Afternoon heating triggers widely-scattered showers/thunderstorms over northern Illinois – few strong storms possible

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As temperatures warm into the middle 80s this afternoon, building cumulus clouds have continued to develop into widely scattered showers/thunderstorms over northern Illinois. Because of the unseasonable heating and the unstable atmosphere, a few of the strongest storms could produce brief downpours and  damaging winds. Individual storm cells will be moving north-northeast at 30 to 40 miles per hour.

The National Storm Prediction Center has issued a Mesoscale Discussion for this area (escalloped area outlined on the headlined map) posted below – they do not anticipate issuing a Severe Thunderstorm Watch unless overall storm activity intensifies.

Mesoscale Discussion 1551
   NWS Storm Prediction Center Norman OK
   0245 PM CDT Tue Oct 09 2018

   Areas affected...Far east Iowa...far northwest Illinois...southern

   Concerning...Severe potential...Watch unlikely 

   Valid 091945Z - 092145Z

   Probability of Watch Issuance...20 percent

   SUMMARY...Isolated damaging wind gusts and possibly a brief tornado
   may accompany the stronger storms. A WW issuance is not expected at
   this time.

   DISCUSSION...Isolated, small, relatively discrete storms have
   developed just south of a differential heating zone within the last
   1-2 hours, where temperatures have climbed into the 80s. A few of
   these storms have exhibited occasional low-level rotation (as was
   the case recently in Dubuque County, Iowa). 

   With mid 60s dewpoints and modest lapse rates in place, up to 1000
   J/kg MLCAPE has recently been realized across the discussion area.
   Still, buoyancy is rather modest, as is deep-layer shear, with 35
   knots of effective bulk shear noted. Deep layer ascent and stronger
   low-level shear are currently located well upstream of the region
   and are not expected to arrive until later this afternoon/early
   evening. As such, in the short term, a few of the stronger cells may
   become organized, perhaps with brief periods of moderate to strong
   low-level rotation. A few marginally severe wind gusts may occur and
   a brief tornado cannot be ruled out.

   Given the isolated and marginal extend of the short term severe
   threat, a WW issuance is not expected at this time, though
   convective trends will be monitored for any increase in threat.
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