WDXS31 PGTW 120900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 24S (IVONE) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5S 74.3E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 751 NM SOUTH OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 26 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A NEARLY
FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) ASSOCIATED WITH 
TROPICAL CYCLONE (TC) 24S (IVONE). CONVECTION OVER THE LLCC IS SHALLOW 
AND FRAGMENTED, WHILE A BROADER AREA OF CONSISTENT DEEP CONVECTION IS 
DISPLACED TO THE SOUTHEAST, WITHIN AN AREA OF CONVERGENT FLOW AND 
HIGHER HUMIDITY. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE 
BASED ON THE 120421Z METOP-B ASCAT SCATTEROMETER IMAGE SHOWING AN 
ELONGATED LLCC, WITH ASYMMETRIC WIND FIELD. THE INITIAL INTENSITY OF 
40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AFOREMENTIONED 
ASCAT PASS, WHICH SHOWS A BROAD PATCH OF 35 KT WINDS THROUGHOUT THE 
SOUTHEASTERN QUADRANT. THE ENVIRONMENTAL ANALYSIS REVEALS WARM (27-28 
C) SEA SURFACE TEMPERATURES (SST), OFFSET BY A WEAKENING OUTFLOW, DRY 
AIR NEARY FULLY SURROUNDING THE LLCC AND STRONG (30+ KTS) 
NORTHWESTERLY VERTICAL WIND SHEAR (VWS).

INITIAL WIND RADII BASIS: 120421Z METOP-B ASCAT SCATTEROMETER DATA

CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR) TO
THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T1.0 - 25 KTS
   CIMSS SATCON: 38 KTS AT 120217Z
   CIMSS ADT: 25 KTS AT 120300Z
   CIMSS AIDT: 33 KTS AT 120500Z
   CIMSS D-MINT: 22 KTS AT 120101Z
   CIMSS D-PRINT: 27 KTS AT 120500Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30+ KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG EASTWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE NORTHWEST

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: TC 24S IS CURRENTLY TRANSITING WESTWARD, JUST 
PRIOR TO TURNING WEST-SOUTHWESTWARD. THE SYSTEM IS FORECAST TO TRAVEL 
ALONG THE WESTERN PERIPHERY OF A STR TO THE SOUTH AND THE STEERING 
PATTERN WILL REMAIN IN PLACE THROUGHOUT THE FORECAST PERIOD. AS THE 
SYSTEM APPROACHES THE AXIS OF THE RIDGE, IT WILL TRANSIT OVER RAPIDLY
COOLING SST, WHICH WILL CONTRIBUTE TO THE WEAKENING TREND THROUGHOUT 
TAU 96. CONCURRENTLY BY TAU 72, TC 24S WILL APPROACH THE BAROCLINIC
ZONE AND WITH RAPIDLY INCREASING VWS WILL BEGIN ITS EXTRATROPICAL
TRANSITION (ETT). BY TAU 96, TC 24S WILL BECOME AN EXTRATROPICAL LOW, 
WHILE BEING LOCATED DIRECTLY UNDERNEATH AN UPPER-LEVEL JET.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE REMAINS IN GOOD
AGREEMENT THROUGHOUT THE INITIAL FORECAST PERIOD, WITH A 100 NM
CROSS-TRACK SPREAD AT TAU 72. THE ONLY OUTLIER BEING ECMWF, WHICH
DOES NOT EXTEND THAT FAR AND INDICATES A SHORTER DISSIPATION
TIMELINE. CROSS-TRACK SPREAD INCREASES TO OVER 200 NM BY THE END OF
THE FORECAST PERIOD, WITH ALL TRACKERS CHARACTERIZED BY THE SAME
TRANSLATION DIRECTION, INDICATING CONSISTENTLY RECOGNIZED STEERING 
PATTERN. INTENSITY FORECAST IS CLOSELY FOLLOWING THE MULTI-MODEL 
CONSENSUS, BUT IS ASSESSED WITH MEDIUM CONFIDENCE, DUE TO MODERATE 
INITIAL UNCERTAINTY. WHILE GFS AND COAMPS-TC ARE INDICATING 35-45 KT
INTENSITY THROUGHOUT TAU 72, HAFS IS SHOWING A POTENTIAL FOR
INTENSIFICATION UP TO 55 KTS BY TAU 24, FOLLOWED BY A GOOD
AGREEMENT WITH OTHER MODELS, IN REGARD TO THE WEAKENING TREND. JTWC
INTENSITY ASSESSMENT IS LEANING CLOSER TO THE GFS, COAMPS-TC AND
THE CONSENSUS GUIDANCE, GIVEN THE ENVIRONMENTAL ANALYSIS DESCRIBED
EARLIER.

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