Heavy Shear over Japan
Posted by JAC on 10/3/2009, 10:35 pm




WDPN34 PGTW 031500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (MELOR) WARNING NR 18//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
  A. TYPHOON (TY) 20W (MELOR), LOCATED APPROXIMATELY 80 NM
NORTH-NORTHWEST OF SAIPAN, HAS TRACKED WESTWARD AT 13 KNOTS
OVER THE PAST SIX HOURS. AT 030924Z, THE HIGHEST SUSTAINED
WINDS OUT OF SAIPAN (PGSN) WERE LOGGED AT 27 KNOTS, GUSTING
TO 37 KNOTS, AND THE HIGHEST REPORTED GUSTS, 43 KNOTS,
OCCURRED AROUND 030911Z. BASED ON A PARTIAL CLOUD-FILLED EYE
IN ANIMATED INFRARED SATELLITE IMAGERY AND WFO PGUM RADAR
ANALYSIS FIXES, THE LOW LEVEL CIRCULATION CENTER IS DETER-
MINDED TO BE DOWNSTREAM OF SAIPAN. THE INITIAL FORECAST
INTENSITY OF 115 KNOTS IS BASED ON DVORAK SATELLITE INTENSITY
ESTIMATES FROM PGTW AND RJTD. THE TYPHOON HAS CONTINUED TO
TRACK GENERALLY WESTWARD BETWEEN 10 AND 13 KNOTS ALONG THE
SOUTHERN PERIPHERY OF THE MID TO UPPER-LEVEL SUBTROPICAL
RIDGE TO THE NORTH.
3. FORECAST REASONING
  A. THE FORECAST REASONING HAS NOT CHANGED SIGNIFICANTLY
SINCE THE PREVIOUS PROGNOSTIC REASONING BULLETIN.
  B. TY 20W WILL CONTINUE TO TRACK TO THE NORTHWEST UNDER
THE INFLUENCE OF THE SUBTROPICAL RIDGE, AND WILL BEGIN TO
TURN PROGRESSIVELY POLEWARD BEYOND TAU 36. A STRONG
BAROCLINIC BOUNDARY TO THE NORTH, AND SUPPORTING UPPER
LEVEL, LONGWAVE TROUGH, WILL SIGNIFICANTLY WEAKEN THE
RIDGING ALLOWING FOR RECURVATURE (AND EXTRATROPICAL
TRANSITION) INTO THE MID-LATITUDE WESTERLIES. THE TYPHOON
WILL ALSO HELP TO ERODE THE RIDGE. RECURVATURE IS EXPECTED
TO BEGIN AROUND 30N AND EXTRATROPICAL TRANSITIONING WILL
BEGIN NEAR TOKYO. A 030000Z UPPER AIR SOUNDING FROM
47909 (28.4N 129.5E) INDICATES VERY STRONG WESTERLIES
EXTENDING FROM THE LOW TO UPPER LEVELS OF THE ATMOPSHERE,
WITH WINDS IN EXCESS OF 50 KNOTS BETWEEN 300 AND 200 MB.
THE ATMOSPHERE ALSO DRIES OUT CONSIDERABLY ABOVE 800 MB.
TIMING OF RECURVATURE, AND EXTRATROPICAL TRANSITION, ARE
CONSISTENT WITH THIS UPPER AIR SOUNDING.
  C. THE SYSTEM WILL BEGIN TO ACCELERATE NORTHEASTWARD AS
IT ROUNDS THE SUBTROPICAL RIDGE AXIS TOWARDS THE MID-
LATITUDES. AGAIN, STRONG WESTERLIES ALOFT, EXTENDING AS FAR
SOUTH AS 28N, WILL PROMOTE SUCH MOTION. EXTRATROPICAL
TRANSITION WILL ALSO BE INITIALIZED BY DRIER AIR AND
EXCESSIVE VERTICAL WIND SHEAR BETWEEN TAU 72 AND 96, AND
WEAKENING IS EXPECTED TO BEGIN AS EARLY AS TAU 48. THE
SYSTEM WILL BE COMPLETELY EXTRATROPICAL BY TAU 120. ALL THE
AVAILABLE CONSENSUS MEMBERS EXCEPT JGSM ARE FORECASTING A
RECURVATURE SCENARIO, AND KEEP THE SYSTEM OVER OPEN WATER
AND SOUTH OF THE KANTO PLAIN.
//
NNNN
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Melor the Monster - a solid Cat5, 914mb and dropping - JAC, 10/3/2009, 9:25 pm
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