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Derecho season is about to begin: Why you should be aware of these potentially devastating windstorms

These violent thunderstorm complexes can be as destructive — and frequent — in the U.S. as landfalling hurricanes

A destructive windstorm sweeps into northeast Philadelphia on June 3, 2020. (Ray Skwire)
correction

A previous version of this article gave an incorrect date for the Cedar Rapids derecho. It occurred on Aug. 10, 2020. This article has been corrected.

Aug. 10, 2020, started like a normal day in Cedar Rapids, Iowa. Temperatures were forecast to peak in the 80s with a chance of afternoon thunderstorms.

By evening, many buildings lay in shambles, the streets were littered with tree debris and several tall transmission towers were erased from the horizon.

Iowa derecho in August was most costly thunderstorm disaster in U.S. history

The culprit? Winds gusting to 130 mph from a derecho, or vicious windstorm, that roared through the Corn Belt. The surprise storms wrought havoc along a 770-mile path, causing $11.2 billion in damage and killing four people. Five million acres of corn and soybeans were flattened, according to the National Oceanic and Atmospheric Administration.

Derecho season is about to begin again. Approximately two-thirds of derechos happen during the months of June, July and August.

Last year’s episode in Iowa, among the most extreme derecho events in U.S. history, was one of several such windstorms that rolled across the United States:

“There should be no lingering doubt regarding the potential impacts [a derecho] can bring to a community or state,” wrote Steve Bowen, a meteorologist and head of catastrophe insight at the Aon Center, in an email. “It is safe to assume that derecho damage in recent decades is annually causing well in excess of $1 billion in direct physical impacts to property, agriculture, and infrastructure.”

In some parts of the country, derechos occur on average more than once per year. In fact, parts of the Midwest and Heartland area are as prone to damaging derechos as Florida is to hurricanes. And with derechos, the footprint of hurricane-force winds can often be more geographically extensive than with a hurricane itself.

Iowa is as prone to destructive derechos as Florida is to hurricanes

Derechos feed on warm, humid air. They start as ordinary thunderstorms but tap into jet stream energy, quickly expanding and swelling into a lengthy band of storms resembling an archer’s bow. The storms take the shape of a backward-C, the middle of the line propelled outward by strong winds descending behind them. Winds often gust upward of 80 mph. According to the American Meteorological Society, derechos produce a swath of winds at least 60 miles wide and more than 400 miles long.

There are two main types of derechos — serial and progressive. Serial derechos usually form along a cold front draped south of a low pressure system, and are often composed of several bowing segments of storms. They’re most common in the spring and fall. Progressive derechos, on the other hand, are most frequent in summer and make up the majority of these windstorms. They’re common on the northern periphery of “heat domes,” or sprawling blobs of high pressure, that sit over the Plains and Midwest during the summertime.

Forecasting derechos is challenging. While meteorologists can sometimes identify overall patterns that could support the formation of a derecho, particularly an progressive one, 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 en route. While hurricanes often come with days of lead time to prepare, derechos may only arrive with minutes’ or, at best, hours’ warning.

Derechos are generally found east of the western Continental Divide, their frequency maximized from the Ozarks to the Mississippi Valley, where an average of four derechos occur every three years. In the Midwest and Great Lakes regions, you’ll get one every year or two. They occur more sporadically on the East Coast, affecting any single location typically two or three times a decade.

Given their frequency, areal coverage and risk of high-end winds, it may be time to treat “derecho season” like hurricane season. Their impacts can be similar.

“From a damage risk perspective, most homes in the traditional ‘Midwest derecho belt’ are not built to easily withstand the 100+ mph wind gusts that derechos can often bring,” Bowen wrote. “There is certainly an appeal to bring more of a hurricane preparedness mind-set from a construction, development, and retrofit point of view in areas facing some of the highest risk for severe convective storms.”

Next week marks the start of the core of derecho season, and it’s only a matter of time before the first vicious windstorm crops up. Mirroring hurricane preparedness in terms of building codes and living in a heightened state of awareness could go a long way to minimizing damage next time a derecho strikes.

“Any means of communication to enhance awareness and preparation, plus the assurance of additional protections for the most vulnerable communities, would be a welcome step forward,” Bowen wrote.

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