From the NWS:
Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1043 AM CDT Thu Aug 10 2017 .UPDATE... 1043 AM CDT Primary concern this morning is with convective/severe potential this afternoon. A well defined upper low can be seen GOES-16 water vapor imagery this morning moving east across central MN and into western WI. Seasonably strong mid-upper level jet can also be seen on water vapor imagery this morning with RAP analysis depicting speed max of >50kt over northern IA at 6km and over 80kt at 250mb. As the upper low moves east this afternoon the associated speed max will overspread our area resulting in some fairly impressive deep layer shear profiles (>40kt 0-6km) for this time of year. Visible satellite imagery shows very little in the way of cloudiness over the immediate area, which should allow for strong heating of the boundary layer today. Moisture isn't great for this time of year, but mid 60F dewpoints pooling along and ahead of the approaching front should be sufficient for MLCAPE values climbing solidly over 1000 j/Kg this afternoon, despite the fairly weak mid level lapse rates. Combination of weak frontal forcing and DCVA in advance of the upper low should provide sufficient for convective initiation this afternoon over southern WI/northwest IL/eastern IA. Various convective allowing models are favoring 19-21z time frame for convective initiation. Once storms develop, the combination of moderate instability and strong deep layer shear profiles favors supercells, which could eventually morph into line segments. Weak low level shear and fairly high LCL heights suggests tornado threat will be minimal, but strong shear could augment for the weaker lapse rates and allow for large hail (possibly up to golf ball size) to be a big threat from the storms. In addition, a localized damaging wind damage threat could develop given the fairly large DCAPE values. Convection should be fairly progressive and probably clear our southeast CWA by around midnight. The severe threat should begin to wane this evening as boundary layer cools/instability weakens. Have updated hourly pop/wx grids to fine tune the timing just a bit and lowered sky cover early in the day, otherwise no big changes with the morning update.