SPC talking about a significant severe Derecho (Chicago)
Posted by JAC on 6/17/2009, 8:30 am



DAY 3 CONVECTIVE OUTLOOK 
  NWS STORM PREDICTION CENTER NORMAN OK
  0224 AM CDT WED JUN 17 2009
 
  VALID 191200Z - 201200Z
 
  ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE MID MS VALLEY TO THE
  OH VALLEY...
 
  ...MID MS/OH VALLEY REGION...
 
  SHORT RANGE MODEL GUIDANCE IS IN GENERAL AGREEMENT WITH STRONG
  SHORTWAVE TROUGH THAT WILL DIG INTO THE MID MS VALLEY REGION EARLY
  IN THE DAY3 PERIOD.  STRONGEST MID LEVEL FLOW IN ASSOCIATION WITH
  THIS FEATURE TRANSLATES INTO THE DOWNSTREAM SIDE OF THE TROUGH AND
  SHOULD EXTEND FROM MO...NEWD INTO OH WITH SPEEDS APPROACHING 70KT BY
  20/00Z OVER IL.  ALTHOUGH FLOW IS EXPECTED TO STAY SOMEWHAT VEERED
  THROUGH A DEEP LAYER...STRONG DEEP LAYER SHEAR ALONG/AHEAD OF
  ADVANCING COLD FRONT STRONGLY FAVORS ORGANIZED SEVERE THUNDERSTORMS
  ACROSS PORTIONS OF THE MID MS VALLEY...EWD ACROSS THE ENTIRE OH
  VALLEY REGION BY LATE EVENING.  FORECAST SOUNDINGS ACROSS IL/IN ARE
  QUITE IMPRESSIVE WITH MLCAPE VALUES ON THE ORDER OF 3500 J/KG WITH
  SFC-6KM FLOW ON THE ORDER OF 50KT.  ADDITIONALLY...VERY HIGH
  PRECIPITABLE WATER VALUES...ROUGHLY 1.75-2.00 INCHES...WILL ENSURE
  THE POTENTIAL FOR HEALTHY UPDRAFTS WITHIN THIS STRONGLY SHEARED
  ENVIRONMENT.  GIVEN THAT LARGE SCALE ASCENT WILL OVERSPREAD AN
  INCREASINGLY BUOYANT PRE-FRONTAL BOUNDARY...ORGANIZED SEVERE
  THUNDERSTORMS SHOULD EASILY DEVELOP BY 18Z THEN SPREAD RAPIDLY ESEWD
  WITHIN MEAN FLOW APPROACHING 50KT.  DAMAGING WINDS APPEAR LIKELY IF
  THIS SCENARIO UNFOLDS WITH THE POTENTIAL FOR SIGNIFICANT
  SEVERE...POSSIBLY A DERECHO.
 
  ..DARROW.. 06/17/2009
16
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Possible Severe Weather this Weekend for Great-Lakes & Ontario - JAC, 6/15/2009, 7:29 am
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