WDXS31 PGTW 160900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (BILLY) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.1S 93.6E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 402 NM SOUTH-SOUTHWEST OF THE COCOS
ISLANDS, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A MOSTLY
EXPOSED LOW LEVEL CIRCULATION (LLC) WITH FLARING CONVECTION ON THE
SOUTHERN PERIPHERY OF THE LLC. A 160329Z ASCAT-C IMAGE SHOWS AN
ELONGATED CIRCULATION CENTER WITH 30-35 KNOT WINDS EXCLUSIVELY IN
THE SOUTHERN SEMICIRCLE WITH SMALL POCKETS OF 35-40 KNOT WINDS.
ANIMATED TOTAL PRECIPITABLE WATER IMAGERY AND MODEL OUTPUT INDICATE
EXTENSIVE DRY AIR PRESENT OVER THE WESTERN AND NORTHERN SEMICIRCLES
ENTRAINING INTO THE LLC. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON A 160709Z AMSR2 89GHZ COLOR COMPOSITE MICROWAVE
IMAGE, WHICH SHOWS SHALLOW BANDING WRAPPING INTO A DEFINED CENTER
WITH DISORGANIZED DEEP CONVECTION CONFINED TO THE SOUTH QUADRANT.
TC 20S IS LOCATED UNDER STRONG NORTHWESTERLY FLOW ON THE LEADING
EDGE OF AN UPPER-LEVEL TROUGH. THE INITIAL INTENSITY OF 40 KTS IS
ASSESSED WITH HIGH CONFIDENCE BASED PRIMARILY ON THE ASCAT-C IMAGE,
HEDGED SLIGHTLY BELOW THE PGTW DVORAK CURRENT INTENSITY ESTIMATE.
ADT ESTIMATES REMAIN UNREALISTICALLY HIGH BASED ON AN IRREGULAR
CENTRAL DENSE OVERCAST PATTERN.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP
LAYERED SUBTROPICAL RIDGE (STR)

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   CIMSS ADT: 63 KTS AT 160740Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 25-30 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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 20S (BILLY) IS CURRENTLY TRACKING WESTWARD
TO WEST-SOUTHWESTWARD ON THE NORTHWESTERN PERIPHERY OF A WEAK
EXTENSION OF THE STR AND IS EXPECTED TO TURN SOUTHWESTWARD THROUGH TAU 
24 IN AN UNFAVORABLE ENVIRONMENT. STRONG POLEWARD OUTFLOW, HIGH 
VERTICAL WIND SHEAR (VWS) AND DRY AIR ENTRAINMENT WILL CONTINUE TO 
STEADILY WEAKEN THE SYSTEM. SINCE TC 20S IS BEING DECAPITATED, IT IS 
FORECAST TO TRACK WESTWARD UNDER THE LOW-LEVEL EASTERLY STEERING FLOW. 
BETWEEN TAU 36 AND TAU 48 PERSISTENT HIGH VERTICAL WIND SHEAR COUPLED 
WITH SIGNIFICANT DRY AIR ENTRAINMENT WILL FULLY ERODE CENTRAL 
CONVECTION AND THE OVERALL STRUCTURE RESULTING IN DISSIPATION NEAR TAU 
48.  

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, WHICH DRIVES THE
SYSTEM SOUTHWARD, NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
WITH THE BULK OF THE TRACKERS SUPPORTING THE JTWC FORECAST TRACK
PHILOSOPHY. SEVERAL MODEL TRACKERS (ECMI, UEMI AND EGRI) DISSIPATE
THE SYSTEM AROUND TAU 36 TO TAU 48. RELIABLE INTENSITY GUIDANCE
INDICATES A LOW PROBABILITY FOR SLIGHT REINTENSIFICATION BY TAU 12
AFTER WHICH GRADUAL WEAKENING AND EVENTUAL DISSIPATION WILL OCCUR. 

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