What you need to know about derechos, an uncommon and destructive weather event
What is a derecho? A derecho is a long-lived line of storms that often produces extreme wind damage, said Gino Izzi, a meteorologist at the National Weather Services office in Chicago. Derechos can last eight hours or longer and travel hundreds of miles (kilometers) across the country with winds typically gusting over 60 mph (97 kmh). There's been instances where derechos have moved from Iowa all the way to Washington D.C., said Izzi. Because of the intense straight-line winds and vast distances a derecho can travel, the damage they cause is sometimes comparable to the destructive forces of tornadoes or hurricanes.
Hurricanes are low-pressure systems that form over tropical or subtropical ocean waters and have organized thunderstorm activity according to the National Oceanic and Atmospheric Administration. Unstable air that is hot and humid is supportive of derecho formation. Derechos can happen any time of year but July and early August are the prime time for this type of extreme weather, said Izzi. Derechos are an uncommon weather event and usually a small number occur each year, but Izzi said several years could pass without a recorded derecho.
There are different types of derechos. Serial derechos tend to form with large powerful storms and typically occur in early spring, fall, and early winter. Progressive derechos are usually smaller and tend to form in summer as they feed off of hot, humid conditions. Hybrid derechos have characteristics of both serial and progressive types. Conditions that are favorable for derechos can also lead to small embedded tornadoes which are violently rotating columns of air that extend down from a thunderstorm and reach the ground. Derechos tend to produce much more widespread damage than tornadoes because they can produce a swath of damage that exceeds 100 miles (160 kilometers) in width whereas the damage from violent tornadoes is typically less than a mile (1.6 kilometers) wide.
Safety tips include using a radio to listen to the forecast, monitoring devices for severe weather alerts from the NWS, and having access to a sturdy shelter. It can go from nice calm weather to 100 miles per hour winds blowing down trees in literally a matter of minutes, said Izzi.
Where do derechos form? Derechos can happen almost anywhere in the US but are most common in the central and eastern regions of the United States. Progressive derechos tend to favor the northern and central Plains eastward into the Midwest, Great Lakes, and Ohio Valley area. That is because domes of heat can form across central parts of the US and derechos tend to form on the northern and northeastern flanks of the dome where there are high levels of atmospheric instability. This also tends to be where the jet stream winds are stronger. Humidity from crops like corn, also known as corn sweat, also magnifies how unstable the hot humid air is.
A 2003 derecho traveled from Arkansas through several southern states including Alabama, Georgia, and South Carolina. Two people died and 11 were hurt. A 2009 storm dubbed a 'Super Derecho' by meteorologists traveled from western Kansas to eastern Kentucky. It caused several deaths and injuries and more than 500 million in damages by the time it had traveled more than 1000 miles (1600 kilometers). A 2020 derecho that traveled from eastern Nebraska across Iowa and parts of Wisconsin and Illinois reached wind speeds of a major hurricane. The National Weather Services Storm Prediction Center reported winds approaching 100 mph (160 kph) in places. In Cedar Rapids Iowa, residents emerged from their homes to find an estimated 100000 trees had been snapped or torn out of the ground. In December 2021, a derecho in the Great Plains and Upper Midwest spawned at least 45 tornadoes, caused widespread damage, and killed at least five people.
Derechos can cause flash flooding Derechos can be associated with a slow-moving or nearly stationary band of thunderstorms. This could potentially lead to heavy rainfall and flash floods that can cause significant damage. Flash floods associated with a derecho that occurred July 4-5, 1969, caused Killbuck Creek in Ohio to rise more than 20 feet (6 meters) above normal level and at least two dozen fatalities according to NOAA.
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