WDXS32 PGTW 112100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.1S 104.4E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 826 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: EASTWARD AT 18 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 18S WITH A BROAD AND PARTIALLY EXPOSED LOWER-LEVEL
CIRCULATION CENTER (LLCC) WITH FLARING DEEP-LAYER CONVECTION
LIMITED TO THE NORTHWEST SECTOR DUE TO HIGH EASTERLY SHEAR. MOIST
AIR FROM THE NORTHWEST ENTRAINS INTO THE SYSTEM AS INDICATED BY
CLOUD BANDS WRAPPING INTO THE CENTER OF THE CYCLONE. THE
ENVIRONMENT IS MARGINALLY FAVORABLE WITH MODERATE VERTICAL WIND
SHEAR FROM THE EAST AT 20-25KTS, WARM SEA SURFACE TEMPERATURE (SST)
OF 29-30C, AND MODERATE WESTWARD OUTFLOW IN THE EASTERN SEMI-CIRCLE
OF THE SYSTEM. THE INITIAL POSITION WAS BASED ON GOES-IO EIR AND
SHORTWAVE INFRARED (SWIR) REVEALING A PARTIALLY EXPOSED AND BROAD
LLCC. THE INITIAL INTENSITY WAS DETERMINED BASED ON AGENCY AND
OBJECTIVE FIXES LISTED BELOW. THE PRIMARY STEERING INFLUENCE AT THE
MOMENT IS THE STRONG NEAR EQUATORIAL RIDGE (NER) CENTERED OVER
BORNEO, AND THE ACCOMPANYING DEEP WESTERLIES ALONG THE INDONESIAN
ISLANDS. HOWEVER, THERE IS A RELATIVELY DEEP TROUGH IN PLACE TO THE
SOUTH, WHICH AS OF YET IS NOT INFLUENCING THE TRACK BUT REMAINS A
POTENTIAL INFLUENCER DOWN THE ROAD.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: ON THE SOUTHERN PERIPHERY OF A NEAR
EQUATORIAL RIDGE (NER) TO THE NORTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T1.5 - 25 KTS
   APRF: T2.0 - 30 KTS
   CIMSS ADT: 35 KTS AT 111730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: LOW
   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 18S IS FORECAST TO TRACK EAST-SOUTHEAST
OVER THE NEXT 72 HOURS WHILE UNDER THE STEERING INFLUENCE OF A NER
TO THE NORTH. FROM TAU 72 ONWARDS, THE TRACK BECOMES HIGHLY
UNCERTAIN DUE TO SIGNIFICANT DIFFERENCES IN THE EVOLUTION OF A
SUBTROPICAL RIDGE (STR) ANTICIPATED TO DEVELOP OVER CENTRAL
AUSTRALIA. AS THE RIDGE DEVELOPS OVER THE CONTINENT, PRIMARILY TO
THE SOUTH OF TC 18S, IT WILL TEND TO BLOCK THE SYSTEM FROM MOVING
FURTHER EAST, AND TEND TO SLOW IT DOWN AS IT APPROACHES THE
NORTHWEST COAST OF AUSTRALIA. A SLOW CURVE TOWARDS THE SOUTH IS
EXPECTED BY THE END OF THE FORECAST PERIOD AS THE SYSTEM MOVES
TOWARDS A BREAK IN THE STR. TC 18S IS FORECAST TO INTENSIFY SLOWLY
BUT STEADILY OVER THE FORECAST PERIOD AS IT STRUGGLES IN THE
NEAR-TERM AGAINST SIGNIFICANT EASTERLY SHEAR (20-25KTS). A FASTER
RATE OF INTENSIFICATION IS EXPECTED BY THE END OF THE FORECAST AS
SHEAR IS FORECAST TO WEAKEN, THOUGH THE RESTRAINED OUTFLOW
ENVIRONMENT SHOULD KEEP A LID ON THE INTENSIFICATION. OTHERWISE,
CONDITIONS ARE ANTICIPATED TO BE FAVORABLE AS THE ENVIRONMENT
REMAINS MOIST AND SEA SURFACE TEMPERATURES ARE EXPECTED TO REMAIN
BETWEEN 29-30C.

MODEL DISCUSSION: JTWC CONSENSUS TRACK MODEL GUIDANCE IS IN GOOD
AGREEMENT IN THE NEAR-TERM, THAT THE STORM WILL TRACK TO THE
WEST-SOUTHWEST OVER THE NEXT 36 HOURS, HOWEVER CROSS-TRACK SPREAD
BEGINS TO OPEN EXPONENTIALLY BEYOND TAU 36. BY TAU 120, THE
CROSS-TRACK SPREAD OPENS TO 1050NM, WITH GALWEM MARKING THE
SOUTHERN EXTENT AND GFS MARKING THE NORTH. CONSENSUS GUIDANCE TENDS
TO PREFER A MORE SOUTHERLY AND SLOWER ROUTE THAN GFS. LARGE
DIFFERENCES IN THE STEERING MECHANISM BEYOND TAU 72 ARE REPRESENTED
IN HOW THE RIDGE OVER AUSTRALIA DEVELOPS; GFS SHOWS THE RIDGE
EXTENDING WESTWARD TO THE SOUTH OF THE SYSTEM, ALLOWING THE SYSTEM
TO TRACK MORE EASTWARD AND CLOSER TO THE COAST NEAR BROOME.
MEANWHILE THE ECMWF GUIDANCE SHOWS THE RIDGE BUILDING AS ONE LARGER
RIDGE TO THE EAST, WHICH WOULD FORCE THE SYSTEM TO TURN TO THE
SOUTH-SOUTHWEST MUCH SOONER IN THE FORECAST PERIOD, BEING IN
GENERAL SIMILAR TO THE OTHER MODELS SUCH AS NAVGEM AND THE UKMET.
THE JTWC FORECAST TRACK LIES ON THE NORTHERN EDGE OF THE GUIDANCE
ENVELOPE THROUGH TAU 72, THEN REMAINS EAST OF THE CONSENSUS AND THE
MAJORITY OF THE GUIDANCE, EXCEPT GFS, THROUGH TAU 120. ENSEMBLE
GUIDANCE ALSO SHOWS AN EXTREME AMOUNT OF UNCERTAINTY, WITH A VERY
WIDE SPREAD IN THE TRACKS. WITH SUCH HIGH UNCERTAINTY, THE JTWC
FORECAST IS SET WITH LOW CONFIDENCE THROUGH THE FORECAST PERIOD.
JTWC INTENSITY CONSENSUS GUIDANCE CONSISTENTLY AND STEADILY
INCREASES TO 50-65KTS BY THE END OF THE FORECAST PERIOD, WITH
MINIMAL SPREAD ACROSS THE GUIDANCE PACKAGE. HOWEVER, IN LIGHT OF
TRACK UNCERTAINTY AND THE IMPACT OF TRACK CHANGES ON THE POTENTIAL
INTENSITY, THE INTENSITY FORECAST IS SET WITH LOW CONFIDENCE.  

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