As heat records continue to break in the U.S., this week has already brought the threat of derechos. That’s left many people listening to weather reports wondering what the heck a derecho is.
We’re here to help.
Derechos are widespread, long-lived wind storms, according to the National Weather Service. They’re associated with bands of fast-moving showers or thunderstorms.
Typically, these storms only occur in North America, per the National Oceanic and Atmospheric Administration, but they have occasionally been reported in other places, including Germany. In Bangladesh and nearby parts of India “Nor’wester” storms behave similarly to something called a “progressive derecho.”
NOAA explained that progressive derechos are “generally only warm season events and are found moving over the north side of upper atmosphere high pressure ridges.” Another type of derecho is the “serial derecho” – described by NOAA as “primarily cool season events,” though they also can occur in the warm season.
“They occur between the upstream trough and downstream ridge. Approximately 40% of all derechos are of the serial type,” said NOAA.
As for progressive derechos, when the jet stream moves toward the North Pole along with the storm track during the Northern Hemisphere shift from spring to summer, pressure patterns move and a ridge of high pressure is positioned over the northern plains and upper mid-west region of the country. NOAA said this location experiences the majority of derechos each year.
“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,” NOAA said. “The other warm season axis extends from the southern Plains into the mid-Mississippi Valley.”
It also said that once a derecho occurs more will likely follow. They don’t always pop up in the same location, however.
This Monday, USA Today warned of a possible derecho across the Midwest, East and South as nearly 200 million Americans were under an extreme heat warning. FOX Weather reported Tuesday morning that a “destructive derecho,” did indeed whip through portions of the northern Plains and Upper Midwest late Monday and early Tuesday, knocking out power for tens of thousands of people. In addition to FOX Weather, AccuWeather said Tuesday that, as its meteorologists predicted, thunderstorms that developed over Montana and the Dakotas Monday afternoon congealed into a derecho “charging through portions of South Dakota and Iowa overnight.”
“Although a derecho can produce destruction similar to the strength of tornadoes, the damage typically is directed in one direction along a relatively straight swath,” said the National Weather Service. “As a result, the term ‘straight-line wind damage’ sometimes is used to describe derecho damage. By definition, if the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho.”
AccuWeather said that the derecho generated reports of wind gusts over 80 mph (up to 99 mph in Sioux Center, Iowa) and that a complex of thunderstorms were expected to continue early Tuesday morning impacting the morning commutes in Chicago, Ill. and Milwaukee, Wisc. It also said that extreme heat over the south-central U.S. and moisture would help to spark severe thunderstorms through the early week.
“The daily risk for severe thunderstorms across the Plains and Midwest will continue Tuesday afternoon and night stretching from Wyoming to portions of far western Iowa and Missouri as the front continues to move southward,” AccuWeather said. By Wednesday, the front is expected to move south and east and the risk of severe weather will continue Thursday in parts of the mid-Atlantic and the Carolinas.
According to a Tuesday report from the National Weather Service, there are risks of severe weather throughout the country this week, including thunderstorms and high heat.