A derecho moves across central Kansas on July 3, 2005. Jim Reed/Corbis via Getty Images
Thunderstorms are common across North America, especially in warm weather months. About 10% of them become severe, meaning they produce hail 1 inch or greater in diameter, winds gusting in excess of 50 knots (57.5 miles per hour), or a tornado.
The U.S. recently has experienced three rarer events: organized lines of thunderstorms with widespread damaging winds, known as derechos.
Derechos occur fairly regularly over large parts of the U.S. each year, most commonly from April through August. Dennis Cain/NOAA
Derechos occur mainly across the central and eastern U.S., 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. Pennsylvania and New Jersey received the brunt of a derecho on June 3, 2020, that killed four people and left nearly a million without power across the mid-Atlantic region.
In the West, derechos are less common, but Colorado – where I serve as state climatologist and director of the Colorado Climate Center – experienced a rare and powerful derecho on June 6, 2020 that generated winds exceeding 100 miles per hour in some locations. And on August 10, 2020, a derecho rolled across Iowa, Wisconsin, Illinois and Indiana, generating rare “particularly dangerous situation” warnings from forecasters and registering wind gusts as high as 130 miles per hour.
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.
A massive derecho in June 2012 developed in northern Illinois and traveled to the mid-Atlantic coast, killing 22 and causing $4 billion to $5 billion in damages.
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 (26 meters per second) or greater – and those intense winds had to extend over a path at least 250 miles (400 kilometers) 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.
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.
Speeding across the landscape
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, D.C. area. This event killed 22 people and caused millions of power outages.
Top: Radar imagery every two hours, from 1600 UTC 29 June to 0400 UTC 30 June 2012, combined to show the progression of a derecho-producing bow echo across the central and eastern US. Bottom: Severe wind reports for the 29-30 June 2012 derecho, colored by wind speed. Schumacher and Rasmussen, 2020, adapted from Guastini and Bosart 2016, CC BY-ND
Only a few recorded derechos had occurred in the western U.S. prior to June 6, 2020. On that day, a line of strong thunderstorms developed in eastern Utah and western Colorado in the late morning. This was unusual in itself, as storms in this region tend to be less organized and occur later in the day.
The thunderstorms continued to organize and moved northeastward across the Rocky Mountains. This was even more unusual: Derecho-producing lines of storms are driven by a pool of cold air near the ground, which would typically be disrupted by a mountain range as tall as the Rockies. In this case, the line remained organized.
As the line of storms emerged to the east of the mountains, it caused widespread wind damage in the Denver metro area and northeastern Colorado. It then strengthened further as it proceeded north-northeastward across eastern Wyoming, western Nebraska and the Dakotas.
In total there were nearly 350 reports of severe winds, including 44 of 75 miles per hour (about 34 meters per second) or greater. The strongest reported gust was 110 mph at Winter Park ski area in the Colorado Rockies. Of these reports, 95 came from Colorado – by far the most severe wind reports ever from a single thunderstorm system.
Coloradans are accustomed to big weather, including strong winds in the mountains and foothills. Some of these winds are generated by flow down mountain slopes, localized thunderstorm microbursts, or even “bomb cyclones.” Western thunderstorms more commonly produce hailstorms and tornadoes, so it was very unusual to have a broad swath of the state experience damaging straight-line winds that extended from west of the Rockies all the way to the Dakotas.
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.
This is an updated version of an article originally published on June 15, 2020.
About the Author
Russ Schumacher, Associate Professor of Atmospheric Science and Colorado State Climatologist, Colorado State University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Related Books
Life After Carbon: The Next Global Transformation of Cities
by Peter Plastrik , John Cleveland
The future of our cities is not what it used to be. The modern-city model that took hold globally in the twentieth century has outlived its usefulness. It cannot solve the problems it helped to create—especially global warming. Fortunately, a new model for urban development is emerging in cities to aggressively tackle the realities of climate change. It transforms the way cities design and use physical space, generate economic wealth, consume and dispose of resources, exploit and sustain the natural ecosystems, and prepare for the future. Available On Amazon
The Sixth Extinction: An Unnatural History
by Elizabeth Kolbert
Over the last half-billion years, there have been Five mass extinctions, when the diversity of life on earth suddenly and dramatically contracted. Scientists around the world are currently monitoring the sixth extinction, predicted to be the most devastating extinction event since the asteroid impact that wiped out the dinosaurs. This time around, the cataclysm is us. In prose that is at once frank, entertaining, and deeply informed, New Yorker writer Elizabeth Kolbert tells us why and how human beings have altered life on the planet in a way no species has before. Interweaving research in half a dozen disciplines, descriptions of the fascinating species that have already been lost, and the history of extinction as a concept, Kolbert provides a moving and comprehensive account of the disappearances occurring before our very eyes. She shows that the sixth extinction is likely to be mankind's most lasting legacy, compelling us to rethink the fundamental question of what it means to be human. Available On Amazon
Climate Wars: The Fight for Survival as the World Overheats
by Gwynne Dyer
Waves of climate refugees. Dozens of failed states. All-out war. From one of the world’s great geopolitical analysts comes a terrifying glimpse of the strategic realities of the near future, when climate change drives the world’s powers towards the cut-throat politics of survival. Prescient and unflinching, Climate Wars will be one of the most important books of the coming years. Read it and find out what we’re heading for. Available On Amazon
From The Publisher:
Purchases on Amazon go to defray the cost of bringing you InnerSelf.comelf.com, MightyNatural.com, and ClimateImpactNews.com at no cost and without advertisers that track your browsing habits. Even if you click on a link but don't buy these selected products, anything else you buy in that same visit on Amazon pays us a small commission. There is no additional cost to you, so please contribute to the effort. You can also use this link to use to Amazon at any time so you can help support our efforts.
