WDPS31 PGTW 281500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18P (ALFRED) WARNING NR 
015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.4S 155.7E
   INITIAL INTENSITY: 105 KTS
   GEOGRAPHIC REFERENCE: 457 NM NORTH-NORTHEAST OF BRISBANE,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 40 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A RAGGED
EYE FEATURE STRUGGLING AGAINST STRONG VERTICAL WIND SHEAR OF
15-20KTS FROM THE NORTHEAST. DRY AIR IS ENTRAINING INTO THE
CIRCULATION FROM THE NORTHWEST. WATER VAPOR IMAGERY REVEALS STRONG
POLEWARD OUTFLOW, BUT ALSO UPPER-LEVEL NORTHERLIES WHICH IS
MINIMIZING DIVERGENCE IN THE UPPER-LEVEL ENVIRONMENT. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A 281100Z NOAA-19
AMSU-B 89GHZ MICROWAVE IMAGE REVEALING A SHEARED AND BROKEN
MICROWAVE EYE WITH THE BULK OF THE DEEP CONVECTION TO THE SOUTH.
THE INITIAL INTENSITY OF 105 KTS IS ASSESSED WITH HIGH CONFIDENCE
BASED ON THE AGENCY AND OBJECTIVE FIXES LISTED BELOW ALONGSIDE
281146Z METOP-C ASCAT WIND SPEED DATA SUPPORTING THE INTENSITY AND
WIND RADII.

INITIAL WIND RADII BASIS: 281146Z METOP-C ASCAT WIND SPEED DATA

CURRENT STEERING MECHANISM: A COMPETING STEERING ENVIRONMENT
BETWEEN A DEEP-LAYER RIDGE TO THE WEST EXTENDING OVER AUSTRALIA AND
A NEAR EQUATORIAL RIDGE TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   ABRF: T5.5 - 102 KTS
   CIMSS SATCON: 102 KTS AT 281300Z
   CIMSS ADT: 109 KTS AT 281130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (18P) WILL TRACK SOUTHWARD
DURING THE REMAINDER OF THE FORECAST PERIOD, CURVING TO THE
SOUTHEAST AFTER TAU 72. THE STEERING ENVIRONMENT WILL REMAIN
COMPLICATED AND COMPETING BETWEEN TWO DEEP-LAYER RIDGES. THE RIDGE
TO THE EAST WILL DRIVE THE SYSTEM SOUTHWARD WHILE THE RIDGE LOCATED
OVER AUSTRALIA SLOWS THE TRACK SPEED, POTENTIALLY CAUSING IRREGULAR
TRACK MOTION NOT CAPTURED IN BETWEEN TAUS. AFTER TAU 48, A
DEEP-LAYER TROUGH WILL BREAK DOWN THE RIDGE OVER EAST AUSTRALIA,
ALLOWING FOR THE TRACK SPEED TO INCREASE SOUTHEASTWARD. AFTER THE
RIDGE PASSES AT THE END OF THE FORECAST PERIOD, THE AUSTRALIAN
RIDGE REBUILDS AND SLOWS TRACK MOTION ONCE AGAIN. THE FORECAST
WARNING CAPTURES A NEARLY QUASI-STATION TRACK BETWEEN TAU 96-120,
BUT THE POSSIBILITY LIES THAT AS THE DEEP-LAYER TROUGH MOVES
WESTWARD AND ERODES THE EASTERN NER THE SYSTEM MAY DOUBLE-BACK
NORTHWESTWARD UNDER THE STEERING INFLUENCE OF THE AUSTRALIAN RIDGE.
THE TC WILL RAPIDLY WEAKEN WHILE IN AN ENVIRONMENT WITH UNFAVORABLY
HIGH VERTICAL WIND SHEAR, MARGINAL DIVERGENCE ALOFT, AND
SIGNIFICANT UPWELLING IN THE EARLY-TERM FORECAST. AS THE SYSTEM
MOVES SOUTHWARD APPROACHING AN UPPER-LEVEL TROUGH BETWEEN TAU
48-96, THE UPPER-LEVEL ENVIRONMENT AND VERTICAL WIND SHEAR WILL
BECOME INCREASINGLY UNFAVORABLE AND CONTINUE WEAKENING TO 55KTS BY
THE END OF THE FORECAST PERIOD.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE HAS GOOD CONFIDENCE
IN THE NEAR-TERM FORECAST, BUT BEGINS TO SPREAD FROM 74NM AT TAU 72
TO 200NM AT TAU 120. THE JTWC MULTI-MODEL CONSENSUS GUIDANCE
REPRESENTS A BIFURCATION AFTER TAU 72 DUE TO UNCERTAINTY IN HOW
STRONGLY THE DEEP-LAYER TROUGH WILL ERODE THE EASTERN NER. THE JTWC
CONSENSUS TURNS SHARPLY NORTHWESTWARD AFTER TAU 96, BUT GIVEN THE
UNCERTAINTY DUE TO THE BIFURCATION THE TRACK FORECAST WAS SLOWED.
THE INTENSITY GUIDANCE IS IN STRONG AGREEMENT THAT RAPID WEAKENING
WILL OCCUR WITHIN THE FIRST 24 HOURS OF THE FORECAST PERIOD, AND
THEN BECOME MORE GRADUAL FOR THE REMAINDER OF THE FORECAST.

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