WDXS32 PGTW 120300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.4S 105.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 725 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 17 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE (TC) 18S WITH AN EXPOSED LOW LEVEL CIRCULATION CENTER
(LLCC) WITH FLARING CONVECTION AND EXPANDING CIRRUS FILAMENTS TO
THE NORTHWEST OF THE LLCC AS THE SYSTEM EXPERIENCES STRONG VERTICAL
WIND SHEAR (VWS) FROM THE EAST. THE SYSTEM IS CURRENTLY TRACKING
SOUTHEASTWARD AS IT IS STEERED ON THE SOUTHERN PERIPHERY OF A NEAR
EQUATORIAL RIDGE (NER) TO THE NORTH. THE TRACK MOTION HOWEVER HAS
BEEN RATHER ERRATIC OVER THE PAST SIX HOURS, WITH THE LLCC DIVING
SOUTHEASTWARD UP TO THE 0000Z HOUR SUBSEQUENTLY STOMPING ON THE
BRAKES AND BECOMING ALMOST STATIONARY THROUGH THE 0200Z HOUR.
FAVORABLE SEA SURFACE TEMPERATURES AND FAVORABLE DIVERGENCE ALOFT
ENCOURAGING STRONG WESTWARD OUTFLOW ARE BEING OFFSET BY THE
ELEVATED VWS, INHIBITING DEVELOPMENT OF A CONSOLIDATED VORTEX. THE
INITIAL POSITION WAS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
EXPOSED LLCC IN THE ANIMATED MSI. THE INITIAL INTENSITY WAS
ASSESSED WITH LOW CONFIDENCE DUE TO AN ABSENCE OF WIND SPEED
MEASUREMENTS, AS THUS BASED ENTIRELY ON AGENCY AND OBJECTIVE
ESTIMATES NOTED BELOW AS WELL AS THE OVERALL RAGGED AND
DETERIORATED STRUCTURE.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: THE SOUTHERN PERIPHERY OF THE NER TO
THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T1.5 - 25 KTS
   APRF: T1.5 - 25 KTS
   CIMSS SATCON: 45 KTS AT 122120Z
   CIMSS ADT: 33 KTS AT 120030Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK 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 WILL CONTINUE TO TRACK EAST-SOUTHEAST
FOR THE NEXT 72 HOURS WHILE UNDER THE INFLUENCE OF A NER TO THE
NORTH. THE SYSTEM WILL SLOW DOWN AFTER TAU 48 AS THE STEERING
GRADIENT WEAKENS AND THE SUBTROPICAL RIDGE (STR) CENTERED OVER
CENTRAL AUSTRALIA RIDGES WESTWARD, CREATING A COMPETING STEERING
PATTERN. BEYOND TAU 72, THE EVOLUTION OF THE STR PATTERN IS HIGHLY
UNCERTAIN, WITH THE ECMWF DEVELOPING A NORTH-SOUTH ORIENTED RIDGE,
WHILE IN THE GFS THE RIDGE REMAINS ORIENTED MORE EAST-WEST. THE
CURRENT FORECAST CALLS FOR THE SYSTEM TO RUN INTO THE NORTH-SOUTH
ORIENTED STR WHICH WILL BLOCK THE STORM FROM TRAVELING FURTHER EAST
AND WHICH WILL SLOW THE SPEED AND REDIRECT THE SYSTEM TO THE SOUTH
THROUGH THE END OF THE FORECAST PERIOD. TC 18S IS FORECAST TO
SLOWLY BUT STEADILY INTENSIFY OVER THE FIRST 72 HOURS OF THE FORECAST 
AS THE SYSTEM STRUGGLES TO CONSOLIDATE THE VORTEX AGAINST ELEVATED VWS 
OF 20-25KTS. AS VWS LESSENS FROM TAU 72-120, INTENSITY
IS EXPECTED TO INCREASE MORE RAPIDLY WITH FAVORABLE SEA SURFACE
TEMPERATURES OF 28-29C AND MOIST ENVIRONMENT, REACHING OF A PEAK OF
75 KNOTS BY TAU 120. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
IN THE NEAR-TERM THAT THE STORM WILL TRACK TO THE EAST-SOUTHEAST
OVER THE NEXT 36 HOURS. HOWEVER, BY TAU 120 THE CROSS-TRACK SPREAD
OPENS TO 820NM, BOUNDED BY GALWEM TO THE SOUTHWEST AND GFS TO THE
NORTHEAST. THE VARIATIONS IN THE JTWC TRACK CONSENSUS MEMBERS CAN
BE EXPLAINED BY THE SIGNIFICANT DIFFERENCES IN THE STEERING
ENVIRONMENT REPRESENTED IN EACH MODEL. PARTICULARLY, ECMWF
REPRESENTS A MORE SOUTHERLY ROUTE CLOSER TO THE JTWC TRACK
CONSENSUS WHICH IS DRIVEN BY A LARGE STR OVER THE AUSTRALIAN
CONTINENT WHICH BLOCKS THE SYSTEM FROM CONTINUING ON ITS EASTWARD
TRACK AND CAUSES IT TO SLOWLY TURN TO THE SOUTHWEST. ON THE OTHER
HAND, GFS SHOWS THE RIDGE EXTENDING TOWARDS THE WEST OVER THE
FORECAST PERIOD, ALLOWING THE SYSTEM TO CONTINUE TO TRACK TO THE
NORTHWEST COAST OF AUSTRALIA BEFORE MARKING ITS TURN SOUTH BY TAU
72. DUE TO UNCERTAINTY IN DETERMINISTIC AND ENSEMBLE MODEL
GUIDANCE, THE TRACK FORECAST IS DETERMINED WITH LOW CONFIDENCE
THROUGH THE FORECAST PERIOD. THE JTWC INTENSITY CONSENSUS GUIDANCE
IS ALSO IN GOOD AGREEMENT THROUGH THE END OF THE FORECAST PERIOD
THAT THERE WILL BE A GRADUAL AND CONSISTENT INCREASE IN INTENSITY
WITH THE EXCEPTION OF COAMPS-TC (GFS VERSION) WHICH SPIKES TO
130KTS BETWEEN TAU 96-120 LIKELY DUE TO AN AMPLIFIED REACTION TO A
DROP IN VWS. HOWEVER, WHILE THE INTENSITY GUIDANCE ENVELOPE IS VERY
TIGHT, THE HIGH UNCERTAINTY IN THE TRACK LEADS TO LOW CONFIDENCE IN
THE INTENSITY, AS A TRACK FURTHER SOUTH WOULD LIKELY LEAD TO A MUCH
STRONGER SYSTEM VERSUS A MORE NORTHWARD TRACK. 

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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