WDXS31 PGTW 210900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 05S (DARIAN) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.9S 90.7E
   INITIAL INTENSITY: 130 KTS
   GEOGRAPHIC REFERENCE: 1148 NM EAST-SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 47 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) REVEALS THAT
TROPICAL CYCLONE DARIAN REMAINS A HIGHLY SYMMETRIC QUASI-ANNULAR
SYSTEM WITH A 20NM EYE AND TIGHT BANDING FEATURES. IT IS EVIDENT
FROM THE ANIMATED VISIBLE IMAGERY LOOP THAT THE SYSTEM IS
EXPERIENCING SLIGHT NORTHEASTERLY SHEAR, AS THE LOW LEVEL
CIRCULATION CENTER (LLCC) CAN BE SEEN IN NORTHEASTERN CORNER OF THE
LARGER UPPER LEVEL CIRCULATION CENTER (ULCC). WITHOUT THE LUXURY OF 
CURRENT MICROWAVE IMAGERY, IT IS
IMPOSSIBLE TO DISCERN THE PRESENCE OF A POSSIBLE EYE WALL
REPLACEMENT CYCLE (ERC) AT THIS TIME. A SLIGHTLY OLDER 202321Z
SSMIS 89GHZ MICROWAVE IMAGE REVEALED A ROBUST INNER EYE WALL WITH A
POSSIBLE SECONDARY EYE WALL BEGINNING TO FORM. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON MSI. THE INITIAL INTENSITY
OF 130 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON AGENCY DVORAK
INTENSITY ESTIMATES AS WELL AS CIMSS AUTOMATED INTENSITY ESTIMATES
RANGING FROM 127-140KTS.

INITIAL WIND RADII BASIS: PARTIAL 210334Z ASCAT-C SCATTEROMETER
DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T7.0 - 140 KTS 
   KNES: T6.5 - 127 KTS
   APRF: T7.0 - 140 KTS 
   CIMSS ADT: 132 KTS AT 210540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE RADIAL
   OTHER FACTORS: UPWELLING OF COOLER WATER

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 05S DARIAN HAS RECENTLY STEADIED UP ON A
GENERALLY WESTWARD TRACK UNDER THE STEERING INFLUENCE OF THE STR TO
THE SOUTH. THE SYSTEM IS STILL IN A FAVORABLE ENVIRONMENT,
CHARACTERIZED BY LOW VERTICAL WIND SHEAR, WARM SEA SURFACE
TEMPERATURES, AND MODERATE RADIAL OUTFLOW. AS A RESULT OF RECENT
SLOWER TRACK SPEEDS, WHICH ALLOWED FOR UPWELLING OF COOLER WATERS
AND BY THE SYSTEMS RECENT MIGRATION OVER RELATIVELY COOLER SSTS,
05S WILL GRADUALLY WEAKEN OVER THE NEXT 24 HOURS. AFTER TAU 24, AS
THE SYSTEM REGAINS CONSISTENT FORWARD PROGRESS AND TRACKS FURTHER
NORTH OF THE 14TH PARALLEL. THE SYSTEM REENTERS RELATIVELY WARMERS
WATERS, ALLOWING FOR A GENERALLY CONSISTENT INTENSITY THROUGH TAU
72. BY THIS TIME, THE STR TO THE SOUTH IS FORECAST TO MOVE OFF TO
THE SOUTHWEST, LEAVING THE SYSTEM UNDER THE INFLUENCE OF A
NEAR-EQUATORIAL RIDGE TO THE NORTH, WHICH WILL DRIVE THE SYSTEM
RATHER AGGRESSIVELY POLEWARD. THIS POLEWARD SHIFT WILL NOT ONLY
DECREASE TRACK SPEEDS WHICH WILL ALLOW FOR INCREASED UPWELLING, BUT
IT WILL ALSO PLACE THE SYSTEM IN INCREASINGLY COOLER SSTS ALL OF
WHICH WILL DECREASE INTENSITY. AS A RESULT, BY TAU 120 THE SYSTEM
WILL BE TRACKING GENERALLY SOUTHWARD AND OVERALL INTENSITY WILL BE
REDUCED TO AROUND 70KTS.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72 THAT 05S WILL TRACK GENERALLY WESTWARD UNTIL A
DRAMATIC POLEWARD SHIFT. AFTER TAU 72 AND THROUGH TAU 120,
SOLUTIONS SPREAD NEARLY 450NM FROM SOUTHEASTWARD TO SOUTHWESTWARD,
FOR THIS REASON THE INITIAL JTWC FORECAST TRACK IS PLACED WITH HIGH
CONFIDENCE, WHILE THE EXTENDED FORECAST TRACK IS PLACED WITH MEDIUM
CONFIDENCE. RELIABLE MODEL INTENSITY GUIDANCE IS IN GOOD AGREEMENT
THAT 05S WILL GRADUALLY WEAKEN THROUGH TAU 36 AFTER WHICH
INTENSITIES STAGNATE UNTIL TAU 72 WHEN THE POLEWARD SHIFT OCCURS
AND WEAKENING RESUMES ONCE AGAIN. COAMPS-TC AND HWRF REMAIN HIGHER
THAN CONSENSUS THROUGHOUT THE FORECAST WHILE DECAY-SHIPS REMAINS
LOWER, WITH THE JTWC FORECAST INTENSITY TAKING THE AVERAGE OF THE
TWO GROUPS. FOR THIS REASON THE JTWC FORECAST INTENSITY IS PLACED
WITH MEDIUM CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
NNNN