WDXS31 PGTW 181500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (FOUR) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.0S 73.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 68 NM SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) INDICATES THAT TC
04S REMAINS MOSTLY EXPOSED, WITH THE ACTUAL CENTER HIDING OUT UNDER
SOME THIN CIRRUS BLOWOFF FROM REDEVELOPING CONVECTION TO THE
NORTHWEST. THE LOW LEVEL CIRCULATION CENTER (LLCC) REMAINS RATHER
ELONGATED AND DISORGANIZED IN THE MSI, WITH SEVERAL SMALL SPINNERS
OBSERVED SHOOTING OUT TO THE EAST, THEN WRAPPING BACK IN TOWARDS
THE CENTER. SURFACE OBSERVATIONS FROM DIEGO GARCIA WERE MISSING FOR
SEVERAL HOURS BUT THE 181255Z OBSERVATION SHOWS WINDS FROM THE 260
DEGREES AT 21KNOTS GUSTING TO 33 KNOTS AND PRESSURE DOWN TO 1000MB,
WHICH SUGGESTS THE LLCC IS TO THE SOUTH-SOUTHEAST OF THE STATION
AND APPROACHING CPA. THE INITIAL POSITION IS ASSESSED WITH MEDIUM
CONFIDENCE, ONLY DUE TO THE FACT THAT THE ACTUAL LLCC REMAINS
OBSCURED, EXACERBATED BY A LARGE SCATTER OF THE AGENCY FIX
POSITIONS AND THE LACK OF RECENT MICROWAVE IMAGERY OR
SCATTEROMETERY DATA. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON A BLEND OF THE SUBJECTIVE AND OBJECTIVE
INTENSITY ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS ARE
MARGINALLY FAVORABLE, THOUGH SHEAR REMAINS A BIT TOO HIGH
SIGNIFICANT NEAR-TERM DEVELOPMENT, COMING IN AT 20-25 KNOTS FROM
THE EAST. SSTS REMAIN WARM AND OUTFLOW REMAINS STRONG TOWARDS THE
WEST.

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: T2.5 - 35 KTS
   DEMS: T2.0 - 30 KTS
   FMEE: T2.5 - 35 KTS
   CIMSS ADT: 37 KTS AT 181200Z
   CIMSS AIDT: 33 KTS AT 181200Z
   CIMSS D-MINT: 24 KTS AT 180852Z
   CIMSS D-PRINT: 26 KTS AT 181200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG WESTWARD
   OTHER FACTORS: DRY MID-LEVEL AIR TO THE EAST. 

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 IS NOW APPROACHING ITS CLOSEST POINT OF
APPROACH TO DIEGO GARCIA AND IS FORECAST TO PASS ABOUT 50NM TO THE
SOUTH WITHIN THE NEXT FEW HOURS. THERE ARE NO SIGNIFICANT CHANGES
IN THE FORECAST PHILOSOPHY, AND TC 04S IS FORECAST TO CONTINUE
TRACKING SOUTHWESTWARD ALONG THE NORTHERN SIDE OF THE STR THROUGH
ENTIRE FORECAST PERIOD. THE PASSAGE OF A MID-LATITUDE TROF FAR TO
THE SOUTH, IS EXPECTED TO RESULT IN A WEAKENING OF THE RIDGE
PATTERN AROUND TAU 24, WHICH WILL RESULT IN A NOTABLE SLOWING OF
FORWARD SPEED STARTING AT TAU 24 AND CONTINUING THROUGH TAU 48. THE
TROF WILL PASS BY QUICKLY, AND BY TAU 48, TC 04S WILL BEGIN TO
ACCELERATE TO THE SOUTHWEST AS THE RIDGE STRENGTHENS AND MOVES
WESTWARD. IN TERMS OF INTENSITY, THE SYSTEM CONTINUES TO STRUGGLE
AGAINST MODERATE EASTERLY SHEAR, AND THE PRESENCE OF DRY MID-LEVEL
AIR, WHICH THE HAFS-A HAS FINALLY PICKED UP ON. THE FORECAST MODELS
SHOW A CYCLICAL PATTERN OF DEEP CONVECTION FORMING UP NEAR THE
CENTER, THEN DISSIPATING, AND REPEAT. WITH EACH CYCLE OF CONVECTIVE
DEVELOPMENT, THE MESOSCALE MODELS SHOW THE LLCC MAKING A RUN AT
VERTICAL ALIGNMENT, ONLY FOR IT TO BE DECOUPLED BY PERSISTENT SHEAR
AND DRY AIR, AND THE CYCLICAL NATURE OF THE DEVELOPMENT WILL KEEP
THE SYSTEM FROM REACHING A HIGH INTENSITY. THE BEST WINDOW FOR
INTENSIFICATION IS OVER THE NEXT 36 HOURS, WHEN THE WEAKNESS IN THE
RIDGE TO THE SOUTH WILL OPEN UP A BRIEF WINDOW FOR SOME ENHANCED
POLEWARD OUTFLOW AND A PREDICTED MOISTENING OF THE ATMOSPHERE.
AFTER TAU 36 HOWEVER, SHEAR IS EXPECTED TO INCREASE ONCE MORE, DRY
AIR WILL ONCE AGAIN MOVE IN AND THE UPPER-LEVEL OUTFLOW WILL BE
MORE RESTRICTED, AND THE SYSTEM WILL FALL BACK TO THE CYCLICAL
PHASE NOTED ABOVE. IT WILL LIKELY MAINTAIN INTENSITY OR WEAKEN VERY
SLOWLY, THROUGH TAU 96, BEFORE STARTING A MORE RAPID DECAY BY THE
END OF THE FORECAST. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD
AGREEMENT, WITH THE EXCEPTION OF NAVGEM, WHICH CONTINUES TO PUSH
THE SYSTEM POLEWARD INTO THE TROF FAR TO THE SOUTH, WHICH IS
UNREALISTIC. THE REMAINDER OF THE GUIDANCE IS CONFINED TO AN
ENVELOPE WHICH GRADUALLY WIDENS TO 120NM AT TAU 120. THE ECMWF AND
EC-AIFS MARK THE NORTHERN EDGE OF THE ENVELOPE, WHILE THE GEFS AND
THE CONSENSUS MEAN (SKEWED BY THE NAVGEM), MARK THE SOUTHERN EDGE.
GFS AND GEFS ARE NOTABLY SLOW AFTER TAU 72, LAGGING OVER 250NM
BEHIND THE ECMWF BY TAU 120. THE JTWC FORECAST IS POSITIONED NORTH
OF THE CONSENSUS MEAN TO ACCOUNT FOR THE INFLUENCE OF NAVGEM.
INTENSITY GUIDANCE IS IN MODERATE AGREEMENT, WITH THE BULK OF THE
GUIDANCE INDICATING A PEAK INTENSITY AROUND TAU 36 OF BETWEEN 35
AND 50 KNOTS FOLLOWED BY A RELATIVELY FLAT TREND OR VERY SLOW
WEAKENING THROUGH THE REMAINDER OF THE FORECAST. THE JTWC FORECAST
IS BASED ON A BLEND BETWEEN THE HAFS-A AND THE HWRF THROUGH THE
ENTIRE FORECAST PERIOD. 

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