Torrents drench Denver as Death Valley recovers from a 1,000-year flood

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Flash flood warnings were posted for parts of Denver on Sunday night as exceptional downpours swept roads, stranded cars and flood relief.

The Denver floods came about 48 hours after a historic deluge in Death Valley, California, on Friday that left about 1,000 people stranded and classified as a 1 in 1,000 year event. And the Death Valley flooding followed three 1 in 1,000 year rain events in the Lower 48 to conclude July and begin August in St. Louis, eastern Kentucky, and southern Illinois.

Excessive rainfall continued to plague parts of the Lower 48 Monday morning, with counties west and south of Chicago under flash flood warnings after seeing up to half a foot of rain.

Each Flood operates in a warmer atmosphere due to human-induced climate change and is capable of releasing more extreme amounts.

In Denver, thunderstorms devastated parts of the northern metro area Sunday night, drenching up to an inch and a half of rain in just 20 minutes. In some areas, rainfall of this intensity is only expected to occur every few hundred years.

Numerous roads were closed, including: part of Interstate 70. The Denver ABC branch described a “traffic nightmare” with drivers stranded on the highway for hours and nearly 20 people in need of rescue.

“It appears our heaviest report came in at 2.5 inches of rain,” said David Barjenburch, a meteorologist with the National Weather Service office in Boulder, although the radar showed the possibility of higher amounts locally.

He explained that most areas in any given location only had storms for about 40 minutes. They were traveling at a speed of about 15 mph.

“This is the highlight [time of year] in terms of monsoon rains,” Barjenburch said, referring to the southwest monsoon — a seasonal wind shift that helps moisture drift north across the desert southwest, the Four Corners region and, at times, the Colorado Front Range. “July, early August is typically our flash flood season. And this time we had a lot of fluid, much more than we normally have here.”

He referred to infamous floods, such as the one that hit Fort Collins in 1997 or the Big Thompson episode, which killed 144 people when a foot of rain poured into the Big Thompson River in just a few hours on July 31, 1976.

According to the National Park Service, the people stranded in Friday’s Death Valley deluge were able to “travel carefully through damaged roads.”

About 1.46 inches of rain fell – just short of the 1.47-inch record. The total corresponds to about three quarters of the amount of rain of a typical year.

The lowest, driest, and hottest location in the United States, Death Valley averages just 0.11 inches of rain in August.

Many cars were damaged by the sudden deluge and ensuing mudslides.

The Park Service reported that the flood destroyed a water system that serves numerous park homes and facilities. It also said many miles of roads were damaged and strewn with debris.

Sudden flooding in Death Valley strands about 1,000 people in national park

Like Denver, the downpours were caused by the southwest monsoon.

Flooding in Northern Illinois

Parts of Illinois west and south of Chicago were also hit by heavy rainfall early Monday, triggering flash flooding in the northwestern and north-central parts of the state. The Weather Service office serving Chicago had received about a dozen reports of flooding, including around Rockford until noon.

“[T]Significant flash flooding has come as close to subway as Rockford and Byron, Illinois, about 90 miles west of Chicago,” said Matt Friedlein, a meteorologist with the Weather Service. “DeKalb and Sycamore … about 70 miles west of Chicago … also saw some flooding.”

Friedlin said Rockford broke its August 8 rain record by 2.62 inches as of 7 a.m. A weather station south of Rockford posted 6.21 inches.

A brief downpour of heavy rain swept through Chicago, but the effects were limited.

Explanation about the exceptional rainfall

The heavy rainfall has been caused by the signature summer moisture that collects along a stalled front that has draped from the Colorado Rockies into the central states, atop a heat dome stretching across the southern US. Such fronts wring the humidity out of the air as if someone were squeezing a washcloth. That can lead to rainfall of 2 to 3 inches — or more — per hour. These fronts also act as train tracks, guiding repeated developing thunderstorms over the same areas.

That was the case eight days ago in St. Louis, where 7.87 inches of rain fell in six hours. That led to flash flooding in the city and cars inundated by rising floodwaters. Just two days later, extreme flooding ravaged eastern Kentucky, killing 37. President Biden, who visited the region on Monday, has pledged the federal government’s assistance in recovery efforts. Another storm dropped up to 14 inches of rain near Effingham, Illinois, late last week.

Biden visits flood-ravaged Kentucky ahead of bill signing this week

As the atmosphere continues to warm, events of this magnitude will become more frequent. That will translate into greater economic losses, damage to fragile and aging infrastructure and danger to the public, especially in urban areas.

In the past two weeks we have observed four rainfall events of 1 in 1000 years. That doesn’t mean that rain falls once every thousand years, but that it should have a 0.1 percent chance in any given year.

A limitation of the 1000-year precipitation measurement is that it is based on historical data and on the assumption that the climate does not change. As the atmosphere continues to warm and its ability to store and transport moisture increases, this metric loses meaning as previously rare events become more common.

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