WDXS31 PGTW 260300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (TWENTY) WARNING 
NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.7S 50.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 418 NM NORTH-NORTHEAST OF ANTANANARIVO,
MADAGASCAR
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 20S NEARLY QUASI-STATIONARY (QS) IN A COMPETING STEERING
ENVIRONMENT CAUGHT AT A NEAR STANDSTILL BETWEEN A NEAR EQUATORIAL
RIDGE (NER) TO THE EAST, WEST, AND NORTH. PERSISTENT CONVECTION IS
BEGINNING TO CONSOLIDATE INTO AN UPPER-LEVEL MICROWAVE EYE-LIKE
STRUCTURE THAT WAS DEPICTED ON A 252147Z ATMS IMAGE. HOWEVER, THE
LOW-LEVEL IS COMPLETELY OBSCURED AND UNABLE TO BE IDENTIFIED. ON
THE UPPER-LEVEL, AN ANTICYCLONE HAS DEVELOPED OVER THE SYSTEM WITH
THE SUBTROPICAL JET TO THE SOUTH ENCOURAGING A STRONG POLEWARD
OUTFLOW CHANNEL. MINIMAL VERTICAL WIND SHEAR (VWS) OF 5-10KTS AND
SEA SURFACE TEMPERATURES OF 29-30C CREATE A POSITIVELY FAVORABLE
ENVIRONMENT FOR INTENSIFICATION. THE INITIAL POSITION WAS ASSESSED
WITH LOW CONFIDENCE BASED ON AN EXTRAPOLATION FROM THE PREVIOUSLY
MENTIONED ATMS IMAGE AND ANIMATED EIR IMAGERY. THE INITIAL
INTENSITY WAS ASSESSED WITH LOW CONFIDENCE BASED ON AGENCY AND
OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT.

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

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 20S IS FORECAST TO REMAIN QS FOR THE NEXT
12 HOURS BEFORE BEGINNING TO SLOWLY CURVE TO THE SOUTH-SOUTHWEST AS
IT ROUNDS THE WESTERN PERIPHERY OF THE NER TO THE EAST. FROM TAU
48-120, THE SYSTEM BEGINS TO CHANGE DIRECTION TO THE SOUTHEAST ON
THE WESTERN PERIPHERY OF THE NER WHICH GRADUALLY MOVES EAST. AS THE
SYSTEM TRAVELS SOUTH, IT IS EXPECTED TO BEGIN SUBTROPICAL
TRANSITION AT TAU 120 DUE TO COOLER SSTS, INCREASED VWS, AND CLOSE
PROXIMITY TO THE JET MAXIMUM. IN THE INTENSITY FORECAST, THERE IS A
MODERATE LIKELIHOOD OF RAPID INTENSIFICATION (RI) TO 95KTS BETWEEN
TAU 60-72 DUE TO CONSISTENTLY LOW VWS, STRONG DIVERGENCE ALOFT, AND
FAVORABLE SSTS THROUGH TAU 60. THE PRIMARY MITIGATING FACTOR TO
EVEN MORE SIGNIFICANT INTENSIFICATION IS THE PROXIMITY TO LAND OVER
THE FIRST 36 HOURS, WHICH HAS THE POTENTIAL TO INTRODUCE DRY AIR
FROM THE MOUNTAINOUS TERRAIN OF NORTHERN MADAGASCAR AND FRICTION
FROM TOPOGRAPHY. AN INCREASE IN VWS TO 20-25KTS FROM TAU 72-120 IS
EXPECTED TO CAUSE THE VORTEX TO SHALLOW AND BECOME EMBEDDED INTO A
JET MAXIMUM AS THE SYSTEM TRAVELS SOUTH, DRASTICALLY REDUCING
OUTFLOW AND ALLOWING FOR THE SYSTEM TO WEAKEN GRADUALLY.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
IN THE NEAR TERM BUT CROSS-TRACK SPREAD BEGINS TO OPEN TO 250NM BY
TAU 96. THERE BEGINS TO BE DEVIATIONS IN THE TRACK SPEED AT THAT
SAME TIME, WITH GFS INCREASING TRACK SPEED RAPIDLY AND UKMET TAKING
A MUCH SLOWER PATH LIKELY DUE TO DISAGREEMENT IN HOW SIGNIFICANTLY
THE VORTEX WILL BE SHEARED AND SHALLOW BY THE END OF THE FORECAST
PERIOD. THE INTENSITY GUIDANCE HAS A LARGE SPREAD BUT GENERALLY
FORECASTS A PEAK BETWEEN 80-100KTS AT TAU 48. THE JTWC FORECAST
PREDICTS A PEAK OF 95KTS BETWEEN TAU 60-72 DUE TO FAVORABLE
OUTFLOW, SSTS, AND VWS. POTENTIAL INTERACTION FROM LAND ALSO WAS
TAKEN INTO ACCOUNT EXPECTING THAT THE PEAK WILL OCCUR ONCE TC 20S 
MOVES AWAY FROM THE COAST OF MADAGASCAR. 

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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