WDXS31 PGTW 212100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 
012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.9S 64.2E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 520 NM WEST-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 29 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DEEP
CONVECTION PERSISTING TO THE WEST OF A LOW-LEVEL CIRCULATION CENTER
(LLCC) EVIDENT IN RECENT SATELLITE MICROWAVE, SAR, AND
SCATTEROMETER IMAGERY. THE SYSTEM IS HIGHLY TILTED TO THE WEST WITH 
HEIGHT DUE TO PERSISTENT, MODERATE TO HIGH EASTERLY VERTICAL WIND 
SHEAR. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON 
211402Z RCM-2 SAR, 211452Z SSMIS 37 GHZ MICROWAVE, AND 211735Z METOP-C
ASCAT IMAGERY, ALL OF WHICH CLEARLY DEPICT THE LLCC TO THE EAST OF THE 
MAIN DEEP CONVECTIVE AREA. THE INITIAL INTENSITY OF 60 KTS IS ASSESSED 
WITH MEDIUM CONFIDENCE BASED ON A CONSENSUS OF THE MULTI-AGENCY 
SUBJECTIVE DVORAK AND AUTOMATED INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: 211402Z RCM-2 SAR AND 211735Z METOP-C
ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   DEMS: T4.0 - 65 KTS
   FMEE: T3.5 - 55 KTS
   FIMP: T3.0 - 45 KTS
   CIMSS ADT: 59 KTS AT 211800Z
   CIMSS AIDT: 69 KTS AT 211700Z
   CIMSS D-MINT: 53 KTS AT 211352Z
   CIMSS D-PRINT: 68 KTS AT 211830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

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 WESTWARD TO
WEST-SOUTHWESTWARD OVER THE NEXT 36 TO 48 HOURS UNDER THE
CONTINUOUS INFLUENCE OF SUBTROPICAL RIDGING TO THE SOUTHEAST. A
SUBTROPICAL RIDGE BUILDING TO THE EAST OF MADAGASCAR IN THE WAKE OF A 
PASSING SHORTWAVE TROUGH WILL BECOME THE PRIMARY STEERING INFLUENCE 
AFTER TAU 48, CARRYING THE SYSTEM WESTWARD TO WEST-NORTHWESTWARD 
THROUGH THE REMAINDER OF THE FORECAST PERIOD. VERTICAL WIND SHEAR 
INDUCED BY PERSISTENT UPPER-LEVEL EASTERLY TO NORTHEASTERLY FLOW OVER 
IS EXPECTED TO WEAKEN THE SYSTEM TO DISSIPATION BY TAU 120, WITH 
FAVORABLE UPPER-LEVEL OUTFLOW AND A TRACK OVER SUFFICIENTLY
WARM WATER LIMITING THE WEAKENING RATE AT LEAST FOR THE NEXT COUPLE OF
DAYS. TC 04S HAS THUS FAR BEEN RESISTANT TO PERSISTENT VERTICAL
WIND SHEAR WITH THE BALANCING SUPPORT OF FAVORABLE OUTFLOW AND
SEA SURFACE TEMPERATURES. 

MODEL DISCUSSION: THE NUMERICAL MODEL GUIDANCE TRACK FORECAST
PACKAGE IS TIGHTLY GROUPED IN BOTH DIRECTION AND SPEED THROUGHOUT
THE ENTIRETY OF THE FORECAST PERIOD. THE CURRENT JTWC TRACK
FORECAST IS CONSISTENT WITH BOTH THE MULTI-MODEL CONSENSUS AND THE
PREVIOUS JTWC TRACK FORECAST. DYNAMIC AND STATISTICAL-DYNAMICAL
INTENSITY GUIDANCE ARE IN GOOD AGREEMENT REGARDING THE OVERALL
WEAKENING SCENARIO, ALTHOUGH COAMPS-TC SUGGESTS A POTENTIALLY
EARLIER DISSIPATION, BY TAU 96 OR SOONER. WHILE THE SYSTEM HAS
RESISTED WEAKENING DUE TO EASTERLY SHEAR SO FAR, REORIENTATION OF THE
FLOW AND CHANGES IN STORM STRUCTURE OVER TIME WILL MAKE THE SYSTEM
MORE VULNERABLE TO MORE RAPID DISSIPATION, LENDING SOME UNCERTAINTY
TO THE CURRENT INTENSITY FORECAST, WHICH IS CONSISTENT WITH THE
MULTI-MODEL INTENSITY CONSENSUS AND PREVIOUS JTWC INTENSITY
FORECAST.

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