Monday’s sudden thunderstorm over Denver tripled the flow of the South Platte River while raising its depth almost 3 feet in about an hour’s time.
The intense storm formed just west of Denver in the foothills late in the day and unleashed about 1.5 inches of rain over portions of south Denver between 7 and 8 p.m., National Weather Service officials said.
“It was like setting a match to a firecracker,” Weather Service meteorologist Mike Baker said of storm and flash floods it caused.
A cold front, lots of moisture in the air and a “very unstable atmosphere” joined to unleash the wild weather, Baker said, which also brought rain, hail, high winds and tornado sightings to the Eastern Plains.
Hail hit several cities and towns in Weld County Monday night, including Greeley, Fort Lupton and Johnstown. Golfball-sized hail pounded the Hudson area, breaking windows.
The storm unleashed most of its fury between 7 and 8 p.m. A tornado funnel cloud was spotted about 3 miles southeast of Aurora in Arapahoe County at about 7:30 p.m. Monday but it did not touch down, according to a Weather Service spotter, and winds were clocked at up to 80 mph.
In Fort Lupton up to half an inch of rain fell in a five-minute period flooding some homes there.
It only took about an hour for the intense rain to more than triple the flow of the South Platte River through Denver Monday night.
A 2-year-old boy in a stroller, who had been on a walk with his mother, was swept by stormwater from a concrete underpass into a ditch feeding into the South Platte near Invesco Field at Mile High.
The river, which has been flowing at about 2,000 cubic feet per second as water is released from Chatfield Reservoir to make way for runoff, swelled to a peak of 6,700 cubic feet per second on Monday night, said Kevin Stewart of the Urban Drainage and Flood Control District.
“It was not an unusual rain,” Stewart said, “although it caused the Platte to rise about 3 feet in an hour with downtown Denver draining real efficiently into the Platte.”
Stewart said it was “a hard-hitting, and fortunately, fast-moving storm, a nuisance storm that fills gutters and not homes – and that’s what it did, except in those tragic situations where it caught people in the wrong place.”
On Monday, the district’s system of metro-area gauges recorded the heaviest rainfall at the Stapleton neighborhood, including a half-inch of rain that fell between 7:35 p.m. and 7:40 p.m. “That’s how hard rain was falling,” Stewart said.
Goldsmith Gulch in southeast Denver, where a young man was presumed to have been swept away Monday, rose 4 feet in 15 to 20 minutes, Stewart said, but didn’t overtop East Yale Avenue and didn’t flood area apartment buildings as it has in the past.
The force of the increasingly swift and rising water would have been “enormously powerful,” said Nate Lord, who coaches kayakers and canoers through the Alexander Dawson School in Lafayette.
To better envision the force, Lord suggested filling a gallon milk jug with water – which would be roughly one cubic foot of water. “Just holding it in front of you is significant,” Lord said.
Then multiply that weight by 2,000 – the initial flow rate of the South Platte before it began to rise – and then again by 6,700, which was the peak rate.
“One has to respect the forces tremendously,” said Lord, who ensures his students are equipped with life vests, helmets and neoprene clothing. “You can’t underestimate the enormity of the power of a river when it’s rising,” coupled with cold water, debris and other obstacles.
Staff writer Kieran Nicholson can be reached at 303-954-1822 or knicholson@denverpost.com.
