WDXS31 PGTW 190900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.6S 69.8E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 184 NM WEST-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 04S CONTINUES TO STRUGGLE AGAINST PERSISTENT
MODERATE EASTERLY SHEAR AND DRY MID-LEVEL AIR, WHICH IS ENTRENCHED
ALONG THE EASTERN FLANK OF THE CIRCULATION. THE RESULT IS THAT THE
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) CONTINUES TO SHOW A
PARTIALLY TO FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC).
HAFS-A MODEL CROSS-SECTIONS CONFIRM THE HIGHLY TILTED NATURE OF THE
VORTEX AND THE FACT THAT THERE LIKELY IS A MID-LEVEL CENTER OF
ROTATION POSITIONED TO THE WEST OF THE LLCC, WHICH CAN BE FAINTLY
SEEN IN THE MSI. A LACK OF RECENT MICROWAVE IMAGERY OR
SCATTEROMETER DATA MEANS THAT THE INITIAL POSITION IS ASSESSED WITH
ONLY MEDIUM CONFIDENCE, FULLY RELIANT UPON THE PARTIALLY EXPOSED
AND IRREGULAR LLCC SEEN IN THE MSI. THE INITIAL INTENSITY IS
ASSESSED WITH MEDIUM CONFIDENCE, BASED ON A BLEND OF THE AVAILABLE
SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED BELOW.
ENVIRONMENTAL CONDITIONS ARE MARGINAL, WITH MODERATE TO STRONG
EASTERLY SHEAR, MARGINAL SSTS AND MODERATE WESTWARD AND POLEWARD
OUTFLOW ALOFT.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   FMEE: T3.0 - 45 KTS
   CIMSS SATCON: 38 KTS AT 190330Z
   CIMSS ADT: 43 KTS AT 190530Z
   CIMSS AIDT: 34 KTS AT 190530Z
   CIMSS D-MINT: 27 KTS AT 190143Z
   CIMSS D-PRINT: 31 KTS AT 190530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE WESTWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: TC 04S WILL CONTINUE TO TRACK SOUTHWESTWARD
THROUGH TAU 48, UNDER THE STEERING INFLUENCE OF THE STR TO THE
SOUTH. A DEEP MID-LATITUDE TROF IS EXPECTED TO QUICKLY PASS FAR TO
THE SOUTH OVER THE NEXT 24 HOURS, AND INDUCE A WEAKNESS IN THE
STEERING RIDGE, LEADING TO A SIGNIFICANT SLOWDOWN IN TRACK SPEED.
THE TROF PASSES BY VERY QUICKLY, HOWEVER THE RIDGE WILL TAKE TIME
TO REBUILD, RESULTING IN A RELATIVELY WEAK STEERING GRADIENT
THROUGH TAU 48. TRACK SPEEDS PICK UP A BIT AFTER TAU 48 AS THE
TRACK SHIFTS TO A MORE WESTWARD COURSE AS THE RIDGE REORIENTS TO A
MORE EAST-WEST ORIENTATION. ENVIRONMENTAL CONDITIONS ARE EXPECTED
TO REMAIN MARGINAL THROUGH MOST OF THE FORECAST PERIOD, WITH A
BRIEF PERIOD OF SLIGHTLY IMPROVED CONDITIONS EXPECTED WITHIN THE
NEXT 24 HOURS. AS THE RIDGE TO THE SOUTH WEAKENS AND THE SYSTEM
SLOWS, SHEAR WILL DECREASE, THE ATMOSPHERE WILL MOISTEN AND THE
SYSTEM WILL TAP INTO A BRIEF WINDOW OF ENHANCED POLEWARD OUTFLOW,
WHICH WILL ALLOW IT TO CONSOLIDATE AND INTENSIFY. AFTER REACHING A
PEAK INTENSITY OF 45 KNOTS AT TAU 24, INCREASING SHEAR AND DRY AIR
INTRUSION WILL LEAD TO A VERY SLOW AND STEADY WEAKENING TREND
THROUGH THE REMAINDER OF THE FORECAST. 

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, WHICH IS STUBBORNLY
INSISTING ON PUSHING THE SYSTEM INTO THE RIDGE TO THE SOUTH,
DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN FAIRLY GOOD
AGREEMENT THOUGH THE ENVELOPE HAS SHOWN INCREASING DISPERSION OVER
THE PAST COUPLE OF RUNS. THERE ARE THREE GROUPINGS OF MODELS IN THE
GUIDANCE PACKAGE. GROUP ONE CONSISTS OF CONSENSUS, EC-AIFS, GALWEM,
AND EGRR WHICH ARE TIGHTLY PACKED AROUND THE CONSENSUS MEAN. GROUP
TWO CONSISTS OF THE GFS AND GEFS AND IS NORTH OF AND MUCH SLOWER
THAN GROUP ONE. AND THE THIRD GROUP COMPRISES ECMWF AND THE ECENS
MEAN, POSITIONED SOUTH AND FASTER THAN GROUP ONE. CROSS-TRACK
SPREAD OPENS UP TO ABOUT 150NM, AND ALONG-TRACK TO 320NM, BY TAU
120 BETWEEN THE GFS AND THE ECMWF. THE JTWC FORECAST IS POSITIONED
NEAR THE CONSENSUS MEAN AND EC-AIFS THROUGH THE ENTIRE FORECAST
PERIOD. INTENSITY GUIDANCE IS IN GOOD AGREEMENT WITH ONLY 10 KNOTS
OF SPREAD BETWEEN THE MODELS. 

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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