WDXS31 PGTW 150900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 29S (TWENTYNINE) 
WARNING NR 015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.0S 121.6E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 237 NM NORTH OF BROOME, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A VERY
COMPACT SYSTEM, WITH DEEP CONVECTION PULSING NEAR THE ASSESSED LOW
LEVEL CIRCULATION CENTER (LLCC). EARLIER VISIBLE IMAGERY DEPICTED A
PARTIALLY EXPOSED LLCC, BUT SUBSEQUENT VISIBLE IMAGERY CLOSE TO
LOCAL SUNSET AS WELL AS A 150519Z NOAA-20 ATMS 89GHZ MICROWAVE
IMAGE SUGGESTS THE LLCC HAS TUCKED UP UNDER A VERY SMALL CENTRAL
DENSE OVERCAST (CDO) FEATURE. SO FAR, POCKETS OF FLARING DEEP
CONVECTIVE HAVE REMAINED RELATIVELY DISORGANIZED BUT ARE STARTING
TO SHOW SIGNS OF DEVELOPING INTO A MORE PERSISTENT CDO. THE INITIAL
POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON BOTH THE EIR AND
THE MICROWAVE IMAGE NOTED ABOVE. THE INITIAL INTENSITY IS ASSESSED
WITH MEDIUM CONFIDENCE, ON THE HIGHER END OF THE SUBJECTIVE DVORAK
CURRENT INTENSITY ESTIMATES NOTED ABOVE. THE SYSTEM IS QUITE SMALL
AND THE BULK OF THE OBJECTIVE ESTIMATES ARE LIKELY BIASED A BIT
LOW. ANALYSIS REVEALS A FAVORABLE ENVIRONMENT. THE SYSTEM IS
SITUATED ROUGHLY ALONG THE UPPER-LEVEL RIDGE AXIS, WITH GOOD
WESTWARD AND POLEWARD OUTFLOW, ENHANCED BY A SHORTWAVE TROUGH
POSITIONED JUST EAST OF PORT HEDLAND. SHEAR IS LIGHT AND SSTS ARE
QUITE WARM, AND ALL THAT IS REQUIRED FOR EXPLOSIVE DEVELOPMENT IS A
BIT MORE AXISYMMETRIZATION TO OCCUR.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF AN EXTENSION OF
THE DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE
SOUTHWEST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   APRF: T2.5 - 35 KTS
   CIMSS SATCON: 39 KTS AT 150510Z
   CIMSS ADT: 37 KTS AT 150610Z
   CIMSS AIDT: 32 KTS AT 150610Z
   CIMSS D-PRINT: 41 KTS AT 150610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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: TROPICAL CYCLONE (TC) 29S (TWENTY-NINE) IS
CURRENTLY DRIFTING WESTWARD ALONG THE NORTHERN SIDE OF AN EXTENSION
OF THE STR WHICH EXTENDS FROM SOUTH OF DARWIN TO A CENTER SOUTHWEST
OF COCOS ISLAND. THIS GENERAL WESTWARD MOTION IS EXPECTED TO
CONTINUE THROUGH THE NEXT 24 HOURS, BEFORE THE APPROACH OF A DEEP
TROUGH WILL BEGIN TO ERODE AND ULTIMATELY BREAK DOWN THE RIDGE,
ALLOWING TC 29S TO BOTH SLOW DOWN AND BEGIN A POLEWARD TURN AROUND
TAU 36. IN TERMS OF INTENSITY, TC 29S IS EXPECTED TO ACHIEVE VORTEX
SYMMETRIZATION WITHIN THE NEXT SIX HOURS OR SO, SETTING THE STAGE
FOR EXPLOSIVE DEVELOPMENT. THE FORECAST CALLS FOR RAPID
INTENSIFICATION (RI) TO BEGIN PRIOR TO TAU 12, WITH THE SYSTEM
INTENSIFYING UP TO A PEAK OF AT LEAST 105 KNOTS BY TAU 36 UNDER
VERY FAVORABLE ENVIRONMENTAL CONDITIONS. SOME MODELS INDICATE THE
POTENTIAL FOR A PEAK INTENSITY OF 10-15 KNOTS HIGHER AND THIS IS
CERTAINLY POSSIBLE DUE TO ENVIRONMENT AND THE SMALL, COMPACT NATURE
OF THE SYSTEM, WHICH LENDS ITSELF TO DRAMATIC PERIODS OF RI. 
HOWEVER, THE PARTY WILL BE SHORT-LIVED, AS SHEAR IS EXPECTED TO
RAPIDLY INCREASE IMMEDIATELY AFTER THE SYSTEM REACHES PEAK
INTENSITY, WITH MODEL FIELDS SUGGESTING A 30 KNOT INCREASE IN SHEAR
BETWEEN TAU 36 AND TAU 60. THE SYSTEM WILL QUICKLY BE DECAPITATED
BY THESE HIGH SHEAR VALUES AND AS THE VORTEX SHALLOWS OUT AND
WEAKENS, THE SYSTEM WILL TURN SHARPLY SOUTHEASTWARD, ALONG THE
LEADING EDGE OF A DEEP TROUGH MOVING IN FROM THE WEST. BY THE END
OF THE FORECAST, THE SYSTEM WILL BEGIN TO TURN SOUTHWARD AND
EVENTUALLY SOUTHWESTWARD ALONG THE LOWER-LEVEL FLOW PATTER. THE
EXACT TIMING OF THIS TURN WILL DETERMINE WHETHER IT OCCURS OFFSHORE
OR ONSHORE AND THERE IS CONSIDERABLE UNCERTAINTY IN THIS PHASE OF
THE FORECAST. REGARDLESS OF WHERE IT OCCURS, THE SYSTEM WILL
DISSIPATE AS A TROPICAL CYCLONE BETWEEN TAU 96 AND TAU 120.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
IN THE SHORT-TERM (THROUGH TAU 36) BUT THE GUIDANCE QUICKLY AND
SHARPLY DIVERGES AFTER THIS POINT. THE GFS AND GEFS MEAN BEGIN TO
DIVERGE AWAY FROM THE REMAINDER OF THE CONSENSUS MEMBERS AT TAU 36,
AND CONTINUE TO TRACK THE SYSTEM WEST-SOUTHWESTWARD, WHILE THE EGRR
MODEL TURNS THE SYSTEM SHARPLY EAST AT TAU 36OPENING UP A
CROSS-TRACK SPREAD OF AROUND 120NM BY TAU 48. THE GUIDANCE
CONTINUES TO DIVERGE AFTER TAU 48, WITH THE GFS-GEFS COMBO
DEPICTING A TURN SOUTHEASTWARD BY TAU 72, WHILE THE GALWEM,
ECMWF-AIFS, NAVGEM AND THE ECMWF ENSEMBLE MEAN SHOWING THE CENTER
JUST OFFSHORE OF NORTHWEST AUSTRALIA BETWEEN BROOME AND ADELE
ISLAND. THE NET EFFECT IS A SPREAD OF OVER 350NM AT TAU 72. BY TAU
120, SPREAD INCREASES EVEN MORE AS THE GFS-GEFS COMBO SHOWS AN
S-SHAPED TRACK, NOW TRACKING SOUTHWEST BY THE END OF THE FORECAST,
WHILE THE OTHER MODELS AS DESCRIBED ABOVE CONTINUE TRACKING THE
REMNANT VORTEX FURTHER INLAND OVER NORTHWEST AUSTRALIA. THE GEFS
AND ECEPS ENSEMBLES DEPICT A HIGHLY UNCERTAIN TRACK FORECAST
SCENARIO, WITH A HIGH DEGREE OF SPREAD, THOUGH THE ECEPS APPEARS
MORE CONFIDENT THAN THE GEFS. THE JTWC FORECAST IS CONSISTENT WITH
THE ECWMF AND EC-AIFS TRACK, BUT CONFIDENCE IS LOW THROUGH THE
FORECAST PERIOD. INTENSITY GUIDANCE IS IN MUCH BETTER AGREEMENT,
WITH NEARLY ALL OF THE PACKAGE INDICATING RI AND A PEAK OCCURRING
AT TAU 36. MULTIPLE RI AIDS CONTINUE TO TRIGGER WITH THIS RUN,
PROVIDING INCREASED CONFIDENCE THAT RI IS VERY LIKELY IN THE
OFFING. THE HAFS-A AND HWRF CONTINUE TO SHOW A HIGHER PEAK
INTENSITY OF AT LEAST 115 KNOTS, THROUGH THE JTWC FORECAST IS
HEDGED SLIGHTLY LOWER AS THE REMAINDER OF THE GUIDANCE PEAKS AROUND
90 KNOTS. 

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