WDXS31 PGTW 310900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27S (COURTNEY) WARNING 
NR 018//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.0S 88.5E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 854 NM SOUTH-SOUTHWEST OF THE COCOS
ISLANDS
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE 27S WITH CONVECTION THAT IS BEING HEAVILY SHEARED TO THE
SOUTHEAST, REVEALING THE PARTIALLY EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC). A 310344Z METOP-C ASCAT IMAGE SHOWED AN ELONGATING
WIND FIELD WITH 55 KNOT BARBS WITHIN THE SOUTHWESTERN PERIPHERY,
EVIDENCE THAT THE SYSTEM IS STILL NEAR TYPHOON STRENGTH.
ENVIRONMENTAL ANALYSIS INDICATES THAT 27S IS IN A HIGHLY
UNFAVORABLE ENVIRONMENT CHARACTERIZED BY HIGH (30-35 KTS) VERTICAL
WIND SHEAR, SIGNIFICANT DRY AIR ENTRAINMENT, AND BORDERLINE (25-26
C) SEA SURFACE TEMPERATURES. THESE FACTORS ARE ONLY OFFSET BY THE
STRONG POLEWARD OUTFLOW, AS THE SYSTEM APPROACHES THE JET. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON
EXTRAPOLATION FROM THE WIND AMBIGUITIES IMAGE CORRESPONDING TO THE
MENTIONED ASCAT-C PASS. THE INITIAL INTENSITY OF 65 KTS IS ASSESSED
WITH MEDIUM CONFIDENCE BASED ON THE ASCAT-C IMAGE AS WELL AS THE
HIGHER END OF THE AGENCY DVORAK CURRENT INTENSITY (CI) ESTIMATES
LISTED BELOW. ADDITIONALLY, A 302326Z RCM-1 SAR WIND SPEED IMAGE
SHOWED A VMAX OF AROUND 80 KTS WITHIN THE SOUTHWESTERN QUADRANT,
SUPPORTING THE SLIGHTLY HIGHER INITIAL INTENSITY ASSESSMENT. CIMSS 
OBJECTIVE ESTIMATES ARE ALL TOO LOW NOW DUE TO THE WEAK CONVECTIVE 
STRUCTURE OF THE SYSTEM.

INITIAL WIND RADII BASIS: 310344Z METOP-C SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) POSITIONED TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   KNES: T3.5 - 55 KTS
   DEMS: T3.5 - 55 KTS
   FMEE: T4.0 - 65 KTS
   FIMP: T3.5 - 55 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 30-35 KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: SIGNIFICANT DRY AIR ENTRAINMENT WITHIN THE
NORTHERN PORTION OF THE SYSTEM

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: SUBTROPICAL TRANSITION IS NOW
FORECAST TO OCCUR BEFORE DISSIPATION.

FORECAST DISCUSSION: TC 27S IS FORECAST TO TRACK FURTHER SOUTHWARD,
ALONG THE SOUTHWESTERN PERIPHERY OF THE STR THROUGH TAU 24. AFTER
TAU 24, THE VORTEX WILL BECOME VERY SHALLOW AND THE STEERING
MECHANISM WILL TRANSITION TO A LOW-LEVEL RIDGE POSITIONED TO THE
SOUTH OF THE SYSTEM. THIS WILL CAUSE 27S TO TURN SHARPLY WESTWARD
THROUGH THE REMAINDER OF THE FORECAST PERIOD. SUBTROPICAL
TRANSITION IS NOW FORECAST TO BE COMPLETE BEFORE DISSIPATION CAN
OCCUR. THE TRANSITION IS CURRENTLY FORECAST TO COMPLETE NO LATER
THAN TAU 48, BUT LIKELY EARLIER AS SHEAR IS EXPECTED TO RISE TO
OVER 50 KTS WITHIN THE NEXT 12 HOURS. REGARDING THE INTENSITY OF
27S, FURTHER WEAKENING THE FORECAST DUE TO RISING SHEAR, COOLING SEA
SURFACE TEMPERATURES, AND INCREASING DRY AIR ENTRAINMENT. AN
INTENSITY OF AROUND 35-40 KTS IS ANTICIPATED AROUND THE TIME
SUBTROPICAL TRANSITION COMPLETES. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
CONCERNING THE TRACK OF 27S WITH THE EXCEPTION OF NAVGEM, WHICH
TRACKS THE VORTEX FURTHER SOUTH THAN THE REMAINDER OF GUIDANCE.
DISREGARDING NAVGEM, THERE IS A 60 NM CROSS-TRACK SPREAD AT TAU 48.
THIS LENDS HIGH CONFIDENCE TO THE JTWC TRACK FORECAST. INTENSITY
GUIDANCE IS IN OVERALL GOOD AGREEMENT WITH ABOUT A 10 KNOT SPREAD
THROUGHOUT THE FORECAST PERIOD. THE JTWC INTENSITY FORECAST IS
PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE AS
WELL.

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