The NAM model is forecasting the jet stream (300mb level) to be screaming at over 100mph when the severe storms initiate this evening. To give you an idea of how intense that is, the April 2011 tornado outbreak also involved a jet stream over 100mph. As the system digs into the south Plains, it will try to break up the jet stream, making a rubber band-type situation. As the force (the low pressure system) digs farther into the rubber band (jet stream), the stress (wind speed) on the rubber band will increase, thus setting the stage for even stronger storms as the wind speeds increase.
Here's the NSSL's WRF forecast for the same timeframe as the jet stream image above.
The images cooperate nicely, with the jet stream enhancing the storms blowing up in Illinois.
We will have much more on the developing situation. Stay with The Weather Centre for the latest severe weather updates on this system.