Sources: SciJinks, The Weather Channel, CNN, NOAA

From the Spanish word for "straight," these windstorms leave wide, long areas of straight-line wind damage. The winds can be as strong as 60 to 100 mph or higher in extreme cases. To be called a derecho, the thunderstorm-generated high winds and wind damage should cover a distance of at least 400 miles long and be at least 60 miles wide.

The occurrence of derechos is divided into two seasons: the "warm" season - May, June, July, and August - during which 70% of all derechos occur, and the "cool" season, comprising the remaining eight months.

Progressive derechos are generally only warm season events and are found moving over the north side of upper atmosphere high pressure ridges (image below). In meteorology, they are said to move "over the top" of the ridge.

Serial derechos, while not rare in the warm season, are primarily cool season events. They occur between the upstream trough and downstream ridge. Approximately 40% of all derechos are of the serial type.

Derechos in the United States typically occur along two axes. One axis extends along the "Corn Belt" from the upper Mississippi Valley into the Ohio Valley. The other warm season axis extends from the southern Plains into the mid-Mississippi Valley. During the cool season, derechos are most likely to occur from eastern Texas into the southeastern states. Although derechos are extremely rare west of the Great Plains, isolated derecho events have occurred in the interior portions of the western United States, especially during spring and early summer.

To meteorologists, they have a distinct look on radar, usually as one or more curved lines of thunderstorms known as a bow echo.

These storms are a particular threat in parts of the Great Plains and Midwest. According to a 2016 study, a distinct corridor from the upper Mississippi Valley through the Ohio Valley is most susceptible to derechos from May through August. Parts of Minnesota, Wisconsin, Iowa, Illinois, Indiana, southern Michigan, Ohio, Kentucky and the Appalachians averaged one to two progressive derechos each May-August over the 18-year period from 1996 through 2013, consistent with prior studies. They can occur during the daytime or in the overnight and early-morning hours, when most people are sleeping.

This animation from NOAA's GOES-East satellite shows lightning from the storms in a derecho moving across the Midwest on August 10, 2020. More than 1 million homes and businesses lost power due to the storms. Credit: NOAA