Severe weather expected to develop this evening- west and south of the Chicago Metro area

Mesoscale Discussion 1717
   NWS Storm Prediction Center Norman OK
   0346 PM CDT Mon Aug 12 2019

   Areas affected...Central/southeast IA...Northern IL...Far northeast
   MO

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

   Valid 122046Z - 122215Z

   Probability of Watch Issuance...80 percent



   SUMMARY...Thunderstorms capable of primarily damaging winds and a
   few tornadoes are possible late this afternoon into the evening.
   Tornado watch issuance is likely prior to 23Z.

   DISCUSSION...An MCV is currently moving through the upper
   Mississippi Valley, with a weakening area of primarily stratiform
   rain noted across northeast IA into northern IL. In the wake of the
   MCV, clearing has been noted across central into southeast IA, with
   cumulus gradually increasing along an outflow/differential heating
   boundary from west-central IL into southeast IA. Lapse rates are
   generally weak across this region, but continued heating of a very
   moist low-level airmass will result in the development of moderate
   to locally strong buoyancy (MLCAPE of 1500-3000 J/kg) from southeast
   IA into northwest/central IL. 

   Forcing for ascent will be somewhat nebulous in the wake of the
   leading MCV, but scattered thunderstorm development is eventually
   expected in the vicinity of the surface boundary and a weak surface
   low across central IA. Area VWPs and the 20Z ILX sounding depict
   sufficient deep-layer shear for organized convection, and initial
   development will include the potential for a few discrete
   supercells. Some increase in low-level flow/shear is expected with
   time into the evening, and a tornado threat will evolve with any
   supercell, particularly in the vicinity of the surface boundary
   which may move slowly northward over the next 2-3 hours. A strong
   tornado will be possible if any supercell can become sustained in
   the zone of enhanced low-level helicity near and just north of the
   boundary. Otherwise, locally severe wind gusts and marginally severe
   hail will be possible with any discrete convection. Most HREF
   members depict some sort of upscale growth this evening into
   portions of central IL, which would pose an increasing risk of more
   widespread damaging winds and potentially QLCS tornadoes. 

   Tornado watch issuance by 23Z is likely in order to cover these
   threats, though earlier watch issuance is possible if deep
   convective initiation appears imminent.
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