Torrents drench Denver as Demise Valley recovers from the 1,000-year Deluge

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Flash flood warnings were posted for parts of Denver Sunday night as extraordinary downpours overwhelmed streets, bogged down cars and forced flood rescues.

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

Excessive rain 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 deluge acts in a warmer atmosphere due to human-caused climate change and can release more extreme amounts.

In Denver, thunderstorms blasted parts of the northern metro area Sunday night, drenching them with up to an inch and a half of rain in just 20 minutes. In some areas, precipitation of this intensity can only be expected every few hundred years.

Numerous roads were closed, including a section of Interstate 70. Denver affiliate ABC described a “traffic nightmare” that left drivers stuck on the interstate for hours and requiring the rescue of nearly 20 people.

“Looks like our heaviest report came in at 2.5 inches of rain,” said David Barjenburch, meteorologist at the National Weather Service’s office in Boulder, although radar showed the possibility of higher amounts locally.

The dark green area on this map represents where more than 2 inches of rain likely fell in less than 30 minutes yesterday.

These include Globeville, City Park, Clayton, Elyria-Swansea, Park Hill + Commerce City.#9wx #COwx

— Chris Bianchi (@BianchiWeather) August 8, 2022

He explained that in most areas, storms only lasted about 40 minutes in any given location. They were moving at about 15 km/h.

“This is the high point [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 sometimes the Colorado Front Range. “July, early August is usually our flash flood season. And this time we had plenty of moisture, much more than we usually have here.”

He was referring to notorious flooding events, such as the one that hit Fort Collins in 1997 or the Big Thompson episode on July 31, 1976, when 30 centimeters of rain poured into the Big Thompson River in a matter of hours, killing 144 people.

People stranded in Death Valley’s deluge on Friday were able to “drive carefully out through damaged roads” over the weekend, according to the National Park Service.

About 1.46 inches of rain fell — just short of the 1.47-inch record. The sum corresponds to about three quarters of the rainfall of a typical year.

Death Valley, the lowest, driest, and hottest place in the United States, rains an average of just 0.11 inches in August.

Many cars were damaged by the sudden torrent and resulting mudslides.

The park service reported that the flooding destroyed a water system that supplies numerous park residences and facilities. It also said many kilometers of roads were damaged and littered with debris.

Flash floods in Death Valley strand about 1,000 people in the national park

As in Denver, its downpours were triggered by the southwest monsoon.

Floods in northern Illinois

Parts of Illinois west and south of Chicago were also hit by heavy rain early Monday, triggering flash flood warnings in the northwest and north of the state. The Chicago Weather Service office had received about a dozen reports of flooding, including through noon in the Rockford area.

“[T]The significant flash floods have come as close to the subway as Rockford and Byron, Illinois, about 90 miles west of Chicago,” said Matt Friedlein, weather service meteorologist. “DeKalb and Sycamore … about 70 miles west of Chicago … also saw some flooding.”

Friedlin said Rockford broke its Aug. 8 rainfall record of 2.62 inches at 7 a.m. A weather station south of Rockford reported 6.21 inches.

Some historical perspectives on the last 2 days of precipitation in #Rockford: The consecutive calendar days (08/07/2022 to 08/08/2022) with a total precipitation total of 2″+ is the first such event since July 23-24, 2010. 4, 70″ fell on 7/23/2010 and 2.81″ fell on 7/24/2010. #ilwx

— NWS Chicago (@NWSChicago) August 8, 2022

A brief heavy rain shower swept through Chicago, but its impact was limited.

Explanation of the exceptional rainfall

The heavy rains were caused by characteristic summer moisture that accumulated along a stalled front draped from the Colorado Rockies into the central states and sat atop a heat dome stretching across the southern United States. Such fronts wring the moisture out of the air like someone wringing out a washcloth. That can result in precipitation rates of 2 to 3 inches — or more — per hour. These fronts also act like railroad tracks, channeling developing thunderstorms over the same areas over and over again.

That was the case eight days ago in St. Louis, where 7.87 inches of rain fell in six hours. This resulted in flash flood emergencies across the city and cars being swamped by rising tides. Just two days later, extreme flooding hit eastern Kentucky, with 37 people now confirmed dead. President Biden, who visited the region Monday, pledged federal government support for the recovery effort. Another storm dropped up to 14 inches of rain near Effingham, Illinois late last week.

Biden is visiting flood-stricken Kentucky this week before signing legislation

As the atmosphere continues to warm, events of this magnitude will occur more frequently. This will lead to increased economic losses, damage to vulnerable and aging infrastructure, and dangers to the public, particularly in urban areas.

In the past two weeks, we have observed four 1-in-1,000 annual rain events. This does not mean that the amount of precipitation occurs once every thousand years, but that it should have a 0.1 percent chance in any given year.

A limitation of the 1,000-year precipitation metric is that it is based on historical data and on the assumption that the climate is not changing. As the atmosphere continues to warm and its ability to store and transport moisture increases, this metric loses its importance as previously rare events become more frequent.

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