WDXS31 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (GAMANE) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.3S 49.2E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 619 NM NORTHWEST OF PORT LOUIS, MAURITIUS
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 05 KTS

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE (TC) 20S (GAMANE) AS OVER LAND IN NORTHERN MADAGASCAR. TC
20S HAS BECOME LESS ORGANIZED WITH CONVECTION BEGINNING TO DISPERSE
AROUND THE OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC) AS
FRICTIONAL EFFECTS START TO TAKE A TOLL ON THE SYSTEM. THE CURRENT
ENVIRONMENT IS ASSESSED AS UNFAVORABLE CHARACTERIZED BY LAND
INTERACTION AND DRY AIR BEING ENTRAINED INTO THE SYSTEM. THE
UPPER-LEVEL ENVIRONMENT IS, IN CONTRAST, ASSESSED AS FAVORABLE WITH
STRONG POLEWARD OUTFLOW ALOFT (AIDED BY AN UPPER-LEVEL JET MAXIMUM
TO THE SOUTH) AND LOW (10-15 KTS) VERTICAL WIND SHEAR (VWS). THESE
FACTORS ARE NOT ENOUGH TO OUTWEIGH THE LAND INTERACTION AND DRY AIR
PRESENCE THOUGH. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON A 271200Z MET-9 VISIBLE SATELLITE IMAGE. THE
INITIAL INTENSITY OF 55 KTS IS ASSESSED WITH LOW CONFIDENCE BASED
ON OBJECTIVE INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: POSITION ONLY (OVER LAND)
   CIMSS DMINT: 53 KTS AT 270954Z
   CIMSS DPRINT: 56 KTS AT 271200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: TC 20S HAS UNDERGONE A MUCH QUICKER
WEAKENING TREND THAN PREVIOUSLY FORECAST. WITH THE SYSTEM TRACKING 
FURTHER WEST THAN ANTICIPATED, OVER THE TSARATANANA MASSIF PEAKS, THE 
VORTEX ALIGNMENT LIKELY TITLED AND INTERMITTENTLY DECOUPLED, 
CONTRIBUTING GREATLY TO THE SIGNIFICANT DROP IN INTENSITY OVER THE 
PREVIOUS 12 HOURS.  THE INTENSITY FORECAST HAS BEEN ADJUSTED 
ACCORDINGLY TO ACCOUNT FOR THE AFOREMENTIONED.

FORECAST DISCUSSION: TC 20S IS FORECAST TO BEGIN A SOUTHWARD TRACK
THROUGH TAU 24 AS IT ROUNDS THE WESTERN PERIPHERY OF STR TO THE EAST 
AND THE STEERING PATTERN BECOMES MORE DEFINED. AFTER TAU 24, TC 20S 
IS EXPECTED TO START ON A SOUTHEASTWARD TRACK AS THE NER PROPAGATES 
EASTWARD. THE SYSTEM IS ANTICIPATED TO EMERGE BACK OVER WATER BETWEEN 
TAU 36 AND TAU 48. TC 20S IS FORECAST TO MAINTAIN THE SOUTHEASTWARD 
TRACK THROUGH THE REMAINDER OF THE FORECAST PERIOD AS IT STARTS TO 
TRANSITION INTO THE BAROCLINIC ZONE. THE JTWC INTENSITY FORECAST HAS 
THE SYSTEM CONTINUING TO WEAKEN AS A RESULT OF FRICTIONAL EFFECTS AND 
DRY AIR OVER MADAGASCAR ENTERING THE SYSTEM THROUGH TAU 36. NEAR TAU 
48, THE SYSTEM IS FORECAST TO REINTENSIFY TO 45 KTS AS IT LOSES THE
LAND EFFECTS THAT ARE HINDERING IT AND SHEAR REMAINS RELATIVELY LOW
(15-20 KTS). TC 20S IS FORECAST TO MAINTAIN 45 KTS THROUGH TAU 72
AS VWS IS NOT EXPECTED TO REACH HIGH VALUES BY THAT TIME. AROUND
TAU 96, THE SYSTEM IS FORECAST TO BEGIN SUBTROPICAL TRANSITION
(WHICH WILL COMPLETE BY TAU 120) AND VWS VALUES WILL RISE TO NEAR
30 KTS ALONG WITH DECREASING RELATIVE HUMIDITY VALUES. IN RESPONSE,
THE JTWC INTENSITY FORECAST HAS TC 20S WEAKENING TO 40 KTS AT TAU
96 AND 35 KTS NEAR TAU 120. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
REGARDING TRACK ALIGNMENT THROUGH TAU 36 WITH A 95 NM CROSS-TRACK
SPREAD, WHICH INCREASES TO AROUND 250 NM AT TAU 72. MODELS CONTINUE
TO DISAGREE ON TRACK SPEED AFTER TAU 72, CAUSING SIGNIFICANT
ALONG-TRACK SPREAD. ECMWF IS AN OUTLIER IN GUIDANCE, SLOWING DOWN
AFTER TAU 72 VICE SPEEDING UP. AS A RESULT, THE JTWC FORECAST TRACK
IS PLACED WITH OVERALL LOW CONFIDENCE. INTENSITY GUIDANCE IS IN
GOOD AGREEMENT THROUGH TAU 24 WITH ALL MODEL WITH THE EXCEPTION OF
COAMPS-TC (NAVGEM BASED) QUICKLY WEAKENING THE SYSTEM TO AROUND 35
KTS. AFTER TAU 24, GUIDANCE SPREADS WITH VARYING AMOUNTS OF
INTENSIFICATION AS THE SYSTEM MAKES ITS WAY BACK OVER WATER. HWRF
HAS THE SYSTEM REACHING A PEAK INTENSITY OF 65 KTS AT TAU 60 WHILE
THE REST OF THE INTENSITY GUIDANCE RANGING FROM 30 TO 45 KTS
THROUGH THE REMINDER OF THE FORECAST INTERVAL. BECAUSE OF THE
SLIGHT CHANGE IN TRACK FORECAST CAUSING LARGE CHANGES IN INTENSITY,
THE JTWC INTENSITY FORECAST IS PLACED WITH OVERALL LOW CONFIDENCE.

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