WDXS31 PGTW 092100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 24S (IVONE) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.4S 83.1E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 876 NM SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 27 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 24S (IVONE) WITH A LARGE AREA OF CONVECTION OBSCURING THE
CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC). NORTHERLY SHEAR
CAN BE SEEN ACTING ON THE SYSTEM ALONG WITH DRY AIR WITHIN THE
NORTHERN SEMICIRCLE, CAUSING THE LOW-LEVEL CLOUD LINES TO BE
EXPOSED IN THAT REGION. THESE TWO FACTORS ARE INHIBITING CONVECTIVE 
DEVELOPMENT WITHIN THE NORTHERN PORTION OF THE SYSTEM. 
ENVIRONMENTAL ANALYSIS INDICATES THAT 24S IS IN A MARGINALLY 
FAVORABLE ENVIRONMENT CHARACTERIZED BY THE DRY AIR AND NORTHERLY 
VERTICAL WIND SHEAR OFFSET BY THE STRONG POLEWARD OUTFLOW AND WARM 
(27-28 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION IS PLACED 
WITH MEDIUM CONFIDENCE BASED ON THE ANIMATED EIR. ADDITIONALLY, A
091616Z METOP-B ASCAT IMAGE REVEALED AN ELONGATED LLCC IN THE
NORTH-SOUTH AXIS, SUPPORTING THE INITIAL POSITION. THE INITIAL
INTENSITY OF 50 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
MENTIONED ASCAT SHOWING A SWATH OF 40 KTS WITHIN THE SOUTHERN
SEMICIRCLE AND ALL AGENCIES AGREEING ON A T3.0 ESTIMATE.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   DEMS: T3.0 - 45 KTS
   FMEE: T3.0 - 45 KTS
   FIMP: T3.0 - 45 KTS
   CIMSS SATCON: 44 KTS AT 091749Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 24S IS FORECAST TO CONTINUE TRACKING
SOUTH-SOUTHWESTWARD ALONG THE WESTERN PERIPHERY OF THE STR THROUGH 
TAU 24. AFTER TAU 24, A BUILDING RIDGE WILL FORM SOUTH OF THE SYSTEM 
AND BECOME THE PRIMARY STEERING INFLUENCE. THIS WILL DRIVE 24S 
WESTWARD THROUGH TAU 72. NEAR TAU 72, 24S WILL BEGIN TO ROUND THE 
RIDGE AND WILL TURN SOUTHWARD. REGARDING INTENSITY, 24S IS FORECAST 
TO MAINTAIN INTENSITY THROUGH TAU 24 AS SHEAR REMAINS RELATIVELY LOW. 
HOWEVER, AFTER TAU 24, SHEAR INCREASES TO AROUND 30 KTS. THE SHEAR 
ALONG WITH DRY AIR ENTRAINMENT WILL CAUSE 24S TO WEAKEN, THOUGH ONLY 
MARGINALLY DUE TO THE ROBUST POLEWARD OUTFLOW CHANNEL. NEAR TAU 96, 
SHEAR WILL INCREASE TO OVER 35 KTS AND DRY AIR WILL COMPLETELY ERODE 
THE VORTEX, CAUSING DISSIPATION BY TAU 120. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72 WITH A 180 NM CROSS-TRACK SPREAD AT TAU 72. AFTER
TAU 72, MODELS DIVERGE AND CROSS-TRACK SPREAD INCREASES TO AROUND
500 NM AT TAU 120. NAVGEM TAKES THE SYSTEM NEARLY STRAIGHT WESTWARD
WHILE THE ECMWF ENSEMBLE MEAN RACES THE VORTEX SOUTHWARD. AS A
RESULT, THE JTWC TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE
THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS
IN MODERATE AGREEMENT, WITH MODELS OVERALL PORTRAYING MARGINAL
WEAKENING THROUGH TAU 72 AND QUICKER WEAKENING AFTERWARD. THE JTWC 
INTENSITY FORECAST IS PLACED WITH MEDIUM CONFIDENCE.

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