A significant severe weather outbreak, including the potential for nighttime tornadoes, is expected Friday night from parts of the lower and mid-Mississippi Valley into the Tennessee and lower Ohio valleys. A potent upper-level disturbance punching out of the Rockies and into the Plains on Friday will enter the Mississippi Valley by Friday night. This will cause a surface low-pressure system to intensify as it tracks into the Great Lakes region overnight, pushing a strong cold front across the central U.S. from the Ohio Valley to East Texas. Warm, humid air from the Gulf of Mexico will surge northward out ahead...