What is a derecho? An atmospheric scientist explains these rare but dangerous storm systems — The Conversation


A derecho moves across central Kansas on July 3, 2005.
Jim Reed/Corbis via Getty Images

Russ Schumacher, Colorado State University

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 two 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 that generated winds exceeding 100 miles per hour in some locations. Derechos have also been observed and analyzed in many other parts of the world, including Europe, Asia and South America.

Derechos are an important and active research area in meteorology. I expect that at least one or two more will occur somewhere in the U.S. this summer. 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.

Animation showing the development and evolution of the 6-7 June 2020 western derecho. Radar reflectivity is shown in the color shading, with National Weather Service warnings shown in the colored outlines (yellow polygons indicate severe thunderstorm warnings). Source: Iowa Environmental Mesonet.

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.

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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.

Chatfield Reservoir’s $171M redo complete, with new storage for #FrontRange cities, farmers — @WaterEdCO

A Colorado Parks and Wildlife officer heads out on patrol at Chatfield Reservoir. A $171 million redesign at the popular lake is now complete, providing more water storage for Front Range cities and farmers. But environmental concerns remain about the project’s impact on hundreds of bird species. June 8, 2020 Credit: Jerd Smith/Water Education Colorado

From Water Education Colorado (Jerd Smith):

Chatfield Reservoir, one of the largest liquid playgrounds in the Denver metro area, will take on a new role this year, storing water under an innovative $171 million deal completed last month between the state, water providers, environmental groups and the federal government.

For millions of boaters, campers, cyclists, runners and bird watchers, the 350,000 acre-foot reservoir that sits southwest of the city is a year-round recreational hot spot, with 1.6 million annual visitors.

But for thirsty Front Range communities and farmers nearby and downstream, including Highlands Ranch, Castle Rock, the Greeley-based Central Colorado Water Conservancy District and six other water providers, Chatfield represents a rare opportunity to transform a reservoir once designed strictly for flood protection into a much-needed water storage vessel, a key goal of the Colorado Water Plan.

Thanks to the redesign, the reservoir will be able to hold an additional 20,600 acre-feet of water, an amount sufficient to serve more than 40,000 new homes or irrigate roughly 10,000 acres of farm land, while maintaining its ability to protect the metro area from flooding, according to the U.S. Army Corps of Engineers.

“It is cool to see it done,” said Randy Ray, manager of the Central Colorado Water Conservancy District and president of the Chatfield Reservoir Mitigation Company, Inc., which oversees the project. “It will be better when it fills up with water.”

Originally built by the Army Corps in 1975 to help control the South Platte River during floods, by the 1990s water agencies and others began looking at ways to actually store water there.

It wasn’t easy. To raise the shore level, hundreds of acres of land along the reservoir’s banks were revegetated to replace low-lying areas that will be inundated as water is stored. The cove that houses the marina was dredged, new boat ramps were built, and new habitat for birds was created downstream in Douglas County.

A 2,100 acre-foot pool of water for environmental purposes was also set aside. It will be used to provide water for recreation and improve flows for the South Platte River through Denver, Ray said.

Though the project has been praised for its multi-purpose nature, it also triggered a long-running battle with the Denver chapter of the Audubon Society, which feared the construction damage to bird habitat would not be adequately repaired in the reservoir’s new design.

The society’s lawsuit to stop the project ultimately failed. But Polly Reetz, the chapter’s conservation chair, said they plan to closely monitor how habitat and birds respond.

“We’re still not convinced it’s going to work,” Reetz said. “They’ve done some good work out there. Plum Creek is much better. But we plan to watch it very carefully and see what happens.”

The project’s $171 million price tag was paid by the cities and farmers who will store water there, with additional funds provided by the Colorado Water Conservation Board and the federal government.

“This project is a great example of federal, state and local authorities working together to address vital water supply issues along the Front Range,” said Army Corps Omaha District Commander Col. John Hudson in a statement.

That the reservoir is a highly valued part of the outdoor recreation scene in metro Denver was clear Monday morning. More than two dozen cars waited patiently to enter the park, campgrounds were brimming with visitors, and paddle boaters and sailors were already gliding across the lake.

Elizabeth Jorde and her son Jeremiah were waiting at the marina, hoping to reserve a slip for their family pontoon boat on Father’s Day.

Jorde said she’s looking forward to seeing what a fuller reservoir will look like on the many days she and her family come out to relax. But she also said the $171 million price tag seemed steep for the amount of water the project will store.

“I was flabbergasted,” she said. “It will be interesting to see if it is worth it.”

For Randy Ray the project will provided 4,274 acre-feet of critical new storage space for the farmers in his district, who anteed up $20 million to help get the deal done.

And he said it is proof that collaborative solutions to Colorado’s looming water shortages can be found.

“We rolled up our sleeves, put our differences aside and got this thing built,” Ray said.

Jerd Smith is editor of Fresh Water News. She can be reached at 720-398-6474, via email at jerd@wateredco.org or @jerd_smith.

Fresh Water News is an independent, nonpartisan news initiative of Water Education Colorado. WEco is funded by multiple donors. Our editorial policy and donor list can be viewed at wateredco.org.

2020 #COleg: @COParksWildlfe: Passage of SB20-003 [State Parks Improvement Appropriation] provides $1 Million in support for #Colorado’s next state park

A view of Fishers Peak from the property that will become Colorado’s next state park. Senate Bill 3 provides $1 million toward the park’s continuing development. Photo credit: Colorado Parks & Wildlife

Here’s the release from Colorado Parks & Wildlife (Rebecca Ferrell):

In the closing hours of the 2020 legislative session, Colorado legislators approved $1 million to support efforts to develop Colorado’s next new state park around iconic Fishers Peak near Trinidad.

The Colorado Legislature’s passage of Senate Bill 3 will provide critical funding to help Colorado Parks and Wildlife (CPW) develop trails and infrastructure on the 19,600-acre property that features Fishers Peak, a landmark that towers over Trinidad and guided pioneers along the Santa Fe Trail in the 1800s.

“We extend our sincere gratitude to the Legislature for recognizing the value of investing in Colorado’s state parks,” said Dan Prenzlow, Director of Colorado Parks and Wildlife. “As we’ve all worked through the many challenges of the COVID-19 outbreak, including budgetary challenges, one constant was the ability for people to center themselves a bit in nature while visiting our parks. Having this investment in our next state park will allow us to provide even more of Colorado’s outdoors to our residents and visitors.”

The funding authorized by this bill will support the early stages of the park’s infrastructure development. A master planning process that will guide future development of the park is also underway, involving community stakeholders and additional project partners The Nature Conservancy, Trust for Public Land, the City of Trinidad and Great Outdoors Colorado. While the property is not yet open to the public, CPW’s goal of providing initial public recreational access to a portion of the property by early 2021 remains in place.

“This initial investment in our 42nd state park would not have been possible without the strong advocacy and support of Governor Polis, Senators Leroy Garcia, Dennis Hisey, and Representatives Daneya Esgar and Perry Will, along with the businesses, local governments in Trinidad and Las Animas County and our non-profit partners,” said Dan Gibbs, Executive Director, Department of Natural Resources. “We came together during a difficult year to provide needed resources to move forward on what is going to be a signature and our second largest state park. I can’t wait to climb the flanks of Fishers Peak or try new mountain bike trails of this fantastic amenity which will serve as a draw for visitors and recreationists alike to explore the Park and other attractions of this incredible region of Southern Colorado.”

Though Senate Bill 3 has undergone several iterations as the state made adjustments for the costs of the COVID-19 pandemic, Gov. Jared Polis, the Colorado Department of Natural Resources and Colorado Parks and Wildlife remain committed to creating a state park with input from the local community, sportspersons and other conservation stakeholders.

#Drought/#Runoff news #SanJuanRiver

West Drought Monitor June 9, 2020.

From The Pagosa Springs Sun (Chris Mannara):

As of June 4, Archuleta County is in “extreme drought,” according to the U.S. Drought Monitor.

The next classification from ex- treme drought is “exceptional drought,” which is the highest standard of drought on the U.S. Drought Monitor.

The southwest portion of the state features counties such as Archuleta, Conejos and Alamosa being fully in extreme drought, while others — such as La Plata, Mineral and Hinsdale — feature a mixture of extreme and severe drought, ac- cording to the U.S. Drought Monitor.

As of Wednesday, the San Juan River had a reported flow of 374 cfs, lower than the average for June 10 of 1,480 cfs.

The highest reported flow total for June 10 came in 1952 when the San Juan River had a reported flow of 4,120 cfs. The lowest total came in 2002 when the river had a flow of just 67.1 cfs.

Community help sought in revising #drought plan — The Pagosa Springs Sun

Swim class on the San Juan River. Photo: Brent Gardner-Smith/Aspen Journalism

From The Pagosa Springs Sun (Chris Mannara):

The drought management plan for Pagosa Area Water and Sanitation District (PAWSD) will be going through some changes soon, and the PAWSD board is seeking input from a variety of community members on the plan’s trigger points.

In an email, PAWSD Manager Justin Ramsey explained that the drought management plan will have triggers that are based on river, lake and/or hydrologic data to encourage or require water use reductions.

Within the 2018 drought management plan, there are voluntary drought management water reductions that go all the way up to level four, or severe, drought management measures.

“It was based on the cumulative amount of water we had in the district, so how much water was in the river, how much water was in the lakes, including Hatcher,” Ramsey said in an interview on Wednesday.

Ramsey added that if totals were below a certain threshold, drought restrictions would occur per the 2018 drought management plan.

However, the basis for the 2018 drought management plan was determined to be “flawed,” Ramsey described.

“As you remember in 2018, we had somewhat of a drought. The river still flowed pretty good, but Hatcher really dropped,” he said. “I can have all the water in the world in the river; if I don’t have water in Hatcher, then we’re still screwed.”

In 2018, drought restrictions were not triggered until late October, Ramsey explained.

“But the lake got very low, and we don’t want that to happen again,” he said. “If we would have based it solely on the lake level, then we would have triggered it much earlier than October.”

The revised drought management plan will be “broken up” with various triggers, Ramsey explained, citing examples such as how much water is in Hatcher Lake, the water in the San Juan River and snow water equivalency, among other things.

“Any of those things could cause a trigger to occur. It’s not going to be a cumulative effect anymore. That’s what the major change is going to be,” he said, “instead of it being cumulative, we’re going to break out each of those little components and say if one of these happens, any of these components, we’re going into it.”

PAWSD is looking for input from people on both the environmental side and business side of the com- munity, Ramsey explained.

Anyone interested in serving on a committee to help with the revising of the plan can contact Ramsey at 731-7641 or justin@PAWSD.org.

What to make of June snowstorms amidst this rapidly #warming #climate? — The Mountain Town News #ActOnClimate #KeepItInTheGround

Glenwood Canyon/Colorado River. Photo credit: Allen Best/The Mountain Town News

From The Mountain Town News (Allen Best):

If the storm that dropped up to a foot of snow in Colorado’s mountains [June 8-9] was unexceptional, it provokes a question about the shifting climate.

How can snowstorms occur in June when temperatures in Colorado have been rising significantly in recent decades?

The short answer is that weather remains variable. The climate — the accumulation of weather over longer periods — has been warming, but not so much as to drown out the noise of short-term variability. On any given day, short-term variability will trump broader trends.

June snowstorms are part of that short-term variability.

Records taken at Aspen, Breckenridge and Climax – the mine between Leadville and Copper Mountain – all show frequent snow during June for the last 70 to 90 years. In Summit County, last week’s snow was good enough to cause skiers to flock to Loveland Pass for a powder party. The largest June snowfall was 16 inches in 1984.

June snow is not weird—yet. But in coming decades, it may be.

“I think there’s a strong likelihood we will be measuring some decline in late season snowfalls in the next 30 years,” says Peter Goble, a climatologist with the Colorado Climate Center.

Statewide temperature 1895 through 2018 via the Colorado Climate Center.

Temperatures have been rising across Colorado for the last 30 years, an average 2 degrees F, but more so in some areas—western Colorado and particularly southwestern Colorado—than others.

It can still get cold—and record-breaking cold at that. But for every one new record low temperature, in Colorado, there are three record high temperatures set, says Goble.

Along the Continental Divide north of Denver, the story is similar to that of ski towns on the Western Slope.

“Yes, June snows have become an endangered species at the 8,000- to 9,000-foot elevation level in the northern Front Range,” said Klaus Wolter, a former long-time staff member of the Earth System Research Laboratory at the National Oceanic and Atmospheric Administration. The same can be said about September snows, which were much more common as recently as the 1990s, he adds.

Wolter, who lives at around 9,000 feet near the old mining town of Ward, west of Boulder, said that it’s striking how big snow events have petered out in recent decades. But they still occur, as witnessed by the foot of snow that fell last week near Red Feather Lakes, northwest of Fort Collins.

And, to add a couple more wrinkles, the precipitation west of Boulder and Fort Collins might look very different west of the Continental Divide. He also points out that May temperatures have actually dropped 3 degrees Fahrenheit at Ward in the last three decades. None of this is simple.

“As we have all just had a crash-course in how science works with covid-19 (and I am sure we will learn plenty more about it than we ever wanted over the next few months), climate change has similar issues where what is considered firm ground at any given point may be a bit shaky after all.”

Firm is the effect on river runoff of warming springs such as this one.

The winter produced an “average+ snowpack,” as Eric Kuhn, the former general manager of the Colorado River Water Conservation District, described it in a mid-April tweet. By then, the predicted inflow of the Colorado River into Lake Powell for April-June had declined to 75% of average.

Since then, the spring splish-splash into Powell, the second biggest reservoir in the Colorado River Basin, has diminished even more, to just 57% of average, according to a report issued Wednesday by the Western Water Assessment.

On the main stem of the Colorado River—including the Roaring Fork, Eagle and Blue rivers—runoff was forecast to be near to slightly below normal. Looking back to mid-winter, there were higher expectations.

Who purloined the precipitation? It was likely a result of above-average temperatures.

Nearly all of Colorado and Utah had temperatures 2 to 4 degrees above normal, and some places of western and southern Colorado had temperatures up to 6 degrees above normal, the report said.

Colorado had average high temperatures in May that ranked among the top 10 highest for the month since 1895, as did Utah.

This is part of a well-defined warming trend in Aspen, Vail and Steamboat Springs, but also Summit County and the Colorado River headwaters in Grand County. The shift is documented on a website sponsored by the Aspen Centers for Environmental Studies. It’s called the Forest Health Index. There you can study temperature, precipitation and other data for those river basins in Colorado that are at least one-third treed. That’s most of them.

In the Roaring Fork River Valley, for example, the average temperature has bobbed up and down year by year since 1980, but there’s been a general rise. Think about a hill in Iowa rather than the face of Maroon Bells. Still, that’s a breath-taking change when compared with climatic shifts of the past.

Less clear are the trends in the average peak streamflow. Precipitation also has giant ups and downs without a remarkable trend. The profiles of the Eagle, Yampa and Blue rivers look similar.

Frost-free days remain greatly variability in the Roaring Fork Valley, as true in other parts of Colorado, but the trend is clearly toward less frost and more warmth. Graphic via Allen Best/The Mountain Town News

The basins also have charts for frost-free days. This jumps around, too, but the trend is toward a longer growing season. That’s true in Aspen, and it’s true in Summit County, too.

Adam McCurdy, director of forest and climate for the Aspen Center for Environmental Studies, says the numbers come from a combination of satellite, radar and station data to reflect the general state of the river basin. The Roaring Fork data, for example, does not reflect precisely the temperatures and precipitation in downtown Aspen. They’re a more general look at Aspen, Basalt and Carbondale. The same would hold true for the upper Colorado River, which includes Winter Park and Kremmling, almost 50 miles apart.

Taking stock of the Colorado River Basin more broadly, scientists have been producing studies that detect a growing role of warming temperatures in the decreased river flows.

Jonathan Overpeck and Bradley Udall several years ago issued a study that found roughly half of the decreased flows in the Colorado in the 21st century were due to higher temperatures. The water was being taken up by increased evaporation but also transpiration by plants. In other words, what fell as snow and rain was returning to the atmosphere.

In a paper published in May in the Proceedings of the National Academy of Sciences, the two climate scientists—Overpeck from the University of Michigan and Udall from Colorado State University —dissect what is going on.

Climate change is causing the Southwest to aridify. (Left) Since the 1930s, increasing temperatures have caused the percentage of precipitation going to evapotranspiration (ET) to increase at the expense of precipitation going to Colorado River flow, resulting in an unprecedented and still ongoing megadrought (shading) starting in 1999 (8). (Right) Higher temperatures have already reduced Colorado River flow by 13%, and projected additional warming, assuming continued high emissions of greenhouse gases, will increase ET while reducing river flow even more through the 21st century. Data on Left are 20-y running means from ref. 5, and data on Right are calculated from Representative Concentration Pathways (RCP) 8.5 multimodel Coupled Model Intercomparison Project–Phase 5 (CMIP5) ensemble temperature increases projected for the Upper Colorado River Basin combined with temperature sensitivity of −9.3%/°C estimated by ref. 5, assuming no change in precipitation. Graphic credit: Jonathan Overpeck/Brad Udall

“It makes sense that longer growing seasons enabled by warming temperatures mean more total evapotranspiration, drier soils and reduced river flows,” they say.

What about increased precipitation? After all, a warming atmosphere can hold more precipitation, about 7% per one-degree increase Celsius (1.8 degree Fahrenheit).

Outside the American Southwest, some areas have been getting more rain and snow. That unevenness holds true even within Colorado. The Durango area has been getting distinctly drier. Precipitation in the Denver-Greeley area, in contrast hasn’t changed all that much.

Overpeck and Udall would have us think of the Dust Bowl, a time during the 1930s on the Great Plains of both hot temperatures and drought. Recent “flash droughts” on the High Plains in 2012 and 2017 highlight how extreme spring and summer temperatures can speed the onset, and worsen the impact, of dry spells and droughts.

Now, it’s fire season, too. Gunnison County this year found itself in “exceptional drought” in May, while the Aspen area has been in moderate drought, points out McCurdy, taking a break from parenting leave and the diaper-changing duties for an infant to talk with a reporter. In this case, Aspen and Vail would be just fine being called something other than exceptional.

Allen Best is a Colorado-based journalist who publishes an e-magazine called Big Pivots. Reach him at allen.best@comcast.net or 303.463.8630.