Thunderstorms are common across North America, especially in warm weather months. About 10 percent of them become severe, meaning they produce hail one inch or greater in diameter, winds gusting in excess of 58 mph, or a tornado.

The United States recently has experienced several rarer events: organized lines of thunderstorms with widespread damaging winds, known as derechos.

Derechos occur mainly across the central and eastern United States, where many locations are affected one to two times per year on average. They can produce significant damage to structures and sometimes cause “blowdowns” of millions of trees.

A powerful derecho unleashed wind gusts over 100 mph in Iowa, as it tore from the Plains through the Upper Midwest on Monday, slamming Des Moines and Chicago. On June 3, 2020, Pennsylvania and New Jersey received the brunt of a derecho that killed four people and left nearly a million without power.

In the West, derechos are less common, but Colorado experienced a rare and powerful derecho on June 6 that generated winds exceeding 100 mph in some locations.

The term “derecho” vaulted into public awareness in June 2012, when one of the most destructive derechos in U.S. history formed in the Midwest and traveled some 700 miles in 12 hours, eventually making a direct impact on the Washington area. This event killed 22 people, caused power outages affecting millions of people and left damage worth $4 billion to 5 billion.

Derechos have also been observed and analyzed in many other parts of the world, including Europe, Asia and South America. They are an important and active research area in meteorology.

Here’s what we know about these unusual storms.

Walls of wind

Scientists have long recognized that organized lines of thunderstorms can produce widespread damaging winds. Gustav Hinrichs, a professor at the University of Iowa, analyzed severe winds in the 1870s and 1880s and identified that many destructive storms were produced by straight-line winds rather than by tornadoes, in which winds rotate. Because the word “tornado,” of Spanish origin, was already in common usage, Hinrichs proposed “derecho” — Spanish for “straight ahead” — for damaging windstorms not associated with tornadoes.

In 1987, meteorologists defined what qualified as a derecho. They proposed that for a storm system to be classified as a derecho, it had to produce severe winds — 57.5 mph or greater — and those intense winds had to extend over a path at least 250 miles long, with no more than three hours separating individual severe wind reports.

Derechos are almost always caused by a type of weather system known as a bow echo, which has the shape of an archer’s bow on radar images. These in turn are a specific type of mesoscale convective system, a term that describes large, organized groupings of storms.

They typically move very fast, with forward speeds of 50 to 70 mph.

Researchers are studying whether and how climate change is affecting weather hazards from thunderstorms. Although some aspects of mesoscale convective systems, such as the amount of rainfall they produce, are very likely to change with continued warming, it’s not yet clear how future climate change may affect the likelihood or intensity of derechos.

Damage comparable to a hurricane

Derechos are challenging to predict. On days when derechos form, it is often uncertain whether any storms will form at all. But if they do, the chance exists for explosive development of intense winds. Forecasters did not anticipate the historic June 2012 derecho until it was already underway.

For the western derecho on June 6, 2020, outlooks showed an enhanced potential for severe storms in Nebraska and the Dakotas two to three days in advance. However, the outlooks didn’t highlight the potential for destructive winds farther south in Colorado until the morning that the derecho formed.

Once a line of storms has begun to develop, however, the National Weather Service routinely issues highly accurate severe thunderstorm warnings 30 to 60 minutes ahead of the arrival of intense winds, alerting the public to take precautions.

Communities, first responders and utilities may have only a few hours to prepare for an oncoming derecho, so it is important to know how to receive severe thunderstorm warnings, such as TV, radio and smartphone alerts, and to take these warnings seriously. Tornadoes and tornado warnings often get the most attention, but lines of severe thunderstorms can also pack a major punch.

Russ Schumacher is an associate professor of atmospheric sciences at Colorado State University, where he also serves as state climatologist. He is also a Jeopardy! champion.

This article was originally published on The Conversation, a website dedicated to unlocking ideas from academic experts.

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