Severe weather possible later this afternoon and evening… Tornado watch extremely likely to be issued for portions of the Chicago area

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Conditions are ripe for severe weather development across the Chicago area this afternoon. The latest mesoscale discussion from the Storm Prediction Center in Norman, Oklahoma indicates that it is almost a certainty that a tornado watch will be issued.

Mesoscale Discussion 0831
   NWS Storm Prediction Center Norman OK
   1250 PM CDT Mon May 27 2019

   Areas affected...portions of eastern IA...northern and central IL
   and northern IN

   Concerning...Severe potential...Tornado Watch likely 

   Valid 271750Z - 271915Z

   Probability of Watch Issuance...95 percent

   SUMMARY...Severe thunderstorms with a threat of tornadoes (some
   potentially strong), large hail and damaging winds will increase
   over the next couple of hours. Watch issuance is likely.

   DISCUSSION...A warm front is rapidly lifting northward and draped
   across northern IL into southern/central IN as of 17z. Dewpoints
   will continue to climb into the mid and upper 60s this afternoon and
   MLCAPE values will steadily increase to around 1500-2000 J/kg over
   the next 2-4 hours. A shortwave trough currently migrating
   east/northeast across IA/MO will provide large-scale forcing, while
   a couple of MCVs from morning convection will result in more subtle
   forcing across the region. Areas downstream of these MCVs as well as
   along the warm front will be a focus for thunderstorm development
   and organization in the next 1-2 hours from eastern IA into northern
   and central IL. Additional storms are expected to develop along the
   northward advancing warm front across central into northern IN. The
   timing of convective initiation further east across IN is a bit more
   uncertain, but severe storms are expected by 21-22z.  

   18z RAOB from DVN showed capping just below 850mb. With the incoming
   shortwave, temperatures aloft are expected to cool and with
   continued heating and moist advection, this cap is expected to
   quickly erode by mid afternoon. In fact, 18z RAOB from ILX is
   uncapped with steep midlevel lapse rates noted in addition to steep
   low level lapse rates and a mean mixing ratio approaching 14 g/kg.
   This thermodynamic environment combined with favorable vertical
   shear will support supercells. The very moist boundary layer
   combined with ideal low level hodographs (noted in DVN RAOB) should
   lead to tornado potential supportive of at least a couple of strong

   A tornado watch will likely be needed by 19z across parts of far
   eastern IA into northern/central IL. Uncertainty in timing further
   east into IN may result in later tornado watch issuance, but expect
   additional watch issuance by 21z.



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