WDXS31 PGTW 180900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (FOUR) WARNING NR 
002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 7.9S 74.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 128 NM EAST-SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A FULLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC), DISPLACED TO THE EAST
OF AN AREA OF LINEAR CONVECTION IN THE VICINITY OF DIEGO GARCIA. A
180335Z ASCAT-B PASS REVEALED A BEAN-SHAPED LLCC, WITH A TROF
EXTENDING TO THE WEST OVER DIEGO GARCIA. WINDS WERE RELATIVELY
LIGHT IN THE NORTHEASTERN SECTOR, WHILE THE STRONGEST WINDS,
APPROACHING 35 KNOTS, WERE LOCATED IN THE SOUTHERN HEMISPHERE AND
UNDER THE CONVECTION TO THE WEST. ELSEWHERE, WINDS REMAINED AROUND
30 KNOTS. AN AMSU-B 89GHZ MICROWAVE IMAGE COINCIDENT WITH THE
SCATTEROMETER PASS REVEALED SHALLOW, BUT WELL-DEFINED, BANDING
FEATURES WRAPPING INTO AN ELONGATED LLCC. THE EXPOSED NATURE OF THE
LLCC SUGGESTS THE PRESENCE OF MORE SIGNIFICANT DRY AIR AND EASTERLY
SHEAR THAN THE MODELS ARE PICKING UP. EARLIER POSITIONS HAVE BEEN
REANALYZED BASED ON THE SCATTEROMETER DATA ABOVE, RESULTING IN A
SIGNIFICANT REPOSITIONING OF THE 180000Z POSITION, APPROXIMATELY
50NM TO THE NORTHEAST OF THE ORIGINAL POSITION AND A RESULTANT
SLOWDOWN IN THE TRACK SPEED. THE INITIAL POSITION IS ASSESSED WITH
HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN THE MSI. THE INITIAL
INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
SCATTEROMETER DATA DISCUSSED ABOVE, AND THE SUBJECTIVE AND
OBJECTIVE FIX ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS ARE
MARGINALLY FAVORABLE, WITH WARM SSTS, GOOD WESTWARD OUTFLOW ALOFT
AND MODERATE EASTERLY SHEAR. HOWEVER, THE EVIDENT DRY MID-LEVEL AIR
AND POTENTIALLY HIGHER MID-LEVEL SHEAR ARE CURRENTLY INHIBITING
DEVELOPMENT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   CIMSS ADT: 39 KTS AT 180600Z
   CIMSS AIDT: 32 KTS AT 180600Z
   CIMSS D-MINT: 29 KTS AT 180128Z
   CIMSS D-PRINT: 32 KTS AT 180600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE WESTWARD
   OTHER FACTORS: DRY MID-LEVEL AIRMASS TO THE EAST.

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: WHILE TC 04S HAS SLOWED DOWN SIGNIFICANTLY
OVER THE PAST FEW HOURS, THERE ARE NO MAJOR CHANGES IN THE OVERALL
TRACK PHILOSOPHY. THE SYSTEM WILL TRACK SOUTHEASTWARD THROUGH THE
FORECAST PERIOD, ALONG THE NORTH TO NORTHWESTERN SIDE OF THE STRONG
STR TO THE SOUTH. HOWEVER, VARIATIONS IN TRACK SPEED ARE EXPECTED
TO CONTINUE, PARTICULARLY BETWEEN TAU 48 AND TAU 72, AS THE RIDGE
TO THE SOUTH IS WEAKENED DUE TO A PASSING MID-LATITUDE TROF. WITH
THE EXCEPTION OF THE NAVGEM MODEL, NONE OF THE REMAINDER OF THE
FORECAST MODELS SHOW MUCH IN THE WAY OF INTERACTION WITH THIS TROF,
OTHER THAN THE SLOWDOWN IN TRACK SPEED. THE TROF PASSES QUICKLY,
AND THE RIDGE WILL QUICKLY RESTRENGTHEN THEREAFTER, RESULTING IN
ANOTHER ACCELERATION IN TRACK SPEED AFTER TAU 72. IN TERMS OF
INTENSITY, THE SYSTEM WILL STRUGGLE THROUGH THE FORECAST PERIOD
WITH DRY AIR INTRUSIONS AND PERIODS OF ELEVATED EASTERLY SHEAR,
INTERSPERSED WITH PERIODS OF MORE FAVORABLE CONDITIONS. PEAK
INTENSITY OF 45 KNOTS IS EXPECTED BETWEEN TAU 48 AND 72 AS THE
SYSTEM IS ABLE TO TAP INTO SLIGHTLY IMPROVED OUTFLOW CONDITIONS
INTO THE BASE OF THE TROF PASSING TO THE SOUTH. AFTER THIS POINT
HOWEVER, MORE SIGNIFICANT SHEAR AND DRY AIR ARE EXPECTED TO QUICKLY
MOVE IN AND STEADILY WEAKEN THE SYSTEM THROUGH THE REMAINDER OF THE
FORECAST PERIOD.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
IN THE CROSS-TRACK DIMENSION, WITH THE EXCEPTION OF THE NAVGEM
TRACKER, WHICH PULLS THE SYSTEM INTO THE TROF PASSING FAR TO THE
SOUTH. THE REMAINDER OF THE GUIDANCE PACKAGE IS CONFINED TO A
GRADUALLY EXPANDING ENVELOPE WHICH REACHES 110NM AT TAU 120. THE
EC-AIFS MODEL MARKS THE NORTHERN EDGE OF THE ENVELOPE, WHILE THE
ECMWF AND ECENS MEAN MARK THE SOUTHERN EDGE. THE MODELS SHOW
SIGNIFICANTLY MORE ALONG-TRACK SPREAD HOWEVER, WITH THE DIFFERENCE
OPENING UP TO 280NM BY THE END OF THE FORECAST, BETWEEN THE GALWEM
AND ECENS MEAN OUT FRONT, AND THE GEFS MEAN BRINGING UP THE REAR.
THE JTWC FORECAST IS POSITIONED A BIT NORTH AND A BIT SLOWER THAN
THE CONSENSUS MEAN THROUGH THE FORECAST PERIOD, WITH MEDIUM
CONFIDENCE. INTENSITY GUIDANCE IS IN GOOD AGREEMENT, WITH ALL
AVAILABLE GUIDANCE INDICATING A PEAK BETWEEN 30-50 KNOTS, WITH
SHIPS-GFS ON THE LOW END AND COAMPS-TC ON THE HIGH END. THE JTWC
FORECAST IS PLACED CLOSE TO THE HAFS-A SOLUTION WITH MEDIUM
CONFIDENCE. 

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