La Nina weather could be the cause of historic Montana flooding
RED LODGE, Montana – Just three months ago, the Yellowstone region, like most of the West, was going through a prolonged drought with little snow in the mountains and scars from a wildfire in Red Lodge ago. a year, when the area was hit by 105 degrees Fahrenheit. heat and fire.
Rivers and streams raged this week with water much higher and faster than even the rare 500-year benchmark flood. Weather-battered residents and government officials rushed to save homes, roads and businesses.
Primarily natural transient forces with some links to long-term climate change combined to trigger the shift from drought to flood, the scientists said.
It was a textbook case of “strange weather,” said Twila Moon, a resident of Red Lodge and deputy chief scientist at the National Snow and Ice Data Center. Her cropped hair was pulled up in a sweatband and she was covered head to toe in mud after helping residents clean up flooded areas.
But these are unique conditions inside the western north, scientists say. Most of the West doesn’t have much snow and will continue to struggle with drought.
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In the Yellowstone region, after a winter with light snow, it finally accumulated a few months ago, wet and cold, probably thanks to the natural weather event La Nina, building up the snowpack in the mountains at above normal levels. Snow fell so hard over Memorial Day weekend that people had to ditch their camping gear and get out of the park as long as they could, said Tom Osborne, a hydrologist who has spent decades in the area.
Things looked good. The drought wasn’t quite a break — in fact, Thursday’s National Drought Monitor still puts 84% of Montana in unusually dry or full-blown drought conditions — but it was better. Then came too much of a wet thing. Heavy rain poured down thanks to a water-laden atmosphere turbocharged by warmer-than-normal Pacific water. And when it sank, it melted. The equivalent of nine inches of rain rolled down the mountain slopes of Montana in some places. Half or more came from melting snow, the scientists said.
All rivers and streams reacted the same way: “They rose to levels far beyond anything ever recorded,” Osborne said. “Hydrologists know that there is nothing that causes more major flooding in the West than a rain-on-snow event.”
A gauge on the Stillwater River near Absarokee, where Osborne lives, normally flows at 7,000 feet (2,100 meters) per second during moderate flooding and 12,400 feet per second during century flooding, a- he declared. A once-in-500-year flood would mean the water is raging at 14,400 feet per second. Preliminary figures show Monday it peaked at 23,700 feet per second, the equivalent of piling three moderate floods on top of each other, according to Osborne.
La Nina may have helped pack more snow in Yellowstone
La Nina conditions occur when parts of the equatorial Pacific Ocean cool, changing global weather patterns. While La Nina may dry out the southwestern United States, it may increase snow and rain in other more northwestern parts of the country and may have contributed to more snow accumulation in the mountain peaks of Yellowstone, according to Upmanu Lall, director of the Columbia Water Center at Columbia. University.
And while Oregon, Washington, Idaho, and Montana had higher snow accumulations due to a cold, wet spring, areas south of those were extremely dry with anemic to missing snow at late spring, said Daniel Swain, a UCLA climatologist and western weather expert.
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Then an “atmospheric river” – long flowing regions in the sky that move large amounts of water – entered the region and dumped rain on the snow at a time when the weather was warm. This rain came from the northern Pacific where the water and air were unusually warm and warmer air holds more rain due to basic physics, Swain said. It’s a small link to climate change, he said.
In the long term, climate change is reducing snow accumulation in the West, according to Guillaume Mauger, a researcher with the Climate Impacts Group at the University of Washington.
“With climate change, we expect less snow and we expect the melt season to be shorter,” Mauger said.
But spring did not follow this long-term pattern.
“What’s extraordinary is the combination of this high snowpack that built up in April, May, with this rainy event and the warmer conditions,” Lall said. “That’s where the floods come from.”
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Lall said an atmospheric river that brought in moisture from the Pacific “is a little harder” to link to climate change.
La Nina may have played a role in many ways. Although there have been La Ninas like this in the past, “we have never seen in human history persistent La Ninas events with such hot global temperatures before. It’s a unique combination,” Swain said. “We already know that La Nina increases the risk of flooding in some places. This increases the amount of active time in certain locations. And then you have warmer oceans and a warmer atmosphere that can supercharge them. »
“So you really can’t tell it’s one thing or the other,” Swain said. “It’s really both. It’s the natural and the unnatural together.
A year ago, climatologists in Montana created the Greater Yellowstone Climate Assessment and it warned of rain and snow events like this, said report co-lead author Cathy Whitlock, professor of Earth Sciences at Montana State University.
But the real flood disaster was far worse, she said.
“Who could predict houses going into rivers and bridges being destroyed,” Whitlock said. “It’s so much worse than you imagine. And that’s partly because the infrastructure isn’t configured for extreme weather events.