Hailstorm rocks Boulder, Colo., turns roadways into icy rivers (2024)

Severe thunderstorm and flash flood warnings peppered the Colorado Front Range, hoisted by forecasters at the local National Weather Service office in Boulder, located just south of downtown. Meteorologists at the office encountered hail, but it was tame compared with what residents in the heart of the city faced.

“At the office we got about a trace, but downtown it was at least a couple inches of hail,” said Zach Hiris, a meteorologist at the office, in a phone call.

The hail wasn’t big — mostly the size of dimes, with a few quarter-sized stones thrown in — but there was a lot of it.

“Like a lot of the storms we’ve had the past few days, it was pretty slow-moving,” Hiris said. “It was also constantly redeveloping a new updraft. It was a really good case of a back building supercell.”

That means that, as the rotating storm was moving forward, it kept redeveloping on its back side. The result? A continuous hailstorm for folks caught up beneath the angry clouds.

“Thankfully we didn’t have a ton of instability, or else impacts would have been a lot higher,” said Hiris about the supercell, which he said “had a bit of rotation.”

Instability is the energy contained in warm, humid air near the ground that’s prone to rise into environments with colder air aloft. Think of it as thunderstorm fuel. The more instability, the taller the storm clouds, and the more robust the updraft, or column of upward-moving warm air feeding the storm. Large hail depends on stronger updrafts carrying moisture high into the sky, where it’s bitterly cold.

Freezing levels Monday in Colorado were around 13,000 feet above sea level, but remember that Denver already sits at an elevation greater than 5,000 feet. That means storms only had to grow 8,000 feet above the ground before they tapped into subfreezing air aloft. Most of the storms were 30,000 feet tall or more.

That said, instability was meager. That meant updrafts weren’t robust enough to support the growth of big hailstones — modest moisture entering the storm was instead converted into billions of small ones.

“We are kind of a local maximum [for hail],” explained Hiris of Boulder’s proneness to hail. “We get hail as soon as we start getting instability in the spring. It continues into the summer. It peaks in that May/June/July period.”

Thunderstorms erupted around 1 or 2 p.m., but the hail left behind lingered well into the evening. A quick-hitting inch or two of rain localized over downtown Boulder also brought street flooding, which picked up and carried piles of hail. A report of 1.77 inches of rain was received in Washington Village, with 1.69 inches off Bluff Street and 1.86 inches off Cedar Avenue on the northwest side of town.

“What amplified the flooding threat was having several days of these slow-moving thunderstorms,” Hiris said. “Having the antecedent wet conditions beforehand amplified the flooding threat. It’s been a really wet spring and start of summer, and that pattern doesn’t look to change much.”

In addition to the hail and high water in Boulder, the Weather Service also received reports of hail and flooding around Colorado Springs on Monday. The city received a record 4.02 inches of rain Monday, producing a two-day total of 5.49 inches. One video from Colorado Springs showed trash cans floating down a street:

Near Canon City, about 35 miles to the southwest of Colorado Springs, hail exceeded the size of golf balls, reaching two inches in diameter. One report described “vehicles stranded” in a foot of hail.

Flooding and hefty hail accumulations also occurred over the weekend in the broader region.

“Just southeast of Fort Collins, they picked up several inches of rain a couple days ago. Same thing south of Denver on the Palmer Divide,” Hiris said.

The worst of the hail, meanwhile, actually hit over the weekend.

“Boulder is viral, but there were more significant accumulating hailstorms a couple days ago in Douglas County and Limon … they had as much as 6 to 8 inches of hail down there,” Hiris said.

More showers and storms could pop up over the Front Range in the coming days. A flood watch is in effect for Colorado Springs through Tuesday evening because of the potential for “excessive rainfall,” the Weather Service office in Pueblo, Colo., wrote.

Jason Samenow contributed to this report.