A major derecho or long-lived linear mesoscale convective system (MCS) event is expected today from eastern IA through northern IL from late afternoon through late evening, respectively. Extreme instability and good deep-layer shear in a classic northwesterly-flow situation will prevail over this region today. While the initial storm mode will likely be supercells over central IA, with the threat of VERY large hail, strong straight-line winds, and isolated brief tornadoes, the cells should quickly congeal into a monster MCS, which will rapidly propagate southeastward towards northern Illinois by late evening. Stay tuned for updates!