WDXS31 PGTW 130900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 24S (IVONE)      
WARNING NR 011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.8S 68.4E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 630 NM EAST OF PORT LOUIS, MAURITIUS
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
AS REVEALED IN ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI), THE
LOW-LEVEL CIRCULATION CENTER (LLCC) REMAINS FULLY EXPOSED, WITH
LOW-LEVEL CLOUD LINES WRAPPING AROUND A BROAD CENTER AND DISORGANIZED 
DEEP CONVECTION SHEARED TO THE EAST AND SOUTHEAST DUE TO HIGH 
WESTERLY VERTICAL WIND SHEAR (VWS). A 130401Z MHS 89GHZ MICROWAVE 
IMAGE DEPICTS A WEAKLY DEFINED LLCC, WITH DEEP CONVECTION DISPLACED 
WELL TO THE EAST AND SOUTH. A 130404Z ASCAT-B IMAGE SHOWS A HIGHLY 
ASYMMETRIC WINDFIELD, WITH AN ELONGATED CIRCULATION CENTER SURROUNDED 
BY 10-20 KNOT WINDS OVER THE NORTHERN SEMICIRCLE AND GRADIENT-
ENHANCED GALE FORCE WINDS DISPLACED 130NM TO THE SOUTH. THE INITIAL 
POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON MSI AND 
EXTRAPOLATION FROM THE ASCAT-B IMAGE. THE INITIAL INTENSITY OF 35 KTS 
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE ASCAT-B DATA. WITH 
THE EXCEPTION OF THE MODERATE POLEWARD OUTFLOW, ENVIRONMENTAL 
CONDITIONS ARE STEADILY DEGRADING, WITH EXTENSIVE DRY AIR OVER THE 
SYSTEM.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   DEMS: T2.0 - 30 KTS
   CIMSS SATCON: 38 KTS AT 130400Z
   CIMSS ADT: 34 KTS AT 130530Z
   CIMSS AIDT: 37 KTS AT 130530Z
   CIMSS D-PRINT: 23 KTS AT 130530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30+ KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: SIGNIFICANT DRY AIR 

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: TROPICAL CYCLONE (TC) 24S IS FORECAST TO TRACK
GENERALLY SOUTHWARD (SOUTH-SOUTHWESTWARD TO SOUTH-SOUTHEASTWARD)
THROUGH TAU 36 ALONG THE WESTERN PERIPHERY OF THE DEEP-LAYER STR TO
THE EAST. AFTER TAU 36, THE SYSTEM WILL RECURVE AND ACCELERATE
SOUTHEASTWARD AS THE STR REORIENTS WITH THE APPROACH OF A
MIDLATITUDE MAJOR SHORTWAVE TROUGH. TC 24S WILL MAINTAIN INTENSITY
THROUGH TAU 12 BUT IS EXPECTED TO INTENSIFY SLIGHTLY FROM TAU 12 TO
TAU 24, WITH A CONSOLIDATING WINDFIELD, DUE TO ENHANCED POLEWARD
OUTFLOW INTO THE JET TO THE SOUTH. MODEST RE-MOISTENING OF THE CORE
WILL OCCUR BUT IS EXPECTED T0 BE SHORT-LIVED AS DRY AIR WILL
OVERWHELM THE SYSTEM AFTER TAU 24 LEADING TO STEADY WEAKENING
THROUGH TAU 48. THE SYSTEM WILL ALSO BEGIN EXTRATROPICAL TRANSITION
(ETT) BY TAU 36 AS IT INTERACTS WITH THE MAJOR SHORTWAVE TROUGH. THE 
SYSTEM IS EXPECTED TO COMPLETE ETT BY TAU 48 AS IT TRACKS UNDER THE 
JET, WITH STRONG (40-45 KNOTS) VWS AND COLD (20C) SST VALUES, AND 
EMBEDS INTO THE FRONTAL ZONE. TC 24S COULD ALSO DISSIPATE PRIOR TO 
COMPLETING ETT. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE REMAINS IN TIGHT 
CROSS-TRACK AGREEMENT THROUGH THE FORECAST PERIOD, WITH A 60NM CROSS-
TRACK SPREAD AT TAU 48. ALONG-TRACK SPREAD INCREASES WITH INCREASING 
UNCERTAINTY AFTER TAU 36 LENDING MEDIUM OVERALL CONFIDENCE TO THE 
JTWC TRACK FORECAST. EPS AND GEFS ENSEMBLE GUIDANCE REMAINS IN GOOD 
AGREEMENT. WITH THE EXCEPTION OF HAFS-A, RELIABLE INTENSITY GUIDANCE 
INDICATES SOME POTENTIAL FOR SLIGHT INTENSIFICATION THROUGH TAU 36, 
WITH GFS, COAMPS-TC (GFS) AND HWRF SHOWING PEAK INTENSITIES OF 42-52 
KNOTS.   

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