WDXS31 PGTW 112100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (ILSA) WARNING NR 
013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.7S 120.5E
   INITIAL INTENSITY: 80 KTS
   GEOGRAPHIC REFERENCE: 164 NM NORTHWEST OF BROOME, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 33 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE 18S (ILSA) IS STILL STRUGGLING SOMEWHAT AGAINST
PERSISTENT SHEAR BUT CONTINUES TO STEADILY, IF SOMEWHAT SLOWLY
INTENSIFY. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS
THAT CONVECTION HAS CONTINUED TO PULSE OVER THE PAST SIX HOURS,
WITH CLOUD TOP TEMPERATURES REACHING AS LOW AS -92C AT TIMES,
DURING THE HEIGHT OF THE PULSES. HOWEVER, THE PULSES HAVE YET TO BE
SUSTAINED FOR MORE THAN A COUPLE OF HOURS AND THE LOW LEVEL VORTEX,
WHILE SHOWING SIGNS OF TRYING TO VERTICALLY ALIGN, HAS YET TO DO SO
FULLY. THE 12Z SOUNDING FROM BROOME SHOWS NORTHERLY FLOW ABOVE
500MB, WHILE THE DARWIN SOUNDING INDICATES STRONG EASTERLY FLOW
ABOVE 400MB. THE SOUNDING DATA, CIMSS ATMOSPHERIC MOTION VECTOR
(AMV) DATA AND JTWC HAND ANALYSIS REVEALS AN UPPER-LEVEL
ANTICYCLONE OVER NORTHERN AUSTRALIA TO THE SOUTHEAST OF DARWIN,
IMPARTING NORTHEASTERLY SHEAR UPON TC 18S, PARTICULARLY IN THE
MID-LEVELS. WHILE UPPER-LEVEL OUTFLOW IS EXPANDING EASTWARD, THE
MID-LEVEL SHEAR REMAINS THE PRIMARY FACTOR INHIBITING VERTICAL
ALIGNMENT OF THE VORTEX. THERE HAVE BEEN NO RECENT MICROWAVE
IMAGES, AND THERE IS A LARGE SPREAD AMONGST THE AGENCY FIX
POSITIONS, THUS THE INITIAL POSITION IS ASSESSED WITH LOW
CONFIDENCE, EXTRAPOLATING FROM AN EARLIER RCM-3 SAR PASS. THE
INITIAL INTENSITY IS ASSESSED AT 80 KNOTS, WELL ABOVE THE MAJORITY
OF THE SUBJECTIVE AND OBJECTIVE FIX DATA, BASED ON AN EARLIER
110950Z SMAP PASS SHOWING 76 KNOTS (1MIN CONVERSION) AND THE
111015Z RCM-3 SAR PASS, WHICH INDICATED A MAX WIND OF OVER 100
KNOTS. CURRENT CIMSS SHEAR ANALYSIS SUGGESTS THAT SHEAR HAS FINALLY
STARTED TO WEAKEN, DOWN TO 12 KNOTS AT 1800Z. HOWEVER, AS NOTED
ABOVE, MID-LEVEL SHEAR IS LIKELY A BIT HIGHER. SST AND OHC VALUES
ARE INCREASING AS THE SYSTEM TRACKS SOUTHWESTWARD AND POLEWARD
OUTFLOW IS IMPROVING, SETTING THE STAGE FOR THE NEAR-TERM ONSET OF
RAPID INTENSIFICATION (RI).

INITIAL WIND RADII BASIS: SAR DATA AND SMAP DATA. 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   KNES: T4.0 - 65 KTS
   APRF: T4.0 - 65 KTS
   CIMSS SATCON: 67 KTS AT 111730Z
   CIMSS ADT: 63 KTS AT 111800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: MODERATE MID-LEVEL SHEAR

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: ALTHOUGH THERE ARE SOME INDICATIONS OF A
RECENT SLOW DOWN AND SOMEWHAT ERRATIC TRACK, TC 18S CONTINUES TO
TRACK GENERALLY SOUTHWESTWARD ALONG THE WEST SIDE OF THE DEEP STR
CENTERED TO THE EAST. AS THE VORTEX CONTINUES TO SYMMETRIZE AND
VERTICALLY ALIGN AND COMMENCE RI, SOME WOBBLING CAN BE EXPECTED BUT
OVERALL, THE TRACK WILL CONTINUE SOUTHWEST AND THE CENTER OF TC 18S
IS EXPECTED TO BE IN THE VICINITY OF ROWLEY SHOALS BY TAU 24. AFTER
THIS POINT THE SYSTEM IS FORECAST TO BEGIN TO ROUND THE RIDGE AXIS,
TURN SOUTH THEN EVENTUALLY SOUTHEASTWARD BY TAU 36. TRACK SPEEDS
SHOULD SLOW A BIT AS THE SYSTEM APPROACHES THE RIDGE AXIS BUT THEN
ACCELERATE AFTER TAU 36 AS THE STEERING GRADIENT TIGHTENS AHEAD OF
AN APPROACHING TROUGH. TC 18S IS EXPECTED TO MAKE LANDFALL ALONG
EIGHTY MILE BEACH BETWEEN TAU 36 AND TAU 48. AFTER LANDFALL THE
SYSTEM WILL STEP ON THE GAS AND RAPIDLY ACCELERATE TOWARDS THE DEEP
INTERIOR OF AUSTRALIA. IN TERMS OF INTENSITY, WHILE TC 18S HAS
STEADILY INTENSIFIED, THE EXPECTED ONSET OF RI HAS YET TO
MATERIALIZE. HOWEVER, CONDITIONS ARE NOW PRIMED, WITH THE SYSTEM
MOVING INTO WARMER WATERS WITH HIGH OHC, WHILE AT THE SAME TIME
SHEAR HAS FINALLY BEGUN TO WEAKEN AND OUTFLOW IS IMPROVING. HENCE,
THE ONSET OF THE LONG-ANTICIPATED RI IS LIKELY IMMINENT. THE
FORECAST CALLS FOR THE SYSTEM TO REACH 125 KNOTS BY TAU 36, BUT A
PEAK AT OR ABOVE 130 KNOTS IS LIKELY TO OCCUR JUST PRIOR TO
LANDFALL, OR JUST PRIOR TO TAU 48. AFTER LANDFALL, THE SYSTEM WILL
SLOWLY WEAKEN OVER THE RELATIVELY FLAT TERRAIN OF THE GREAT SANDY
DESERT, BEFORE MORE RAPIDLY WEAKENING AFTER TAU 60. THE SYSTEM IS
EXPECTED TO DISSIPATE FULLY BETWEEN TAU 72 AND 96 OVER THE CENTRAL
PORTION OF AUSTRALIA. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN OVERALL GOOD AGREEMENT,
ESPECIALLY IN TERMS OF CROSS-TRACK SPREAD THROUGH LANDFALL.
CROSS-TRACK SPREAD INCREASES DRAMATICALLY AFTER TAU 72 AS THE
MODELS BEGIN TO LOSE THE VORTEX. ALONG-TRACK SPREAD IS MINIMAL
UNTIL THE SYSTEM REACHES THE INFLECTION POINT AT THE RIDGE AXIS,
THEN STEADILY INCREASES FROM THERE TO THE END OF THE FORECAST, WITH
ABOUT 85NM OF ALONG-TRACK SPREAD AT LANDFALL. OVERALL CONFIDENCE IN
THE TRACK IS HIGH THROUGH LANDFALL, DECREASING TO LOW AFTER TAU 72.
INTENSITY GUIDANCE REMAINS IN OVERALL GOOD AGREEMENT, THOUGH
SOMEWHAT SURPRISINGLY THE NORMALLY BULLISH HWRF HAS BACKED OFF
SIGNIFICANTLY ON THE ULTIMATE PEAK INTENSITY, NOW SHOWING A PEAK OF
JUST 100 KNOTS. THE REMAINDER OF THE GUIDANCE IS SOMEWHAT HIGHER,
NEAR 115 KNOTS, WHILE THE RI ENHANCED CONSENSUS PEAKS AT 120 KNOTS
AND THE CTCX DETERMINISTIC GUIDANCE PEAKS AT 125 KNOTS. ALL
AVAILABLE RI GUIDANCE INCLUDING RICN, DTOPS, RIPA, AND RIDE ARE
TRIGGERED WITH THIS RUN, WITH PEAKS IN THE 130-135 KNOT RANGE.
ADDITIONALLY, THE CTCXEPS ENSEMBLE PROBABILITY CONTINUES TO
INDICATE A 100 PERCENT PROBABILITY OF RI BETWEEN TAUS 18-42. IN
LIGHT OF THE SIGNIFICANT INDICATORS OF NEAR-TERM RI ONSET AND THE
LARGE NUMBER OF RI GUIDANCE MEMBERS TRIGGERING, THE JTWC CONTINUES
TO PREDICT RI AND A PEAK IN THE 130 KNOT RANGE, JUST PRIOR TO
LANDFALL, WITH HIGH CONFIDENCE. 

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