Derechos are rare west of the Rocky Mountains and in central and south Florida. Over the Mississippi and Ohio valleys, the storms form once or twice a year, most often in June or July. The Atlantic Coast region, east of the Appalachian Mountains from southern New England to northern Florida, experiences a derecho about once every two to four years.
One of the largest derechos on record occurred in July 2011 with a squall line that stretched from the Dakotas and Nebraska in the west to Maryland and Virginia in the east.
Wednesday's storms are expected to begin about 6 p.m. EDT and extend to about 2 a.m. on Thursday. Meteorologists are forecasting wind gusts up to 75 mph.
"It is possible that these two systems may merge, but the (computer) models are indicating that one will track over the lower Great Lakes region, from southern Wisconsin and northern Illinois across Michigan, and then the models are indicating the possible development of a new convective storm system over eastern Ohio that will track southeastward across the Appalachians," Pryor said.