My husband and son were driving on 80 outside of Iowa City when they came upon a semi blown over. They helped him out and into our truck to wait for paramedics and police. The man is okay! @KCCINews @NWSSevereTstorm @WHO13news @NWSDesMoines @IAStormChasing

— Finding Jesus Christ (@findingJesus15) August 10, 2020

There’s one reason why truckers should be aware of derechos — they can toss tractor-trailers like toys.

It happened to several drivers in the Midwest on Aug. 10, 2020, during one of the most extreme derechos in U.S. history.

A derecho is a violent windstorm. More specifically, it’s a widespread, long-lasting windstorm associated with a band of rapidly moving rain showers or thunderstorms.

Although a derecho can produce destruction similar to the strength of tornadoes, the damage typically is extended in one direction along a relatively straight path. As a result, the term “straight-line wind damage” is sometimes used to describe derecho damage. A tornado’s damage has a more circular pattern due to its rotating winds.

In some parts of the country, derechos occur on average more than once per year. The footprint of hurricane-force winds from a derecho can cover a larger geographic area than some hurricanes.

Based on National Weather Service (NWS) records, derechos are generally found east of the Continental Divide, occurring mostly from the Ozarks to the Mississippi Valley, where an average of four derechos occur every three years. In the Midwest and Great Lakes regions, one pops up every year or two. They happen more sporadically on the East Coast, striking typically two or three times a decade. Overall, weaker derechos are more common than extremely strong ones.

To classify as a derecho, damage must be incurred either continuously or intermittently over a swath of at least 400 miles and a width of 60 miles or more. This is according to the American Meteorological Society definition. The damage must be caused by winds of at least 58 mph along most of the derecho’s length. This strength meets the NWS criteria for severe winds.

Related: The best weather apps for truckers: Part 2

Because of a trailer’s large size, flat surfaces and tall stature, it’s like a sail. Wind has plenty of surface area on which to push. A sudden gust combined with a reactive steering move can lead to an instant rollover, whether deadheading (carrying an empty trailer), hauling a light load or pulling a full load.

For drivers, or anyone who doesn’t take derechos seriously, consider the Midwest derecho mentioned earlier. It wrought havoc along a 770-mile path over a period of 14 hours, producing wind gusts as high as 130 mph. The storm caused damage in at least six states, totaling $11.2 billion, and killed four people. Five million acres of corn and soybeans was flattened, according to the National Oceanic and Atmospheric Administration.

Primary derecho season has just begun. Climatologically, two-thirds of derechos in the U.S. happen during meteorological summer, from June through August.

Radar images of Aug. 10, 2020 derecho in the Midwest. (Images: NWS)

Derechos feed on warm, humid air. They start as ordinary thunderstorms but tap into jet stream energy, swelling quickly into a lengthy band of storms resembling an archer’s bow or backward “C.” The middle of the line is propelled outward by strong winds descending behind it. The word derecho comes from the Spanish for “straight ahead” or “direct” and was chosen to distinguish between wind damage caused by derechos and tornadoes.

Forecasting derechos is challenging. While meteorologists can sometimes identify overall patterns that could support the formation of a derecho,  it’s usually not until one actually gets going that forecasters can begin alerting the public. By then, winds of over 60 or 70 mph may already be on the way. While hurricanes often come with days of lead time to prepare, derechos may arrive with only minutes or, at best, hours of lead time. This is why truckers should pay close attention to forecasts and changing weather conditions while on the road.

Click here for more FreightWaves articles by Nick Austin.

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