Severe Weather Threat Rapidly Increasing Tonight

A brand new severe thunderstorm watch has just been posted for much of Iowa and Northern Illinois. Strong to severe clusters of thunderstorms are developing behind a strong warm front and an area of strong convergence. With a hot and humid air mass in place, the atmosphere is primed for further development this evening and into the overnight. The severe thunderstorm watch is in effect until 11:00pm CDT.

Below are the current CAPE values, basically a general telling of the amount of energy a T-storm has to work with at the given time. It can be increased by surface warming and environmental factors.

Below is current (5:00CDT) Bulk-Richardson number across the Midwest, a basic composite value for severe weather.

Below is the Craven-Brooks severe weather composite value.
It is the product of 100mb MLCAPE and 0-6km magnitude of the vector difference (m/s; often referred to as "deep layer shear") accounts for the compensation between instability and shear magnitude. Using a database of about 60,000 soundings, the majority of significant severe events (2+ inch hail, 65+ knot winds, F2+ tornadoes) occur when the product exceeds 20,000 m3/s3.