WDXS32 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.0S 109.4E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 565 NM NORTH-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 19 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 18S WITH A PARTIALLY OBSCURED LOW-LEVEL CIRCULATION
CENTER (LLCC) AND DEEP CONVECTION FLARING TO THE WEST. A 121807Z
AMSR2 COLOR-ENHANCED 89GHZ MICROWAVE IMAGE DEPICTS VERY BROAD
SPIRAL BANDS WRAPPING INTO A DEFINED AND FULLY EXPOSED LLCC TO THE
EAST OF A PATCH OF DISORGANIZED CONVECTION. CIRRUS FILAMENTS EXPAND
FROM THE CONVECTION CENTER PRIMARILY TOWARDS THE WEST, AS THE
UPPER-LEVEL VORTEX STRUGGLES TO OVERCOME THE INFLUENCE OF THE
STRONG 30KT EASTERLY VERTICAL WIND SHEAR (VWS). ALSO RESULTING FROM
VWS, WEAK WESTWARD OUTFLOW IS BLOWN OF THE LLCC, WHICH IS
EXHIBITING SIGNIFICANT VERTICAL TILT WITH HEIGHT. DESPITE THE
UNFAVORABLE ELEMENTS, SEA SURFACE TEMPERATURES AND STRONG
WESTERLIES TO THE NORTH OF THE STORM ENHANCING VORTICITY WHICH
SHOULD HELP TO STEADY IMPROVE THE VORTEX STRUCTURE OVER THE NEXT 24
HOURS. THE STEERING ENVIRONMENT IS PRIMARILY DRIVING THE SYSTEM TO
THE EAST-SOUTHEAST ON THE SOUTHERN PERIPHERY OF A STRONG NEAR
EQUATORIAL RIDGE (NER) TO THE NORTH. THE INITIAL POSITION WAS
ASSESSED WITH HIGH CONFIDENCE BASED ON ANIMATED EIR REVEALING THE
PARTIALLY OBSCURED LLCC. THE INITIAL INTENSITY WAS ASSESSED WITH
LOW CONFIDENCE BASED ON AGENCY AND OBJECTIVE FIXES LISTED BELOW.
SATCON ASSESSES THE SYSTEM AS 44 KTS, HOWEVER IT IS SUSPECTED THAT
THE WIND FIELD IS SIGNIFICANTLY WEAKER IN THE SOUTHERN HEMISPHERE
OF THE SYSTEM, BASED ON ANALYSIS OF EARLIER SCATTEROMETER DATA
WHICH SHOWED WINDS AT OR BELOW 25 KTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: THE SOUTHERN PERIPHERY OF A NEAR
EQUATORIAL RIDGE TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS
   CIMSS SATCON: 44 KTS AT 121630Z
   CIMSS ADT: 33 KTS AT 121730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   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 CONTINUE TO TRACK
EAST-SOUTHEAST ON THE SOUTHERN PERIPHERY OF AN NER TO THE NORTH
OVER THE NEXT 48 HOURS BEFORE SLOWING TRACK SPEED AND CURVING TO
THE SOUTH UNTIL TAU 72. A SUBTROPICAL RIDGE (STR) EXPANDING OVER
THE AUSTRALIAN CONTINENT IS ANTICIPATED TO REDIRECT THE SYSTEM TO
THE SOUTHWEST THROUGH THE END OF THE FORECAST PERIOD. THE FORECAST
TRACK REMAINS UNCERTAIN, ESPECIALLY AFTER TAU 72, DUE TO MODERATE
DISAGREEMENT IN HOW STRONGLY THE RIDGE WILL INFLUENCE THE SYSTEM
AND WHETHER IT WILL HAVE A NORTH-SOUTH ALIGNMENT FORCING THE SYSTEM
TO TRACK FARTHER SOUTH OR WILL HAVE AN EAST-WEST ALIGNMENT ALLOWING
THE STORM TO TRACK FARTHER EAST AND IMPACT THE NORTHWESTERN COAST
OF AUSTRALIA. LITTLE IN THE WAY OF INTENSIFICATION IS EXPECTED
THROUGH THE FIRST 24 HOURS OF THE FORECAST AS MODERATE TO HIGH
SHEAR WILL CONTINUE TO IMPACT THE SYSTEM. SHEAR BEGINS TO DROP OFF
WHICH WILL ALLOW FOR A MORE RAPID PACE OF INTENSIFICATION, TO A
PEAK OF 80KTS AT THE END OF THE FORECAST PERIOD. 

MODEL DISCUSSION: THE JTWC CONSENSUS TRACK GUIDANCE HAS IMPROVED
OVER THE LAST 12 HOURS, HOWEVER THE CROSS-TRACK SPREAD IS STILL
LARGE AT 450NM BY TAU 120. THE TRACK DIRECTION HAS ALSO OVERALL
TAKEN A MORE SOUTHERLY TURN SHOWING MORE AGREEMENT IN THE
NORTH-SOUTH ORIENTATION OF THE STR OVER AUSTRALIA, DESPITE GFS
BOUNDING THE SPREAD TO THE EAST AND GALWEM BOUNDING THE TRACK TO
THE WEST. THE JTWC FORECAST TRACK GENERALLY CONFORMS TO THE ECMWF
TRACK AND THE CONSENSUS MEAN, FASTER THAN THE GFS BUT SLOWER THAN
THE UKMET AND GALWEM. DUE THE LARGE SPREAD IN THE DETERMINISTIC AND
ENSEMBLE GUIDANCE, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST
TRACK. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT THROUGH TAU 84,
WITH THE EXCEPTION OF COAMPS-TC SPIKING AT TAU 60 AND INTENSIFYING
TO 110KTS BY TAU 96 BEFORE DROPPING OFF AT THE END OF THE FORECAST.
ADDITIONALLY, THE HAFS-A GUIDANCE SUGGESTS A PEAK INTENSITY ABOVE
110 KNOTS, AND THE COAMPS-TC ENSEMBLE INTENSITY PROBABILITIES SHOW
A LOW (30 PERCENT) CHANCE OF RAPID INTENSIFICATION AFTER TAU 96.
THE INTENSITY FORECAST IS ASSESSED WITH LOW CONFIDENCE ON THE BASIS
THAT TRACK DEVIATIONS CAN CAUSE SIGNIFICANTLY DIFFERENT RESULTS IN
INTENSITY AND THE RELATIVELY LARGE SPREAD IN THE INTENSITY
GUIDANCE. 

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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