WDXS31 PGTW 090900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 03S (THREE) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.1S 69.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 225 NM WEST-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN ELONGATED
AND ASYMMETRIC LOW-LEVEL CIRCULATION CENTER (LLCC) WITH FLARING
CONVECTION THAT IS SHEARED TO THE WEST. A 090430Z METOP-C ASCAT
PASS REVEALED PRIMARILY 25-30 KT WINDS WITH A PATCH OF 35 KT WINDS
CONFINED TO THE SOUTHWESTERN QUADRANT. THE ENVIRONMENT IN ASSESSED
TO BE MARGINALLY FAVORABLE CHARACTERIZED BY MODERATE (15-20 KTS)
VERTICAL WIND SHEAR (VWS) AND DRY AIR TO THE NORTH OF THE SYSTEM,
OFFSET BY WARM (27-28 C) SEA SURFACE TEMPERATURES (SST) AND STRONG
WESTWARD OUTFLOW. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON THE AFOREMENTIONED 090430Z METOP-C ASCAT DATA
AND ANIMATED MSI. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH
HIGH CONFIDENCE BASED ON THE SAME ASCAT DATA AND PGTW DVORAK
INTENSITY ESTIMATE OF T2.5.

INITIAL WIND RADII BASIS: 090430Z METOP-C ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: THE SUBTROPICAL RIDGE (STR) TO THE
SOUTH OF THE SYSTEM

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   CIMSS D-PRINT: 33 KTS AT 090600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG WESTWARD
   OTHER FACTORS: DRY AIR TO THE NORTH

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 03S IS FORECAST TO TRACK
WEST-SOUTHWESTWARD ALONG THE NORTHERN PERIPHERY OF THE STR THROUGH
TAU 24. NEAR TAU 24, THE STR BRIEFLY WEAKENS DUE TO AN INCOMING
UPPER-LEVEL TROUGH, CAUSING TRACK SPEEDS TO SLOW. THE STR REBUILDS
BACK IN TO THE SOUTHWEST OF THE VORTEX NEAR TAU 36-48. AS THIS
OCCURS, 03S WILL TRACK MORE NORTHWESTWARD ALONG THE NORTHEASTERN
PERIPHERY OF THE BUILDING STR. THE TRAJECTORY WILL THEN REMAIN
GENERALLY WESTWARD THROUGHOUT THE REMAINDER OF THE FORECAST PERIOD.
REGARDING INTENSITY, 03S IS EXPECTED TO NEARLY MAINTAIN AN
INTENSITY OF AROUND 35 KTS THROUGH TAU 96, WITH SLIGHT
STRENGTHENING TO 40 KTS POSSIBLE BETWEEN TAU 36-48. INTENSITY WILL
BE HINDERED BY THE PERSISTENT EASTERLY VWS AND ASYMMETRICAL WIND
FIELD. THE STRONGEST WINDS WILL REMAIN WITHIN THE SOUTHERN
SEMI-CIRCLE THROUGHOUT THE FORECAST. NEAR TAU 96, VWS IS EXPECTED
TO INCREASE TO 30+ KTS, AIDING IN DISSIPATION AT APPROXIMATELY TAU
120. ONCE THE SYSTEM CROSSES THE 60E LONGITUDE, SSTS ARE EXPECTED
TO DROP BELOW 26C, FURTHER AIDING IN WEAKENING 03S. 

MODEL DISCUSSION: DETERMINISTIC MODELS ARE IN GOOD AGREEMENT
THROUGH TAU 48, WITH AN 85NM CROSS-TRACK SPREAD AT THAT TIME. AFTER
TAU 48, MODELS BEGIN TO DIVERGE, LEADING TO UNCERTAINTY IN THE
LATTER HALF OF THE JTWC TRACK FORECAST. GALWEM AND UK MET ARE
OUTLIERS, STALLING THE SYSTEM NEAR TAU 72, WHILE THE REMAINDER OF
GUIDANCE CONTINUES ON A WESTWARD TRACK. THE JTWC TRACK FORECAST IN
PLACED MORE IN LINE WITH THE ECMWF AND GFS ENSEMBLE MEANS WITH
MEDIUM CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER.
INTENSITY GUIDANCE IS IN MODERATE AGREEMENT, WITH ALL MODELS
KEEPING THE SYSTEM BETWEEN 30-40 KTS THROUGH TAU 48. THE GFS THEN
WEAKENS THE SYSTEM MORE QUICKLY THAN THE REMAINDER OF GUIDANCE,
SUGGESTING 25 KTS AT TAU 72. ON THE OTHER HAND, HAFS-A AND HWRF
MAINTAIN 35 KTS THROUGH TAU 96. THE JTWC INTENSITY FORECAST REMAINS
GENERALLY STABLE AT 35 KTS, ALIGNING WITH THE LATTER MODEL GUIDANCE
WITH OVERALL MEDIUM CONFIDENCE.

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