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Burn Scars, Monsoon Instigate Flash Flooding In Colorado Canyons

Charred trees line the Poudre River in the Cameron Peak burn scar.
Luke Runyon
Charred trees line the Poudre River in the Cameron Peak Fire's burn scar in the river's canyon.

Last week, flash flooding in Poudre and Glenwood canyons resulted in multiple fatalities and road closures. Burn scars from the Cameron Peak and Grizzly Creek fires played a big role in the dangerous water flows, but the monsoon was also a factor.

Storms that typically cause flash floods have calm winds in the upper atmosphere and high levels of water vapor. The calm winds keep the storm in one place, concentrating the rain in one area. The air is also typically saturated, or holding as much water as possible.

However, the storm above Poudre Canyon last Tuesday didn’t contain that much moisture. Colorado Climate Center’s Peter Goble said it was a shockingly normal storm.

“Based on the ingredients we saw in the atmosphere Tuesday evening, I don't think we should expect this to be, say, a freak event,” he said.

Because of the funneling effect in canyons, and burned areas not soaking up moisture, a standard storm turned into a deadly flood. Goble said he wouldn’t be surprised if this happens again this summer.

Storms like those last week are part of a pattern of afternoon showers in July and August. The pattern is a result of monsoons, which are typically strongest in the southwestern U.S., but do reach into Northern Colorado.

Monsoons occur when the sun beats down on the land in the Southwest. The land heats up and the air above it expands. Meanwhile, the ocean is being hit by that same sunlight, but because it’s water, it heats up slower than the land.

That creates a pressure difference: the air over the land is at low pressure and the air over the ocean is at high pressure. The ocean air moves toward the land and brings all of its rain along.

Logs and debris engulf a bridge in Poudre Canyon on Wednesday, July 21, the morning after flash flooding triggered multiple mudslides.
Colorado Parks and Wildlife
Colorado Parks and Wildlife
Logs and debris engulf a bridge in Poudre Canyon on Wednesday, July 21, the morning after flash flooding triggered multiple mudslides.

Goble said last week’s storms got their moisture from the Gulf of Mexico, but that’s not always the case in Colorado. He said crops in the central Great Plains can actually exhale enough water vapor to create storms here as well.

“Just because you’re in monsoon season, doesn’t mean you’ll see a monsoonal pattern, it’ll kind of come and go,” he said. “It’s also a little bit fickle. We’ve had really poor monsoons the last couple years, so it’s really nice to see a better monsoon taking place at least so far this year.”

While the monsoon is annual and quite predictable, individual storms are not. Aaron Hill, an atmospheric scientist at Colorado State University, said scientists are good at measuring the ingredients for a storm, like water vapor. But, “It’s not as simple as, ‘Tell me where it’s going to rain today,’” he said. “That’s not really that possible at the moment in meteorology.”

Weather models can predict a thunderstorm based on the ingredients in the air, like the water vapor saturation. But they can’t identify exactly where and how hard it will strike. Hill and his team at CSU are actively trying to improve the computer models.

“That’s where my research is really focused on how we can leverage artificial intelligence to improve these forecasts and on using these tools to better inform forecasters so they can communicate the threat better to the public,” Hill said.

The researchers look at big data sets of past rainfall events. They’re trying to train a computer to identify what in the environment made the storm so severe.

Examining those trends becomes even more difficult as climate change comes into play. Hill said storms themselves may not change at all.

“It’s not exactly a perfect relationship of ‘climate change is going to do this to storms,’” he said. “Climate change may increase our risk of fires, which then increase our vulnerability to flash flooding.”

Both Hill and Goble said thunderstorms and flash floods are incredibly difficult, but important, to predict. They said forecasters do their absolute best, but the whole prediction process has a lot of room for improvement.

I am the 2021 American Association for the Advancement of Science Mass Media Fellow at KUNC. My goal is to tell the true stories of science — and make them understandable and fascinating for all.
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