When the thunderstorm crossed the boundary, it turned east and tracked just north of it into Southwest Illinois. Tornadoes were reported in Johnson and Pettis counties before the storm entered the NWS St. Louis county warning area. A brief tornado also was reported 3 miles north of California in Moniteau county. Tornadoes were then reported near Fulton, Wright City, Wentzville, Lake St. Louis, and Granite City Illinois. Recent research on tornadoes has shown thunderstorms that interact with boundaries are more likely to produce tornadoes.
Boundaries are an added source of instability and rotation for tornadic thunderstorms.
See the surface map below for the location of the boundaries on this day. In addition to the tornadoes, this large thunderstorm produced widespread hail along its track with numerous reports of golfball to baseball sized hail in both Columbia and the northern suburbs of St. Louis. Many homes and businesses were damaged by the hail, as were 22 planes at Lambert International Airport. A low pressure center is located over western Kansas. A stationary front extends east from the low through Missouri and Illinois.