WDXS31 PGTW 312100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27S (COURTNEY) WARNING 
NR 019//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.8S 88.8E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 885 NM SOUTH-SOUTHWEST OF THE COCOS
ISLANDS
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A BARREN
VORTEX NOW DEVOID OF DEEP CONVECTION DUE TO A COMBINATION OF HIGH
VERTICAL SHEAR (55 KT) AND COOLING SEA SURFACE TEMPERATURES OF NOW
24 TO 25 DEGREES CELSIUS. GIVEN THE LACK OF DEEP CONVECTION AND
APPROACH TOWARD THE SUBTROPICAL JET TO THE SOUTH, 27S IS UNDERGOING
SUBTROPICAL TRANSITION AT THIS TIME. THE INITIAL INTENSITY IS SET
AT 55 KT BASED ON A 311610Z ASCAT-C SCATTEROMETER PASS SHOWING
WINDS UP TO 50 KT IN THE SOUTHWESTERN QUADRANT, ACCOUNTING FOR
ASCAT'S KNOWN LOW BIAS AT THESE WIND SPEEDS. OBJECTIVE DVORAK
INTENSITY ESTIMATES ARE TOO LOW DUE TO THE LACK OF CONVECTION,
WHICH IS TYPICAL FOR A COMPACT STORM IN ITS DECAY PHASE OVER COOLER
WATER.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SURFACE-LEVEL SUBTROPICAL RIDGE TO THE
SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T3.0 - 45 KTS
   FIMP: T3.5 - 55 KTS
   CIMSS SATCON: 38 KTS AT 311617Z
   CIMSS ADT: 32 KTS AT 311515Z
   CIMSS AIDT: 37 KTS AT 311715Z
   CIMSS D-PRINT: 29 KTS AT 311715Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 30+ KTS
   SST: 24-25 CELSIUS
   OUTFLOW: WEAK POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   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 27S (COURTNEY) IS IN
SPIN-DOWN MODE WITHIN A HOSTILE ENVIRONMENT AND WILL STEADILY DECAY
OVER THE NEXT COUPLE OF DAYS. THE STORM MOTION IS NOW SLOWING AS
THE VORTEX BECOMES SHALLOWER AND COMES UNDER THE INFLUENCE OF THE
SURFACE SUBTROPICAL RIDGE TO ITS SOUTH, IMPARTING AN EASTERLY
STEERING FLOW IN THE LOWER TROPOSPHERE. THIS WILL RESULT IN A
GRADUAL TURN SOUTHWESTWARD OR WESTWARD IN THE NEAR TERM, WHICH MAY
BE ALREADY BEGINNING. THE JTWC FORECAST DEPICTS THIS TURN SLIGHTLY
SOONER (FARTHER NORTH) THAN THE PREVIOUS FORECAST, FOLLOWING THE
SHIFT IN THE MODEL GUIDANCE ENVELOPE. THE JTWC FORECAST TERMINATES
IN 24 HOURS DUE TO THE EXPECTATION THAT 27S WILL COMPLETE
SUBTROPICAL TRANSITION BY THEN, THOUGH THE CIRCULATION MAY RETAIN
GALE-FORCE WINDS SLIGHTLY BEYOND THAT TIME.

MODEL DISCUSSION: THE DYNAMICAL MODELS DISAGREE SLIGHTLY ON THE
TIMING AND LATITUDE OF THE TURN WESTWARD DUE TO THE SHALLOWING
VORTEX, BUT AGREE THAT SUCH A TURN IS IMMINENT, AND THE JTWC TRACK
FORECAST IS CLOSE TO THE MULTI-MODEL CONSENSUS. THE JTWC INTENSITY
FORECAST IS ALSO CLOSE TO THE MULTI-MODEL CONSENSUS, WHICH IS IN
TIGHT AGREEMENT ON STEADY DECAY DURING THE NEXT 24 HOURS.

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