WDXS31 PGTW 152100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (BILLY) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.6S 95.3E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 336 NM SOUTH-SOUTHWEST OF THE COCOS
ISLANDS, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
SYMMETRICAL BALL OF CONVECTION OBSCURING THE LOW LEVEL CIRCULATION
CENTER (LLCC), THE OUTER BANDS OF WHICH ARE EVIDENT TO THE
NORTHWEST SIDE OF THE CONVECTIVE MASS. A PARTIAL 151455Z GPM 89GHZ
IMAGE SHOWED MODERATE CONVECTION ACROSS THE SOUTHEASTERN QUADRANT
OF THE CIRCULATION AND SHOWED INDICATIONS OF A WEAK MICROWAVE
EYE-LIKE FEATURE, PROVIDING MEDIUM SUPPORT TO THE INITIAL POSITION.
THE INITIAL INTENSITY OF 55 KNOTS IS ASSESSED WITH HIGH CONFIDENCE,
HEDGED SLIGHTLY HIGHER THAN THE PGTW AND APRF DVORAK CURRENT
INTENSITY ESTIMATES IN FAVOR OF THE ADT AND SATCON ESTIMATES AND
EXTRAPOLATION OF A 151139Z SMAP PASS WHICH INDICATED MAXIMUM WINDS
OF 56 KNOTS. ANALYSIS REVEALS A SOMEWHAT MARGINAL ENVIRONMENT WITH
ROBUST POLEWARD OUTFLOW INTO THE BASE OF AN UPPER-LEVEL TROUGH AND
WARM SSTS BEING OFFSET BY MODERATE TO HIGH WESTERLY VWS AND
SIGNIFICANT DRY AIR TO THE WEST AND NORTH.

INITIAL WIND RADII BASIS: PERSISTENCE WITH 151139Z SMAP DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   ABRF: T3.0 - 45 KTS
   CIMSS SATCON: 53 KTS AT 151524Z
   CIMSS ADT: 59 KTS AT 151740Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 25-30 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 HAS OVER THE PAST 12 HOURS SHIFTED TO A
MORE WEST-SOUTHWESTWARD TRACK, ALONG THE NORTHWESTERN PERIPHERY OF
A DEEP STR CENTERED TO THE SOUTHEAST. OVER THE NEXT 36 HOURS, A
STRONG MID-LATITUDE TROUGH WILL MOVE IN FROM THE WEST, ERODE THE
RIDGE AND INDUCE A WEAKNESS, ALLOWING FOR TC 20S TO TURN ONTO A
MORE SOUTHWESTWARD TRACK. SO FAR, THE STRONG POLEWARD OUTFLOW INTO
THE BASE OF THE UPPER-LEVEL TROUGH TO THE SOUTH,  IS OFFSETTING THE
MODERATE TO HIGH VWS AND TC 20S HAS BEEN ABLE TO MAINTAIN A STEADY
INTENSITY OF 55 KNOTS. HOWEVER, THE CONTINUED ENCROACHMENT OF
MID-LEVEL DRY AIR FROM THE WEST AND NORTH, COMBINED WITH THE
PERSISTENT SHEAR, WILL BEGIN TO IMPACT THE SYSTEM IN THE NEAR-TERM.
ONCE THE SHEAR PUSHES THE CONVECTION OFF THE CENTER, THE DRY AIR
WILL RAPIDLY ENVELOPE THE CORE, WITH RAPID WEAKENING WILL QUICKLY
ENSUE. AS THE SYSTEM WEAKENS BELOW 40 KNOTS, ROUGHLY BY TAU 48, THE
STEERING LEVEL WILL LOWER DOWN TO THE 850-700MB LAYER, AND A STRONG
LOW TO MID-LEVEL RIDGE WILL ASSUME STEERING, PUSHING TC 20S ONTO A
WESTWARD TRACK THROUGH TAU 72. THE SYSTEM IS FORECAST TO DISSIPATE
AS A TROPICAL CYCLONE NO LATE THAN TAU 72 AS IT BECOMES ENVELOPED
IN DRY AIR AND CONVERGENT UPPER-LEVEL FLOW.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE SHOWS TWO DISTINCT
SCENARIOS WITH NAVGEM AND THE ECMWF MODELS SUGGESTING A SHARP TURN
POLEWARD IN THE NEAR-TERM FOLLOWED BY A RECURVE TO THE SOUTHEAST,
INTO THE APPROACHING TROUGH. MEANWHILE THE REMAINDER OF THE
CONSENSUS MEMBERS SUPPORT THE JTWC TRACK INDICATING A SOUTHWEST
THEN WESTWARD TRACK ON THE NORTH SIDE OF THE LOW LEVEL STEERING
RIDGE. ENSEMBLE TRACK GUIDANCE (ECMWF AND GFS) SHOWS A SIMILAR
PATTERN, WITH THE GFS MEMBERS GENERAL AGREEMENT WITH THE
DETERMINISTIC MODEL, WHILE THE ECMWF DEPICTS A BIFURCATION IN THE
ENSEMBLE MEMBERS, WITH ROUGHLY HALF TRACKING SOUTH AND HALF TURNING
WEST. THE JTWC FORECAST IS PLACED ON THE NORTHERN SIDE OF THE
GUIDANCE ENVELOPE WITH MEDIUM CONFIDENCE. THE BULK OF THE INTENSITY
GUIDANCE INDICATES STEADY WEAKENING, WITH THE EXCEPTION OF
COAMPS-TC, WHICH INDICATES NEAR-TERM INTENSIFICATION TO 65 KNOTS
PRIOR TO SHARP WEAKENING. THE JTWC FORECAST TRACKS THE CONSENSUS
MEAN WITH HIGH CONFIDENCE. 

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