WDXS31 PGTW 230900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 
015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 10.2S 59.5E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 607 NM NORTH-NORTHEAST OF PORT LOUIS,
MAURITIUS
   MOVEMENT PAST 6 HOURS: WESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIAL
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) DISPLACED TO THE EAST
OF AN AREA OF PERSISTENT DEEP CONVECTION. A 230514Z ASCAT-B PASS
REVEALED A WELL-DEFINED LLCC WITH 35-40 KNOT WINDS SURROUNDING THE
BULK OF THE CENTER, AND MOST EXPANSIVE TO THE SOUTHEASTERN
QUADRANT. ANIMATED WATER VAPOR IMAGERY AND THE CURRENT CIMSS AMV
ANALYSIS INDICATE AN UPPER-LEVEL ANTICYCLONE POSITIONED FAR TO THE
EAST, IMPARTING EASTERLY DEEP-LAYER FLOW AND SHEAR OVER TC 04S, BUT
ALSO PROVIDING STRONG POLEWARD OUTFLOW, ALONG WITH SOME WEAKER
WESTWARD OUTFLOW. 200MB WINDS CONTINUE TO SPLIT UPSTREAM OF TC 04S
PROVIDING FOR A LOCALIZED REDUCTION IN THE SHEAR MAGNITUDE, WHICH
IS ALLOWING THE CONVECTION TO CONTINUALLY CYCLE, BRIEFLY COVER THE
LLCC AND THEN BACK OFF AGAIN, WHICH IS THE PATTERN SEEN OVER THE
LAST SEVERAL DAYS. THE INITIAL POSITION IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE ASCAT PASS AND THE PARTIALLY EXPOSED LLCC
IN THE MSI. THE INITIAL INTENSITY IS ASSESSED WITH HIGH CONFIDENCE,
SET ABOUT 5-10 KNOTS LOWER THAN THE AVERAGE OF THE OBJECTIVE AND
SUBJECTIVE FIX ESTIMATES NOTED BELOW IN LIGHT OF THE
HIGH-RESOLUTION SCATTEROMETER DATA. ENVIRONMENTAL CONDITIONS ARE
MARGINAL, WITH MODERATE TO HIGH EASTERLY SHEAR, MARGINAL SSTS NEAR
26C AND STRONG POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   DEMS: T3.5 - 55 KTS
   FMEE: T3.0 - 45 KTS
   CIMSS SATCON: 49 KTS AT 230500Z
   CIMSS ADT: 55 KTS AT 230600Z
   CIMSS AIDT: 47 KTS AT 230600Z
   CIMSS D-MINT: 47 KTS AT 230228Z
   CIMSS D-PRINT: 40 KTS AT 230600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR INTRUSION FROM THE EAST. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 96
HOURS. 

FORECAST DISCUSSION: TC 04S HAS BEEN TRACKING SLIGHTLY SOUTH OF DUE
WEST OVER THE LAST DAY, ALONG THE NORTHWESTERN PERIPHERY OF A
DEEP-LAYER STR TO THE SOUTHEAST AND TOWARDS A WEAK BREAK IN THE
RIDGE PATTERN IN THE VICINITY OF LA REUNION. THE RIDGE IS EXPECTED
TO BUILD AND MOVE WESTWARD TO A POSITION OVER NORTHERN MADAGASCAR
OVER THE COURSE OF THE FEW DAYS. AFTER TAU 12, TC 04S WILL RESUME A
MORE WEST-NORTHWESTWARD TRACK ALONG THE NORTHERN SIDE OF THE
STEERING, TRACKING NORTH OF MADAGASCAR AND TOWARDS THE EAST COAST
OF AFRICA THROUGH THE REMAINDER OF THE FORECAST. THE SYSTEM HAS
STEADILY WEAKENED OVER THE PAST 24 HOURS UNDER THE INFLUENCE OF
INCREASING SHEAR AND REPEATED INTRUSIONS OF DRY MID-LEVEL AIR.
GOING FORWARD, THE SYSTEM IS FORECAST TO EXPERIENCE REPEATED
EPISODES OF DRY AIR ENTRAINMENT INTERSPERSED WITH BRIEF PERIODS OF
ENHANCED MOISTENING WHICH WILL ENABLE CYCLICAL BURSTS IN CONVECTIVE
ACTIVITY. THIS IN TURN IS DEPICTED AS HIGHLY CYCLICAL VARIATIONS IN
DEEP-LAYER SHEAR IN THE MODEL GUIDANCE, AS THE CONVECTIVE FLARE-UPS
WILL BRIEFLY PUSH BACK AGAINST THE SHEAR FOR A TIME, BEFORE THE
ENTIRE CYCLE REPEATS. IN AGGREGATE, THE SYSTEM WILL MAINTAIN ITS
CURRENT INTENSITY FOR THE NEXT DAY OR SO, THOUGH SHORT-DURATION
EXCURSIONS TO 45 KNOTS ARE POSSIBLE. MORE SIGNIFICANT DRY AIR
INTRUSION AND INCREASES IN SHEAR ARE EXPECTED AFTER TAU 48, WHICH
WILL LEAD TO A MORE GENERALIZED WEAKENING THROUGH THE END OF THE
FORECAST PERIOD. THE REMNANTS OF TC 04S, EVEN THOUGH THEY WILL BE
BELOW THE BASIN WARNING THRESHOLD, WILL CONTINUE TO TRACK TOWARDS
THE COAST OF EAST AFRICA, LIKELY MAKING LANDFALL ON THE COAST OF
TANZANIA OR KENYA AROUND TAU 120. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
OVERALL, WITH MODERATE CROSS-TRACK SPREAD INCREASING UP TO 180NM BY
TAU 96. GFS IS POSITIONED ON THE SOUTHERN EDGE OF THE GUIDANCE
ENVELOPE, WHILE NAVGEM IS ON THE NORTHERN SIDE. THE JTWC FORECAST
IS POSITIONED NEAR THE CONSENSUS MEAN AND THE ECMWF DETERMINISTIC
TRACKER THROUGH THE FORECAST PERIOD WITH HIGH CONFIDENCE. INTENSITY
GUIDANCE IS IN GENERAL AGREEMENT, WITH THE MESOSCALE MODELS
INDICATING A TIGHTLY PACKED ENVELOPE, CONFINED TO 35-40 KNOTS
THROUGH TAU 72, THEN DROPPING OFF TO 25-30 KNOTS BY TAU 96. THE
JTWC FORECAST IS PLACED NEAR THE MEAN OF THE MESOSCALE MODELS
THROUGH THE FORECAST PERIOD WITH HIGH CONFIDENCE. 

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