WDXS31 PGTW 180300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (FOUR) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 7.7S 74.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 107 NM EAST-SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
CONSOLIDATED LOW-LEVEL CIRCULATION CENTER (LLCC) PARTIALLY OBSCURED
BY PERSISTENT DEEP CONVECTION. A 172302Z SSMIS 91 GHZ MICROWAVE
IMAGE REVEALS LOW-LEVEL CLOUD BANDING TIGHTLY WRAPPING INTO THE
LLCC. TROPICAL CYCLONE (TC) 04S HAS STRUGGLED TO INTENSIFY DUE TO
NORTHEASTERLY FLOW HINDERING THE FORMATION OF AN EQUATORWARD
OUTFLOW CHANNEL, DESPITE ALREADY HAVING ROBUST WESTWARD AND POLEWARD 
OUTFLOW ENHANCED BY THE WESTERLIES TO THE SOUTH. ENVIRONMENTAL 
ANALYSIS INDICATES A MARGINALLY FAVORABLE ENVIRONMENT CHARACTERIZED BY 
LOW TO MODERATE (15-20 KTS) VERTICAL WIND SHEAR, WARM (28-29 C) SEA 
SURFACE TEMPERATURES, AND STRONG DIVERGENCE ALOFT. THE INITIAL 
POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON EXTRAPOLATION FROM 
THE AFOREMENTIONED 172302Z SSMIS 91 GHZ MICROWAVE IMAGE. THE INITIAL 
INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE 
AGENCY DVORAK FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE (STR) TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   FMEE: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 04S IS EXPECTED TO TRACK WEST-SOUTHWESTWARD
OVER THE FORECAST PERIOD, ALONG THE NORTHERN PERIPHERY OF THE
STEERING STR TO ITS SOUTH AND PASSING APPROXIMATELY 64 NM SOUTH OF
DIEGO GARCIA JUST BEFORE TAU 12. SLOW INTENSIFICATION WILL OCCUR
BETWEEN TAU 12-36, PEAKING AT 50 KTS AT TAU 36-48. AFTER TAU 48,
INCREASING (20-25 KTS) NORTHERLY VERTICAL WIND SHEAR AND MID-LEVEL DRY
AIR ENTRAINMENT WILL LEAD TO A SLOW WEAKENING TREND THROUGH THE 
REMAINDER OF THE FORECAST PERIOD. 04S WILL MAINTAIN INTENSITY AT 
MINIMAL TROPICAL STORM STRENGTH THROUGH TAU 96-120, WITH DISSIPATION 
NOT EXPECTED WITHIN THE FORECAST PERIOD.

MODEL DISCUSSION: DETERMINISTIC TRACK MODEL GUIDANCE IS IN TIGHT
AGREEMENT THROUGH TAU 24, WITH NAVGEM AS THE ONLY OUTLIER. AFTER
TAU 24, TRACK CERTAINTY BEGINS TO DIVERGE, WITH A 175NM CROSS-TRACK
SPREAD AT TAU 96 AND A 220NM CROSS-TRACK SPREAD AT TAU 120.
ENSEMBLE MODELS -- GEFS AND ECENS -- ARE IN MODERATE AGREEMENT,
LENDING MEDIUM CONFIDENCE TO THE JTWC TRACK FORECAST. INTENSITY
MODELS ARE IN MODERATE AGREEMENT THAT 04S WILL INTENSIFY THROUGH
TAU 48 BEFORE INDICATING A SLOW DECLINE NEAR THE END OF THE FORECAST 
PERIOD. AS A RESULT, THE JTWC INTENSITY FORECAST IS PLACED WITH 
OVERALL MEDIUM CONFIDENCE.

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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