|D4 means Day 4, D5 means Day 5, etc.|
(I realize they say D4 and D5, But remember that Day 1 is today, day 2 is tomorrow, etc.)
Several models have combined to indicate a low pressure system will move through the area with a front. With strong lifting, convergence, instability combined, numerous storms are expected to possibly develop out ahead of the cold front. This will occur both days. Deep layer shear will also be a supporter of strong storms.
D6 which is not outlined is when the cold front will move into the Great Lakes. While there will likely be severe storms in that region, the models have diverged too much to outline a specific area at this time.