WDXS31 PGTW 150900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (BILLY) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.0S 97.2E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 291 NM SOUTH OF THE COCOS ISLANDS,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION (LLC) WITH PERSISTENT CORE CONVECTION
OBSCURING THE CENTER. A 150252Z 89GHZ AMSU-B MICROWAVE IMAGE SHOWS
FRAGMENTED DEEP CONVECTIVE BANDING LIMITED TO THE SOUTHERN
SEMICIRCLE, DEFINED SHALLOW BANDING IN ALL QUADRANTS. 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 MSI AND MICROWAVE IMAGERY. THE INITIAL
INTENSITY OF 55 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
PGTW DVORAK CURRENT INTENSITY ESTIMATE AND THE CIMSS ADT ESTIMATE.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   APRF: T2.5 - 35 KTS
   CIMSS ADT: 55 KTS AT 150540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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 20S (BILLY) IS CURRENTLY TRACKING WESTWARD
ON THE NORTHWESTERN PERIPHERY OF A WEAK EXTENSION OF THE STR AND IS
EXPECTED TO TRACK GENERALLY WEST-SOUTHWESTWARD THROUGH TAU 24.
PERSISTENT MODERATE TO HIGH VERTICAL WIND SHEAR COUPLED WITH
INCREASING DRY AIR ENTRAINMENT, OFFSET SOMEWHAT BY MODERATE
OUTFLOW, WILL STEADILY ERODE CENTRAL CONVECTION AND THE OVERALL
STRUCTURE RESULTING IN STEADY WEAKENING. AFTER TAU 24, A BROAD
MIDLATITUDE TROUGH WILL BREAK DOWN AND REORIENT THE STR PRODUCING A
SOUTHWESTWARD TRACK THROUGH TAU 48. THE SYSTEM WILL CONTINUE TO
WEAKEN DUE TO HIGH VERTICAL WIND SHEAR AND DRY AIR ENTRAINMENT.
AFTER TAU 48, TC 20S WILL TURN WESTWARD UNDER THE LOW-LEVEL STR,
WHICH IS EXPECTED TO BUILD IN TO THE SOUTH WITH DISSIPATION NEAR
TAU 72.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS BIFURCATED WITH TWO
DISTINCT MODEL GROUPINGS. NVGI AND ECMI SHOW A SHARPER SOUTHWARD
RECURVE INDUCED BY THE AFOREMENTIONED MIDLATITUDE TROUGH'S PASSAGE 
WHILE THE REMAINDER OF THE GUIDANCE SUPPORTS THE JTWC FORECAST TRACK. 
RELIABLE INTENSITY GUIDANCE SHOWS A GENERAL CONSENSUS WITH ALL MEMBERS 
INDICATING VARIOUS DEGREES OF WEAKENING THROUGH THE FORECAST PERIOD 
RESULTING IN MEDIUM CONFIDENCE.

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