WDXS31 PGTW 192100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.6S 68.2E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 271 NM WEST-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
RECENT TOTAL PRECIPITABLE WATER IMAGERY SHOWS THAT THE SYSTEM HAS
CLEARLY MOISTENED OVER THE PAST 12 HOURS, WITH A SYMMETRIC CORE OF
DEEP MOISTURE. HOWEVER, MODEL ANALYSES DO SHOW A NARROW SWATH OF
DRY AIR ALONG THE EASTERN PERIPHERY, WHICH WILL IMPACT THE
INTENSITY FORECAST AFTER TAU 24. ANIMATED ENHANCED INFRARED (EIR) 
SATELLITE IMAGERY DEPICTS A CONSOLIDATING LOW-LEVEL CIRCULATION, WITH
IMPROVED DEEP CONVECTIVE BANDING AND CORE CONVECTION, FUELED BY
EXPANDING EASTWARD OUTFLOW AND ROBUST EQUATORWARD AND POLEWARD
OUTFLOW. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED
ON A 191637Z ASCAT-C BULLSEYE IMAGE, WHICH SHOWS A WELL-DEFINED
CENTER, WITH 35-40 KNOT WINDS. THE INITIAL INTENSITY OF 40 KTS IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE ASCAT-C DATA. SUBJECTIVE
DVORAK ESTIMATES ARE SLIGHTLY HIGHER AT 45 KNOTS, WITHIN THE NARROW
RANGE OF UNCERTAINTY. THE ASCAT-C IMAGE, WHICH DEPICTS EXTENSIVE GALE 
FORCE WINDS OVER THE SOUTHWESTERN QUADRANT, WAS ALSO USED TO REFINE 
THE 34-KNOT WIND RADII ANALYSIS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
ENTRENCHED TO THE SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   FMEE: T3.0 - 45 KTS
   CIMSS ADT: 41 KTS AT 191800Z
   CIMSS AIDT: 32 KTS AT 191700Z
   CIMSS D-MINT: 38 KTS AT 191343Z
   CIMSS D-PRINT: 44 KTS AT 191830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 27-28 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 WILL TRACK GENERALLY 
WEST-SOUTHWESTWARD THROUGH TAU 72 ALONG THE NORTHWESTERN PERIPHERY OF 
THE STR. AFTER A BRIEF PERIOD OF INTENSIFICATION AND CONSOLIDATION, 
TC 04S WILL SUCCUMB TO INCREASING DRY AIR ENTRAINMENT AND INCREASING 
VERTICAL WIND SHEAR (30-35 KNOTS) AFTER TAU 24, WHICH WILL PRODUCE A 
STEADY WEAKENING TREND THROUGH THE REMAINDER OF THE FORECAST. THE 
SYSTEM IS EXPECTED TO DISSIPATE BY TAU 120 WHILE TURNING WESTWARD.    

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72, WITH A 140NM CROSS-TRACK SPREAD AT TAU 72. AFTER
TAU 72, GUIDANCE DIVERGES QUICKLY WITH A 350NM CROSS-TRACK SPREAD
AT TAU 120 AND SOME MINOR ALONG-TRACK DIFFERENCES. PROBABILISTIC
GUIDANCE FROM THE 191200Z GEFS AND EPS ENSEMBLES ALSO SHOW A LARGE
SPREAD IN SOLUTIONS AFTER TAU 72, WHICH LENDS LOW CONFIDENCE TO THE
JTWC TRACK FORECAST. RELIABLE INTENSITY GUIDANCE CONTINUES TO SHOW
A SLIGHT INTENSIFICATION WITHIN THE NEXT 24 HOURS, WITH A STEADY
WEAKENING TREND AFTER TAU 24.       

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