOUS RUNS. CONSISTENT WITH THE PREVIOUS TRENDS, THE CURRENT
FORECAST IS WEIGHTED TOWARD THE SOUTHERN GROUPING BUT IS PLACED
NEAR THE MULTI-MODEL CONSENSUS AND ONLY SLIGHTLY TO THE NORTH OF
THE PREVIOUS JTWC FORECAST IN THE EXTENDED PERIOD. DYNAMIC AND
STATISTICAL-DYNAMIC MODELS ARE IN GOOD AGREEMENT REGARDING THE
INTENSITY TREND, THOUGH THE GFS AND COAMPS-TC ANTICIPATE AN EARLIER
START TO THE WEAKENING TREND, BY TAU 36, WHILE HAFS DEPICTS A MORE
RESISTANT SYSTEM PERSISTING IN THE NEAR-TERM. THE CURRENT INTENSITY
FORECAST IS NEAR BUT SLIGHTLY BELOW THE CONSENSUS AND CONSISTENT WITH 
THE PREVIOUS FORECAST.

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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