Possibility of severe thunderstorms downstate overnight

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The National Storm Prediction Center has issued a Mesoscale Outlook detailing reasons why severe thunderstorms could develop overnight across southern and central portions of Illinois, including the southernmost portion of the Chicago area well south of Interstate-80 (escalloped area shown on the headlined map).

The technical discussion follows below…

Mesoscale Discussion 1305
   NWS Storm Prediction Center Norman OK
   1040 PM CDT Thu Aug 16 2018

   Areas affected...Parts of east central Missouri and southern/central

   Concerning...Severe potential...Watch unlikely 

   Valid 170340Z - 170545Z

   Probability of Watch Issuance...20 percent

   SUMMARY...Isolated to widely scattered strong to severe storm
   development is possible through Midnight to 2-3 AM CDT.  While the
   severe weather threat seems limited enough that a watch is not
   currently anticipated, trends will continue to be monitored.

   DISCUSSION...A mid-level closed low now over central Iowa is
   forecast to gradually turn eastward into the middle Mississippi
   Valley through 06-09Z.  As it does, it appears that associated
   forcing for ascent will continue to support scattered vigorous
   thunderstorm activity, which is already ongoing in an arcing band
   across northern through west central Illinois.  Ahead of this band,
   the boundary layer remains relatively warm with surface temperatures
   in the upper 70s to around 80f in a narrow corridor, from around the
   St. Louis area northeastward through central Illinois.  Coupled with
   dew points in the lower/mid 70s, this appears to be supporting
   moderate to large CAPE on the order of 2000-3000 J/kg.  

   Although shear is still somewhat modest, the eastward progression of
   a belt of 30-40 kt flow in the 700-500 mb layer may contribute to at
   least some strengthening before the mid-level forcing and associated
   convection progresses through the lingering instability axis
   overnight.  This could be accompanied by at least a short-lived
   period with some increase in potential for storms capable of
   producing severe hail and perhaps locally strong surface gusts.

   ..Kerr/Thompson.. 08/17/2018
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