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From dry to deluge, how heavy snow, rain flooded Yellowstone


Rapids along the road between Yellowstone and Teton National Parks. Credit: Unsplash/CC0 Public Domain

Just three months ago, the Yellowstone region, like most of the West, dragged through a prolonged drought with little snow in the mountains and wildfire scars at Red Lodge from a year ago when the area was hit by heat of 105 degrees Fahrenheit. (40.5 Celsius) and fire.

Rivers and creeks raged this week with water much higher and faster than even the rare 500-year flood. Weather-ravaged residents and government officials raced to save homes, roads and businesses.

Mostly natural volatiles with some connections to long-term climate change, combined to drive the transition from drought to flood, scientists say.

It was a textbook example of “weird weather,” said Red Lodge resident and deputy chief scientist at the National Snow and Ice Data Center, Twila Moon. Her cropped hair was in a sweatband and she was covered in mud from head to toe as she helped residents clear flooded areas.

But these were conditions unique to the northern interior, scientists say. Most of the west doesn’t have much snow and will continue to struggle with drought.

In the Yellowstone area, after a winter of light snow, it finally built up a few months ago, wet and cold, probably thanks to the natural weather La Nina, which has pushed snow cover in the mountains above normal levels. Snow fell so hard over Memorial Day weekend that people had to leave their camping gear and leave the park while they could, said Tom Osborne, a hydrologist who has spent decades in the area.

Things looked good. The drought hadn’t completely stopped — in fact, Thursday’s National Drought Monitor still places 84% ​​of Montana under unusually dry or full-blown drought — but it was getting better. Then came too much of a damp thing. Heavy rains poured in thanks to a water-laden atmosphere powered by warmer-than-normal Pacific water. And when it poured, it melted. The equivalent of nine inches (23 centimeters) of rain poured down the mountainsides of Montana in places. Half or more was from the melting snow, scientists said.

All the rivers and streams reacted the same: “They rocketed to levels much higher than ever before,” Osborne said. “Hydrologists know that nothing causes greater flooding in the West than a rain-on-snow event.”

One meter on the Stillwater River near Absarokee, where Osborne lives, normally flows at 7,000 cubic feet (200 cubic meters) per second during moderate flooding and flows at 12,400 cubic feet (350 cubic meters) per second in a 100-year-old flood, he said. A flood once every 500 years would mean the water rushing at 14,400 cubic feet (410 cubic meters) per second. Preliminary numbers show it peaked on Monday at 23,700 cubic feet (670 cubic meters) per second, the equivalent of stacking three moderate floods together, according to Osborne.

La Nina conditions occur when parts of the equatorial Pacific Ocean cool, changing global weather patterns. While La Nina could dry out the U.S. Southwest, it could increase snow and rain in other more northwestern parts of the country and may have helped pack more snow on Yellowstone’s mountaintops, according to Upmanu Lall, the director of the National Institute for Public Health and the Environment. Columbia Water Center at Columbia University.

And while Oregon, Washington, Idaho and Montana had greater snowfalls from a cold, wet spring, areas south of that were extremely arid with anemia to missing late spring snow, said UCLA climate scientist and western weather expert Daniel Swain.

Then an “atmospheric river” — long flowing streams of air that displace large amounts of water — entered the area and poured rain on the snow at a time when the weather was warm. That rain came from over the North Pacific Ocean, where the water and air were unusually warm and warmer air holds more rain because of the basic physics, Swain said. That’s a small link between climate change, he said.

According to Guillaume Mauger, a research scientist at the University of Washington Climate Impacts Group, long-term climate change is reducing snow cover in the West.

“With climate change, we expect less snow and we expect the melt season to be shorter,” Mauger says.

But spring didn’t follow that long-term pattern.

“What’s extraordinary is the combination of that high snow pack that built up in April, May, along with this rainfall and the warmer conditions,” Lall said. “That’s where the flood comes from.”

Lall said an atmospheric river carrying moisture from the Pacific is “a little more difficult” to link to climate change.

La Nina may have played a role in several ways. While there have been La Ninas like this in the past, “never in human history have we seen sustained La Nina events with global temperatures this warm. That’s a unique combination,” Swain said. “We already know that La Nina increases the risk of flooding in some places. In some places, it increases the amount of active weather. And then you have warmer oceans and a warmer atmosphere that can ramp that up.”

“So you really can’t just say it’s one or the other,” Swain said. “It’s really both. It’s the natural and the unnatural together.”

A year ago, Montana climate scientists created the Greater Yellowstone Climate Assessment and it warned of these kinds of rain-and-snow events, said lead author Cathy Whitlock, a professor of earth science at Montana State University.

But the real flood disaster was much worse, she said.

“Who can predict houses going into the rivers and bridges being destroyed,” Whitlock said. “It’s so much worse than you can imagine. And that’s partly because the infrastructure isn’t prepared for extreme climate events.”

Warm water forecast: Drought increases in West, floods ebb

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Quote: From Dry to Flood, How Heavy Snow, Rain Flooded Yellowstone (June 2022, June 17) Retrieved June 17, 2022 from https://phys.org/news/2022-06-deluge-heavy-yellowstone.html

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