The Storm Prediction Center has issued a "Particularly Dangerous Situation" Tornado Watch for eastern Iowa, southwestern Wisconsin and northwestern Illinois this afternoon.
It looks like it could be a rough day for severe weather in northern Illinois. The Storm Prediction Center has issued a relatively rare "high risk" for northern Illinois and Indiana, including the Rockford and Chicago areas.
However, that brings us to today's forecast. There's a very big consequence of the lack of storms this morning--the entire purpose of thunderstorms is to re-stabalize the atmosphere by convectively removing pre-existing instability. Had we had thunderstorms move through this morning, they probably would have done just that--cooled and dried out the lower levels of the atmosphere a bit and leave us less unstable today. But that didn't happen. Now we get another day of heating an already conditionally unstable airmass and this will destabilize the air even more...setting up today's potential for severe weather.
Let's look at the setup this morning. A low pressure center has been getting its act together in the central Plains all night. Here's the surface mesoanalysis from 12Z this morning:
You can click on that image to make it larger. One of the striking features of this sounding is actually the wind profile--almost uniform wind direction all the way through the troposphere. This enhances the possibility of these derecho/straight-line wind events as storms will quickly tend to organize in a line perpendicular to a uniform wind profile. The surface dewpoint this morning is in the low 60s in Davenport and predicted to rise. We also see relatively high dewpoints all the way up to around 850mb and near saturation above 500mb (that may have something to do with outflow aloft from the storms to the north. All in all this sounding shows a lot of moisture in the air--meaning heavy rain is definitely a possibility.
We also have a fair bit of instability. The sounding profile between about 800mb and 500mb shows a pattern indicative of what is called an "elevated mixed layer" (EML). This layer of the atmosphere is marked by extremely steep lapse rates (temperatures falling off rapidly with height--basically dry adiabatic) and well-mixed moisture content. This actually leads to this layer being relatively dry--you can see that the dewpoint is well below the temperature for much of this layer. The presence of an EML enhances the potential for very strong downdrafts--all that moisture being lofted in the thunderstorm can evaporate into that dry layer, causing a lot of cooling within that layer. This cooled air becomes negatively buoyant and sinks down to the ground, creating powerful downdrafts and organizing a "cold pool" beneath the storms. This kind of setup again favors organized, bow-echo sorts of storms.
Another concern is the potential for tornadoes. The wind profile in that sounding doesn't show a lot of low-level directional shear, but this will change as the surface low moves closer. Here's the surface analysis from 17Z this morning: