WDXS31 PGTW 281500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (GAMANE)     
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.1S 47.6E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 288 NM NORTH OF ANTANANARIVO, MADAGASCAR
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS LOW-LEVEL
CLOUD LINES WRAPPING INTO A SMALL, WELL-DEFINED LOW-LEVEL
CIRCULATION (LLC) POSITIONED JUST OFF THE WEST COAST OF MADAGASCAR.
FLARING, ISOLATED DEEP CONVECTION IS PRESENT NORTHEAST OF THE LLC
DUE TO CONVERGENT LOW-LEVEL FLOW AND DIFFLUENT WESTERLY UPPER-LEVEL
FLOW. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON
THE MSI AND LIMITED SURFACE OBSERVATIONS. OVER THE PAST 12 HOURS,
THE SYSTEM WEAKENED RAPIDLY AND TRACKED WESTWARD TO THE INITIAL 
POSITION, WITH INCREASINGLY DISORGANIZED CONVECTION. A 280624Z
ASCAT-B IMAGE REVEALS AN ELONGATED, DEFINED CIRCULATION CENTER,
WITH PRIMARILY 15-20 KNOT WINDS EXCEPT FOR A WEDGE OF 30-35 KNOT
WINDS OVER THE NORTHEAST QUADRANT ALONG THE NORTHWEST COAST OF
MADAGASCAR, ASSOCIATED WITH THE ISOLATED AREA OF DEEP CONVECTION.
THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH HIGH CONFIDENCE
BASED ON RECENT ASCAT DATA.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: LOW-LEVEL NEAR-EQUATORIAL RIDGE (NER)
POSITIONED TO THE NORTH AND NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: WEAK EASTWARD
   OTHER FACTORS: FRICTIONAL EFFECTS ASSOCIATED WITH LAND
                  INTERACTION

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: SIGNIFICANT REPOSITIONING OF THE
CENTER AS WELL AS THE INTRODUCTION OF A DISSIPATION FORECAST
SCENARIO VICE SUBTROPICAL TRANSITION.

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 20S IS DRIFTING SLOWLY
TOWARD THE WESTERN COAST OF MADAGASCAR AND IS EXPECTED TO MAKE
LANDFALL WITHIN THE NEXT FEW HOURS THEN TRACK SOUTHEASTWARD ACROSS
NORTHERN MADAGASCAR UNDER THE STEERING INFLUENCE OF THE LOW-LEVEL
NER POSITIONED TO THE NORTHEAST. THE SYSTEM WILL LIKELY WEAKEN
FURTHER AS IT CROSSES THE MOUNTAINOUS TERRAIN, WITH THE REMNANTS
REEMERGING OVER WATER NEAR TAU 24. THE SYSTEM IS EXPECTED TO SLOWLY
CONSOLIDATE TO A PEAK INTENSITY OF 35 KNOTS BY TAU 48 UNDER
MARGINALLY FAVORABLE CONDITIONS. HOWEVER, UPPER-LEVEL CONDITIONS
ARE EXPECTED TO DEGRADE AS AN UPPER-LEVEL TROUGH SHIFTS OVER AND TO
THE EAST OF MADAGASCAR, WITH INCREASING VERTICAL WIND SHEAR (30-35
KNOTS) ASSOCIATED WITH THE UPPER-LEVEL WESTERLIES. ADDITIONALLY, AS
THE WEAK SYSTEM TRACKS SOUTHEASTWARD A STRONG HIGH WILL BUILD SOUTH
OF THE SYSTEM, LEADING TO A SLOWER EASTWARD TRACK AND EVENTUAL
STALL AND TRACK BACK THE WEST.         

MODEL DISCUSSION: WITH THE EXCEPTION OF THE UKMET ENSEMBLE MEAN
(UEMN), NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT WITH THE JTWC
FORECAST PHILOSOPHY LENDING MEDIUM OVERALL CONFIDENCE TO THE JTWC
TRACK FORECAST. BOTH ECMWF AND GFS INDICATE A SHARP TURN TO THE
WEST OR WEST-NORTHWEST AS THE STRONG HIGH BUILDS TO THE SOUTH. THE 
BULK OF THE ECMWF AND GFS ENSEMBLES ALSO SUPPORT THE JTWC TRACK 
PHILOSOPHY. RELIABLE INTENSITY GUIDANCE GENERALLY SHOWS A DIP IN THE 
NEXT 24 HOURS WITH A SLIGHT REINTENSIFICATION TO 30-35 KNOTS. THE GFS 
ENSEMBLE (GEFS) SHOWS ONE SOLUTION BRIEFLY INCREASING TO 35 KNOTS BUT 
THE LARGE MAJORITY BELOW TROPICAL STORM STRENGTH. THE ECMWF ENSEMBLE 
IS A BIT MORE BULLISH AND SHOWS ABOUT 50 PERCENT OF THE SOLUTIONS 
ATTAINING TROPICAL STORM STRENGTH. CONSEQUENTLY, THERE IS SIGNIFICANT 
UNCERTAINTY IN THE PEAK INTENSITY AND ABILITY FOR THE SYSTEM TO 
SURVIVE ITS TREK ACROSS MADAGASCAR AND TO REINTENSIFY EAST OF 
MADAGASCAR.         

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