Extreme weather? Yes, and the events were interrelated: The convergence of hot and dry, flash floods and bad air were manifestations of a fast-changing #climate in #Colorado and beyond — Big Pivots

From Big Pivots (Allen Best):

Heat waves in June and again in July. Monster wildfires on the West Coast that made the air in Denver, Salt Lake City and Cheyenne unhealthy. Then fast floods that produced debris flows, blocking Colorado’s major east-west artery, Interstate 70 through Glenwood Canyon.

They’re separate but related, said Western Water Assessment’s Seth Arens in a Nov. 18 review of the year’s extreme weather events in Colorado, Wyoming and Utah. He called them a convergence of climate hazards.

“These are related; they are not independent,” he said.

Arens described the concept of compound hazards, where the occurrence of one hazard can create conditions for another hazard.

“None of these climate hazards act in isolation and many of them occur with one another,” said Arens, a research scientist based in Salt Lake City.
For example, extreme temperatures and low precipitation lead to drought, which begets increased wildfire risk. The wildfires produce poor air quality. And after the fires, there’s an increased risk of flash floods.

We’re talking visions of Armageddon here—and, at least until the smoke disappeared in September, it seemed that Colorado had arrived in perdition.
Most of the attention about the heat dome of June focused on the West Coast, and rightfully so. Temperatures of 116 in Portland? People literally baked to death in uncooled apartments.

But it was hot in Utah, Colorado and Wyoming, too. Arens said he narrowed his study of temperature records to places with records of at least 50 years. Even so, he found many all-time records and dozens of daily records. On just one day, June 15, Arens found that 79% of daily temperature records in Utah, 53% in Colorado, and 83% in Wyoming were broken. And so it went for two more days before the heat relented.

In July, more records were set, including an all-time record in Grand Junction of 107 degrees. Utah’s all-time record of 117 degrees was tied at St. George. Later in July, all-time high temperatures were set in three Wyoming tow and cities, the highest being 109 degrees in Weston.

Colorado Basin Forecast Center Drought Monitor map November 30, 2021.

Heat devoid of precipitation equals drought. These Western states have had that. Much of the focus has been on the Colorado River Basin.

Runoff in the Colorado River during water year ending October 2021 was 50% of average. Flows into Lake Powell were 28% of median.

In August, Powell reached the lowest elevation since it began filling after completion of the Glen Canyon Dam in 1963. It’s now less than 30% of capacity. That puts it at 157 feet below full pool.

The Bureau of Reclamation, in its two-year projections, estimates the river will drop another 20 feet by spring 2022. Already, there are questions of when the reservoir level will drop below what is called minimum pool, the least amount of water needed to be able to produce electricity. That could happen by early 2023.

Might a big snow year boost water levels in Powell, averting the chain reaction of problems? Perhaps, although Arens offered little encouragement. With the La Niña weather pattern, the odds again are for the Colorado River Basin receiving below-average snowfall.

Weather prediction has a big gap, Arens added. Weather can be predicted with some confidence beyond two weeks, and the climate can be predicted after about 9 years. But between, not much can be said with confidence.

Back to last summer: The air quality was again miserable, approaching the air quality index of 200 in Salt Lake City on July 12. In Denver it was 192. These are all levels where even healthy people are advised to stay indoors. It was marginally better that day in Cheyenne, at 119.

Along the Wasatch Front of Utah, Salt Lake City exceeded the EPA standard for ozone air quality 23 days and Ogden 16 days. In Colorado, Denver exceeded the limit 25 days, Boulder 28, and Colorado Springs 20.

Colorado Department of Transportation
CDOT crews continue clearing mudflow from Interstate 70 in Glenwood Canyon July 2021 — Colorado Department of Transportation

Then there were the flash floods and mudslides. The torrential rains were not everywhere, but in places they sent rocks and mud rushing and tumbling down onto highways. The most notorious was in Glenwood Canyon, where the hillsides were burned last year in the 32,631-acre Grizzly Gulch fire.

These big rains had some beneficial effect, too. Soil moistures recovered, meaning the melting snow next spring will not automatically get sopped up like an absorbent paper towel. Arens also noted that rainstorm had a satisfying result in a Glen Canyon side canyon where the receding waters of Powell had left 18 to 20 feet of silt.The storm sent all the silt downstream, revealing parts of the canyon that David Brower had known.

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