WDXS31 PGTW 191500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.6S 69.0E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 227 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:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY CONTINUES TO
DEPICT A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC),
WITH FLARING CONVECTION BEGINNING TO REFORM ALONG THE WESTERN FLANK
OF THE LLCC, CONTINUING THE CYCLE SEEN OVER THE PAST COUPLE DAYS.
SHEAR REMAINS RELATIVELY HIGH HOWEVER, AND ALONG WITH THE DRY AIR
TO THE EAST, IS HINDERING VORTEX CONSOLIDATION AND INTENSIFICATION.
A 190813Z GMI 36GHZ MICROWAVE IMAGE REVEALED A SMALL BUT
WELL-DEFINED LOW EMISSIVITY REGION MARKING THE LLCC, WITH A LARGE
AREA OF DEEP MOISTURE AND CONVECTION FORMING CONVECTIVE BANDS ALONG
THE ENTIRE WESTERN HALF OF THE CIRCULATION. THE EASTERN HALF FOR
THE MOST PART CONTAINS ONLY SHALLOW BANDING FEATURES WRAPPING INTO
THE LLCC, DUE TO THE PRESENCE OF THE DRY MID-LEVEL AIR. THE INITIAL
POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC
AND AN EXTRAPOLATION OF THE CENTER DERIVED FROM THE MICROWAVE
IMAGERY. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE,
BASED ON AN AVERAGE OF THE OBJECTIVE AND SUBJECTIVE INTENSITY
ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS REMAIN STEADY, AND
MARGINALLY FAVORABLE, WITH WARM SSTS, AND STRONG POLEWARD OUTFLOW
OFFSET BY PERSISTENT MODERATE EASTERLY SHEAR.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) ENTRENCHED TO THE SOUTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   DEMS: T3.0 - 45 KTS
   FMEE: T3.0 - 45 KTS
   FIMP: T3.0 - 45 KTS
   CIMSS SATCON: 48 KTS AT 191030Z
   CIMSS ADT: 43 KTS AT 191130Z
   CIMSS AIDT: 36 KTS AT 191130Z
   CIMSS D-MINT: 32 KTS AT 190840Z
   CIMSS D-PRINT: 36 KTS AT 191130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY MID-LEVEL AIR PRIMARILY IN THE EASTERN
PORTION OF THE CIRCULATION. 

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 04S HAS CONTINUED TO
TRACK WESTWARD ALONG THE NORTHERN SIDE OF THE STR OVER THE PAST 12
HOURS. THE SYSTEM HAS SLOWED DOWN HOWEVER, AS A WEAKNESS HAS
DEVELOPED IN THE RIDGE DUE TO PASSAGE OF THE A TROF FAR TO THE
SOUTH. THE WEAKENED STEERING GRADIENT IS EXPECTED TO PERSIST FOR
ANOTHER 12 TO 24 HOURS, BEFORE THE RIDGE BUILDS BACK IN AND MOVES
SLOWLY WESTWARD, THOUGH TRACK SPEEDS ARE EXPECTED TO REMAIN FAIRLY
LOW THROUGH TAU 72. AFTER TAU 72, THE RIDGE IS FORECAST TO
STRENGTHEN AND PUSH WESTWARD IN RESPONSE TO STRONG TROFING OVER THE
MOZAMBIQUE CHANNEL, WHICH WILL RESULT IN A FLATTER, MORE WESTWARD
AND SLIGHTLY FASTER TRAJECTORY FOR TC 04S THROUGH THE END OF THE
FORECAST PERIOD. THE SYSTEM HAS MAINTAINED ITS CURRENT INTENSITY
FOR THE LAST COUPLE OF FORECASTS, AS THERE HAVE BEEN NO MAJOR
CHANGES IN THE OVERALL ENVIRONMENT. HOWEVER, THE LATEST CIMSS AMV
ANALYSIS DEPICTS STRENGTHENING POLEWARD OUTFLOW, AROUND A BUILDING
200MB RIDGE TO THE EAST. THE IMPROVED OUTFLOW ENVIRONMENT SHOULD
OFFSET THE MODERATE EASTERLY SHEAR JUST ENOUGH TO ALLOW FOR A BRIEF
PERIOD OF INTENSIFICATION, UP TO 45 KNOTS BY TAU 24. BUT THE
ENVIRONMENT WILL QUICKLY CHANGE AND BY TAU 36, SHEAR IS EXPECTED TO
INCREASE, ACCOMPANIED BY AN INFLUX OF DRY MID-LEVEL AIR AND
CONVERGENT FLOW ALOFT, THE COMBINED EFFECTS OF WHICH WILL START A
SLOW WEAKENING TREND FROM WHICH TC 04S WILL NOT BE ABLE TO RECOVER.
THE SYSTEM WILL SLOWLY SPIN DOWN AFTER TAU 48 AND BEGIN TO
DISSIPATE BY THE END OF THE FORECAST PERIOD. 

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE HAS
STARTED TO SHOW INCREASED DISPERSION, PARTICULARLY AFTER TAU 48.
THE ECMWF HAS FLIPPED FROM BEING ON THE NORTHERN SIDE OF THE
GUIDANCE ENVELOPE TO NOW TAKING THE SYSTEM ON A STRAIGHT
SOUTHWESTWARD COURSE FROM THE BEGINNING OF THE FORECAST, CLOSE TO
WHAT THE NAVGEM HAS BEEN DEPICTING ALL ALONG. THE GFS HAS ALSO
SHIFTED SOUTH AND REJOINED THE MAIN GROUPING OF MODELS, LEAVING THE
GEFS MEAN AS THE SOLE OUTLIER TO THE NORTH. CROSS-TRACK SPREAD HAS
INCREASED TO 300NM BETWEEN THE GEFS MEAN AND THE ECMWF, THOUGH THE
INNER GROUPING OF MODELS (CONSENSUS MEAN, GFS, GALWEM, ECENS MEAN,
EC-AIFS) ARE CONFINED TO A 90NM ENVELOPE, ROUGHLY CENTERED ON THE
JTWC FORECAST TRACK. INTENSITY GUIDANCE IS IN GOOD AGREEMENT AND
ALL MODELS ARE TIGHTLY GROUPED ABOUT THE MEAN WITH LESS THAN FIVE
KNOT DEVIATIONS IN THE PEAK INTENSITY. 

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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