Track shifting west
Posted by JAC on 9/16/2009, 1:55 pm
12 HOUR SUMMARY AND ANALYSIS.
      A. SUPER TYPHOON (STY) 15W (CHOI-WAN), LOCATED APPROXIMATELY
335 NM SOUTH OF IWO TO, HAS TRACKED WEST-NORTHWESTWARD AT 07 KNOTS
OVER THE LAST 06 HOURS. RECENT ANIMATED INFRARED SATELLITE IMAGERY
SHOWS THAT STY 15W HAS MAINTAINED A WELL DEFINED EYE OVER THE PAST
12 HOURS, WITH AN IMPRESSIVE SYSTEM OF DEEP CONVECTIVE BANDING
WRAPPING INTO THE CENTER. INTENSITY ESTIMATES HAVE REMAINED AT 7.0
FROM PGTW, KNES AND RJTD OVER THE PAST 12 HOURS. STY 15W HAS
MAINTAINED A STEADY WEST-NORTHWESTWARD TRACK ALONG THE SOUTHWESTERN
PERIPHERY OF A MID-LEVEL SUBTROPICAL RIDGE (STR) OVER THE PAST 12
HOURS. THE FORECAST HAS SHIFTED MORE WESTWARD IN THE INITIAL 24
HOURS THAN THE PREVIOUS FORECAST TO ACCOUNT FOR THE MORE WESTWARD
TRACK. TRACK SPEEDS HAVE REMAINED FAIRLY CONSISTENT OVER THE PAST 12
HOURS, RANGING FROM 08 TO 10 KNOTS. A TROPICAL UPPER TROPOSPHERIC
TROUGH CELL TO THE NORTHEAST HAS CONTINUED TO PROVIDE GOOD EASTWARD
OUTFLOW, IN ADDITION TO AN UPPER LEVEL POINT SOURCE ANTICYCLONE OVER
STY 15W PROVIDING STRONG RADIAL OUTFLOW OVER THE PAST 24 HOURS. THE
MULTIPLE OUTFLOW MECHANISMS IN CONCERT WITH HIGH VALUES OF OCEAN
HEAT CONTENT AND SEA SURFACE TEMPERATURES HAVE ALLOWED FOR STY 15W
TO MAINTAIN INTENSITY OVER THE PAST 12 HOURS.
3. FORECAST REASONING.
   A. THE FORECAST PHILOSOPHY HAS CHANGED SLIGHTLY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE, WITH THE FORECAST TRACK
REMAINING MORE WESTWARD IN THE EARLY PORTION OF THE FORECAST.
   B. STY CHOI-WAN WILL CONTINUE TO TRACK WEST-NORTHWESTWARD ALONG
THE SOUTHWESTERN PERIPHERY OF THE STR MENTIONED IN PARA 2.A THROUGH
TAU 24. AN APPROACHING MID-LATITUDE TROUGH WILL HELP TO RE-ALIGN THE
STR AND CREATE A WEAKNESS TO THE NORTH OF STY 15W, CAUSING A MORE
NORTHWARD TRACK AROUND THE WESTERN EXTENT OF THE STR. AS STY 15W
MOVES NORTHWARD, INCREASED INTERACTION WITH THE MID-LATITUDE
WESTERLIES WILL BEGIN TO WEAKEN CHOI-WAN AND CAUSE A MORE
NORTHEASTWARD TRACK. ADDITIONALLY, THE INCREASED GRADIENT FLOW WILL
LEAD TO TRACK SPEEDS INCREASING THROUGH TAU 72. EXTRATROPICAL
TRANSITION WILL BEGIN AROUND TAU 72. MODEL GUIDANCE SHOW A WIDE
VARIABILITY IN THE INITIAL FORECAST PERIOD, AND THE CURRENT FORECAST
IS BASED ON PERSISTENCE, FAVORING EGRR AND ECMF AIDS, WHICH HAVE A
LATER RECURVATURE.
    C. BEYOND TAU 72, MODEL GUIDANCE COMES INTO VERY GOOD AGREEMENT
WITH THE A NORTHEASTWARD TRACK AHEAD OF AN APPROACHING MID-LATITUDE
TROUGH, AS STY 15W BEGINS EXTRATROPICAL TRANSITION AROUND TAU 72.
STY 15W WILL HAVE A STEADY INCREASE IN TRACK SPEED AND COMPLETE
EXTRATROPICAL TRANSITION BY TAU 120.//
NNNN
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TY 0914 (Choi-wan), Issued at 00:45 UTC, 16 September 2009 - hanna, 9/16/2009, 1:26 pm
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