Storm systems brewing over the upper Mississippi Valley late Wednesday afternoon may spawn the season's first so-called "derecho" storm stretching about 400 miles in length, said meteorologists with the National Oceanic and Atmospheric Administration (NOAA).
Derecho -- pronounced "de-reh-choh" and which is Spanish for "straight" -- is a special designation for a fast-moving line of thunderstorms stretching at least 400 kilometers (249 miles) in length. The storms typically pack tropical storm-force or higher winds.
A second system forecast for Wednesday evening over Mississippi and the Ohio Valley region could reach near-derecho proportions, with a squall line estimated to grow to about 300 kilometers (186 miles), research meteorologist Ken Pryor, with NOAA's weather and climate prediction center in College Park, Md., told Discovery News.
"You can think of a derecho as analogous to a tropical cyclone over land. The impacts are very similar. There are damaging winds that cover a significant area," Pryor said.
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.
"We're considering here the development of two separate storm systems, but both of those have the potential to become derechos," he added.