WDXS31 PGTW 092100
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: 11.0S 62.4E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 620 NM NORTH-NORTHEAST OF PORT LOUIS,
MAURITIUS
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
PARTIALLY EXPOSED LLCC ON THE EASTERN SIDE OF A YOUNG BURST OF DEEP
CONVECTION, NEARLY CIRCULAR IN SHAPE WITH CLOUD TOPS COOLER THAN -80
DEGREES CELSIUS. OUTER BANDING IS EVIDENT THROUGHOUT THE WESTERN
SEMICIRCLE, BUT NONE TO THE EAST DUE TO ONGOING EASTERLY MID-LEVEL
SHEAR OF ABOUT 20 KT, DENOTED BY CIRRUS STREAMING BENEATH THE
UPPER-LEVEL OUTFLOW INTO THE LLCC AREA FROM THE EAST IN EIR AND
WATER VAPOR IMAGERY. A 091436Z SMOS PASS SHOWED 35-40 KT WINDS
WRAPPING THROUGHOUT THE WESTERN SEMICIRCLE, WITH WEAKER WINDS TO
THE EAST. THE INITIAL INTENSITY IS THUS SET AT 40 KT, SUPPORTED BY
A 1202Z D-MINT ESTIMATE OF 40 KT AND AGENCY DVORAK INTENSITY
ESTIMATES AS HIGH AS T2.5 (35 KT).

INITIAL WIND RADII BASIS: SMOS DATA

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   DEMS: T2.0 - 30 KTS
   FMEE: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE WESTWARD
   OTHER FACTORS: DRY AIR MASS TO THE EAST

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: TROPICAL CYCLONE 04S IS CURRENTLY A VERTICALLY
TILTED VORTEX, LEANING WESTWARD WITH HEIGHT WITH A PARTIALLY EXPOSED 
LLCC. RECENT SATELLITE TRENDS SUGGEST AN IMPROVEMENT IN DEEP 
CONVECTIVE COVERAGE, PARTIALLY OVERSPREADING THE LLCC, AND VERTICAL 
SHEAR IS EXPECTED TO DECREASE FROM AROUND 20 KT TO AROUND 10 KT 
DURING THE NEXT 24-36 HOURS. WITH SEA SURFACE TEMPERATURES EXPECTED 
TO REMAIN WARM AT 28-29 DEGREES CELSIUS, THIS SHOULD RESULT IN MORE 
FAVORABLE CONDITIONS THAT FACILITATE AXISYMMETRIZATION AND 
INTENSIFICATION OF THE CYCLONE DURING THIS PERIOD. THE JTWC INTENSITY 
FORECAST SHOWS INTENSIFICATION TO 60 KT BY 36 HOURS. DURING THIS 
TIME, 04S WILL BE STEERED GENERALLY WESTWARD BY A BUILDING 
SUBTROPICAL RIDGE TO ITS SOUTH. AFTER 36 HOURS, THE STRONGER RIDGE IS 
EXPECTED TO INTENSIFY MID-LEVEL EASTERLIES PRESSING ON THE VORTEX, 
RESULTING IN INCREASING VERTICAL SHEAR ONCE AGAIN DURING THE 36-72 
HOUR PERIOD. NUMERICAL MODELS ARE SPLIT ON HOW WELL THE VORTEX WILL 
REMAIN ALIGNED DURING THIS RENEWED PERIOD OF SHEAR, WITH SOME MODELS 
LIKE HAFS-A AND COAMPS-TC KEEPING THE CYCLONE AT 65+ KT INTENSITY 
DURING THIS TIME, WHILE OTHER MODELS SUCH AS ECMWF AND GFS SHOW DECAY 
AS VORTEX TILT REAPPEARS AND DRY AIR GETS PUSHED IN FROM THE EAST. 
THE JTWC FORECAST HEDGES BETWEEN THESE SOLUTIONS, SHOWING A PEAK 
INTENSITY OF 60 KT AND THEN LEVELING OFF AS SHEAR RESTRENGTHENS 
DURING THE 48-72 HOUR PERIOD. THE DYNAMICAL MODEL TRACK GUIDANCE 
SHOWS 04S APPROACHING NORTHERN MADAGASCAR IN AROUND 96 HOURS, BUT IT 
IS UNCLEAR WHETHER LANDFALL WILL OCCUR. REGARDLESS, A TRACK CLOSE TO 
OR OVER THE STEEP TOPOGRAPHY OF THIS REGION SHOULD SIGNIFICANTLY 
DISRUPT THE INNER CORE OF THE CYCLONE, RESULTING IN WEAKENING DURING 
THIS TIME. THE JTWC TRACK FORECAST IS CLOSE TO THE NORTHERN TIP OF 
THE ISLAND, AND SHOWS WEAKENING DURING THE 72-120 HOUR PERIOD. THE 
CYCLONE WILL LIKELY TRACK INTO THE MOZAMBIQUE CHANNEL THEREAFTER, 
WHERE MODELS CURRENTLY EXPECT SOME VERTICAL SHEAR TO PERSIST. ANY
REINTENSIFICATION IN THE CHANNEL WOULD LIKELY DEPEND SIGNIFICANTLY
ON HOW MUCH THE VORTEX IS DISRUPTED BY LAND INTERACTION AND HOW
MUCH DRY AIR ENTRAINS INTO THE AMBIENT ENVIRONMENT DUE TO THE
PERSISTENT SHEAR. THERE IS THUS LOW CONFIDENCE IN THE INTENSITY
FORECAST DURING THIS PERIOD, WITH THE JTWC FORECAST SHOWING
WEAKENING TO 35 KT BY 120 HOURS.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS THROUGHOUT THE FORECAST PERIOD, NUDGED
SLIGHTLY NORTH AND FASTER. THE JTWC INTENSITY FORECAST IS CLOSE TO
THE MULTI-MODEL CONSENSUS, EXCEPT AROUND 72 HOURS WHEN IT IS ABOUT
10 KTS WEAKER. THE INTENSITY GUIDANCE HAS RELATIVELY HIGH SPREAD,
VARYING FROM 45 TO 95 KT AT PEAK INTENSITY IN 48-72 HOURS.

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