WDXS31 PGTW 240900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 
017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.2S 55.6E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 661 NM NORTH OF PORT LOUIS, MAURITIUS
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 25 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 04S CONTINUES TO DISPLAY A VERY CYCLIC,
NEARLY DIURNAL, TREND IN TERMS OF CONVECTIVE DEVELOPMENT AND
STRUCTURAL CHARACTERISTICS. OVER THE PAST TWO DAYS, DEEP CONVECTION
REFORMS AT THE CONVECTIVE MAXIMUM AND THE LOW LEVEL CIRCULATION
CENTER (LLCC) BRIEFLY MOVES UP BENEATH THE EASTERN EDGE OF THE
CONVECTION, CLOSER TO THE MID-LEVEL CENTER (MLC). SUBSEQUENTLY, THE
CONVECTION IS SHEARED OFF TO THE WEST BY THE PERSISTENT EASTERLY
SHEAR BY AROUND 0900Z, LEAVING BEHIND AN EXPOSED LLCC. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS THIS SAME EXACT
PATTERN REPLAYING CURRENTLY, WITH THE LLCC LOCATED ON THE EXTREME
EASTERN EDGE OF AN AREA OF FLARING CONVECTION. THE LATEST CIMSS AMV
ANALYSIS AND MESOSCALE MODEL GUIDANCE SHOWS THE OUTFLOW-LEVEL FLOW
SPLITTING UPSTREAM (EAST) OF TC 04S, DUE TO THE LOCALIZED
CONVECTIVE OUTFLOW PUSHING BACK AGAINST THE SHEAR, AGAIN IN A VERY
CYCLIC PATTERN. A 240453Z ASCAT-B PASS REVEALED A SMALL LLCC, WITH
35-40 KNOT WINDS MOSTLY CONFINED TO THE SOUTHWESTERN QUADRANT AND
ISOLATED AREAS TO THE SOUTH AND SOUTHEAST. THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE ASCAT DATA. THE INITIAL
INTENSITY IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE OBJECTIVE
AND SUBJECTIVE ESTIMATES NOTED BELOW, CONFIRMED BY THE ASCAT DATA.
ENVIRONMENTAL CONDITIONS ARE MARGINAL, WITH SSTS HOVERING NEAR 26C,
MODERATE OUTFLOW ALOFT AND MODERATE TO HIGH EASTERLY SHEAR.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED NEAR LA REUNION. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   FMEE: T2.5 - 35 KTS
   FIMP: T2.5 - 35 KTS
   CIMSS SATCON: 37 KTS AT 240330Z
   CIMSS ADT: 34 KTS AT 240600Z
   CIMSS AIDT: 34 KTS AT 240600Z
   CIMSS D-MINT: 31 KTS AT 240314Z
   CIMSS D-PRINT: 31 KTS AT 240600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: MODERATE WESTWARD OUTFLOW. 

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: TC 04S WILL CONTINUE TO TRACK WEST-NORTHWEST
ALONG THE NORTHERN SIDE OF THE STR TO THE SOUTH THROUGH THE
FORECAST PERIOD. SLIGHT INCREASES IN TRACK SPEED ARE EXPECTED BY
THE END OF THE FORECAST, AS THE RIDGE MOVES WEST OVER MADAGASCAR,
MOVES CLOSER TO TC 04S AND STRENGTHENS THE STEERING GRADIENT. IN
TERMS OF INTENSITY, THE CYCLES OF CONVECTIVE DEVELOPMENT,
DISSIPATION AND REDEVELOPMENT ARE EXPECTED TO CONTINUE FOR THE NEXT
24 TO 36 HOURS, AND TC 04S WILL GENERALLY MAINTAIN 40 KNOT
INTENSITY OR WEAKEN SLIGHTLY DURING THIS PERIOD. BY TAU 36
HOWEVER, A MUCH LARGER AND MORE ROBUST INFUSION OF DRY AIR IS
FORECAST TO MOVE IN FROM THE NORTH, AND WILL COMPLETELY ENGULF TC
04S. AFTER THIS POINT, THE DRYNESS OF THE ATMOSPHERE WILL FINALLY
BREAK THE CONVECTIVE CYCLE AND THE SYSTEM WILL STEADILY WEAKEN
THROUGH THE END OF THE FORECAST PERIOD. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS VERY GOOD
AGREEMENT, WITH MINIMAL CROSS- OR ALONG-TRACK SPREAD THROUGH THE
FORECAST PERIOD. THE JTWC FORECAST TRACK IS PLACED NEAR THE
CONSENSUS MEAN AND ECMWF TRACKER WITH HIGH CONFIDENCE. INTENSITY
GUIDANCE IN ALSO IN GOOD AGREEMENT, WITH ALL MODELS EXCEPT THE
SHIPS-GFS CONFINED TO A NARROW, FIVE KNOT, ENVELOPE CENTERED ON THE
CONSENSUS MEAN, WHICH SHOWS DISSIPATION TO 30 KNOTS BY TAU 72,
LENDING HIGH CONFIDENCE TO THE JTWC FORECAST INTENSITY. 

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