WDXS31 PGTW 182100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (FOUR) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.2S 71.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 81 NM SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 04S CONTINUES TO STRUGGLE TO CONSOLIDATE DUE
TO PERSISTENT EASTERLY VERTICAL WIND SHEAR. ANIMATED ENHANCED
INFRARED (EIR) SATELLITE IMAGERY DEPICTS A PARTIALLY EXPOSED
LOW-LEVEL CIRCULATION CENTER (LLCC), WITH DEEP CONVECTION CONFINED
TO THE WESTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON THE EIR SATELLITE IMAGERY. THE INITIAL
INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
PGTW SUBJECTIVE DVORAK ESTIMATE, WHICH IS REASONABLE BASED ON RECENT 
ASCAT DATA SHOWING 30-35 KNOT WINDS AND THE CIMSS ADT ESTIMATE.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
ENTRENCHED TO THE SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   CIMSS ADT: 34 KTS AT 181800Z
   CIMSS AIDT: 30 KTS AT 181700Z
   CIMSS D-PRINT: 25 KTS AT 181800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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 04S WILL TRACK GENERALLY WESTWARD THROUGH
THE FORECAST PERIOD UNDER THE STEERING INFLUENCE OF THE STR TO THE
SOUTH. AFTER TAU 72, THE SYSTEM WILL TURN SLIGHTLY WEST-NORTHWESTWARD 
AS THE STR RE-ORIENTS WNW-ESE IN RESPONSE TO A DEEP MIDLATITUDE 
TROUGH DIGGING OVER THE MOZAMBIQUE CHANNEL AND SOUTHWESTERN 
MADAGASCAR. OVERALL, ENVIRONMENTAL CONDITIONS ARE EXPECTED TO REMAIN 
MARGINAL TO UNFAVORABLE THROUGH THE FORECAST PERIOD WITH THE 
EXCEPTION OF A BRIEF PERIOD OF MARGINALLY FAVORABLE CONDITIONS. 
DURING THE TAU 12 TO TAU 36 PERIOD, VERTICAL WIND SHEAR (VWS) IS 
EXPECTED TO DECREASE SLIGHTLY ALLOWING THE SYSTEM TO MOISTEN AND 
CONSOLIDATE, WITH ENHANCED POLEWARD VENTING INTO THE WESTERLIES. 
AFTER PEAKING AT 40-45 KNOTS NEAR TAU 36, VWS WILL INCREASE TO 30-35 
KNOTS WITH SIGNIFICANT DRY AIR ENTRAINMENT, WHICH WILL LEAD TO STEADY 
WEAKENING THROUGH THE REMAINDER OF THE FORECAST PERIOD.      

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, WHICH TRACKS THE 
SYSTEM INTO THE STR TO THE SOUTH, DETERMINISTIC AND PROBABILISTIC 
GUIDANCE REMAINS IN FAIR AGREEMENT LENDING MEDIUM CONFIDENCE TO THE 
JTWC TRACK FORECAST. RELIABLE INTENSITY GUIDANCE IS IN FAIR 
AGREEMENT, WITH SLIGHT INTENSIFICATION DURING THE NEXT 48 HOURS AND 
SLOW, STEADY WEAKENING THROUGH TAU 120. HAFS-A INDICATES A PEAK 
INTENSITY OF 45 KNOTS AT TAU 24 WHILE COAMPS-TC (GFS) SHOWS A PEAK 
INTENSITY OF 50 KNOTS AT TAU 48. DUE TO THE GENERALLY MARGINAL 
CONDITIONS, THERE IS MEDIUM CONFIDENCE IN THE JTWC INTENSITY 
FORECAST, WITH INCREASING POTENTIAL FOR DISSIPATION POSSIBLE AFTER 
TAU 72.       

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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