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Yellowstone floods show forecast errors as climate warms

BILLS, Mon. – The weather forecast for the Yellowstone National Park area on the morning of June 12 seemed pretty tame: Warmer temperatures and rain showers would accelerate snowmelt in the mountains and could cause “minor flooding” with no mention of the danger to humans.

By nightfall, after several inches of rain had fallen on a deep spring snowpack, there they were Record-breaking floods.

Streams of water flowed away from the mountains. Swollen rivers with boulders and trees blared through the cities of Montana in the next days. The floods swept away houses, obliterated bridges and forced the Evacuation of more than 10,000 Tourists, park employees and residents near the park.

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Climate experts and meteorologists say the discrepancy between the destruction and predictions during what is expected to take months to clean up underscores a problematic aspect of climate change: Models used to predict the impact of storms don’t always keep up with increasingly devastating rainstorms, hurricanes, heat waves and other events.

“These rivers had never reached these levels. We were literally flying blind, not knowing what the impact would be,” said Arin Peters, a senior hydrologist at the National Weather Service.

Hydrological models used to predict floods are based on long-term, historical records. But they don’t reflect changes in climate that have occurred over the past decade, said meteorologist and Weather Underground founder Jeff Masters.

“These models will not be enough to deal with a new climate,” Masters said.

Another extreme weather event where the models fell short was Hurricane Ida, which hit Louisiana last summer and then stalled over the east coast – Flooding parts of Pennsylvania, New Jersey and New York with unprecedented rainfall causing massive flooding.

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The Weather Service had warned of a “serious situation” that could become “catastrophic,” but the forecast 3 to 6 inches (8 to 15 centimeters) of rain for New York, New Jersey and Pennsylvania were well short of the 9 to 10 inches ( 23 to 25 centimeters) that fell.

The deadly June 2021 heatwave that scorched the Pacific Northwest was another example. Warmer weather had been expected but no temperatures of up to 116 degrees (47C) that tumbled previous records and killed an estimated 600 or more people in Oregon, Washington State and western Canada.

The surprise flooding in Yellowstone led to a nighttime scramble to close roads and bridges that were swept away by the water and rushed evacuations that left some people missing. Miraculously, no one died as more than 400 homes were damaged or destroyed.

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When Yellowstone experienced rockfalls from the rains, park rangers closed a busy road between the town of Gardiner and the park’s headquarters in Mammoth Hot Springs, Wyoming. Later it was washed out in numerous places.

The rain and snowmelt have been “too much too fast and you’re just trying to get out of the way,” said Tim Townsend, Yellowstone’s deputy chief ranger.

If the road hadn’t been closed, “we would undoubtedly have had fatalities,” Park Superintendent Cam Sholly said.

“The road looks absolutely fine and then it’s like an 80-foot drop straight into the river,” Sholly said. “No way, if someone was driving in the rain at night they would have seen that and could have stopped.”

Rock Creek, which flows through the town of Red Lodge and is normally calm and sometimes only ankle deep, became a raging river. When the Weather Service issued a flood warning for the creek, the water had already burst its banks and started tearing down bridges.

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By the time the warning was issued, “we already knew it was too late,” said Scott Williams, a commissioner for Carbon County, Montana, which borders Yellowstone.

Red Lodge resident Pam Smith was alerted to the flooding by something in her basement before dawn. It was her clothes dryer, floating in the water streaming through the windows.

While trying to salvage souvenirs, Smith slipped on the wet kitchen floor and fell, crushing a bone in her arm. She recalled holding back tears as she trudged through flood water with her partner and 15-year-old granddaughter to reach her pickup truck and get to safety.

“I went blank,” Smith said. “I was angry and I thought, ‘Why didn’t anyone warn us? Why wasn’t there a knock at the door? Why didn’t the police come over and say there’s flooding, you need to get out?’”

Local authorities say sheriff’s deputies and others knocked on doors in Red Lodge and a second flooded community. However, they acknowledged that not everyone was reached, as numerous rivers and streams burst their banks, inundating areas previously never known to be inundated.

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While no single weather event can be conclusively linked to climate change, scientists said the Yellowstone flooding was consistent with changes already documented at the park in warm temperatures.

Those changes include less snowfall in midwinter and more precipitation in spring — setting the stage for flash floods when rain falls on top of the snow, said Montana State University climate scientist Cathy Whitlock.

Warming trends mean spring floods are becoming more frequent — even as the region suffers from a long-term drought that keeps much of the rest of the year dry, she said.

Meister and other experts noted that computer modeling of storms has become more sophisticated and generally more accurate than ever. But extreme weather is inherently difficult to predict, and as such events occur more frequently, there are far more chances for forecasters to be wrong.

The rate of the most extreme rainstorms has increased by a factor of five, Masters said. An event with a 1% chance of occurring in a given year — commonly referred to as a “one-in-100-year” event — now has a 5% chance of occurring, he said.

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“We’re literally rewriting our weather history book,” said Professor Jason Furtado of the University of Oklahoma Meteorology.

This has far-reaching implications for local authorities and emergency officials who rely on weather reports in their disaster response response. If they are not warned, they cannot act.

But the National Weather Service is also keen to avoid inappropriate alerts and maintain public confidence. So if the service’s models show only a small probability of a catastrophe, that information is unlikely to be included in the forecast.

Weather Service officials said the agency’s response to the Yellowstone flooding is being analyzed to determine if changes are needed. They said early warnings that river levels were rising helped officials prepare and avert the loss of life, even if their advice could not predict the severity.

Computer-based forecast models are regularly updated to account for new meteorological trends due to climate change, Peters said. Even with these refinements, events like the Yellowstone flood are still considered unlikely and so often don’t make it into forecasts based on what the models consider most likely to occur.

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“It’s really difficult to offset the feeling that you have that it could get really bad, but the chances of it getting really bad are so slim,” said Peters. He added that the dramatic switch from drought to flooding was difficult even for meteorologists to reconcile, calling it a “weather whiplash”.

To better communicate the potential for extreme weather, some experts say the weather service needs to change its forecasts to inform the public of dangerous, low-probability events. This could be achieved with more detailed daily forecasts or some sort of color-coded system for alerts.

“We’ve been slow to provide this information,” said Gary Lackmann, an atmospheric scientist at North Carolina State University. “You put it on people’s radar and they might think about it and it might save lives.”

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Hanson reported from Helena, Montana.

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Follow Matthew Brown: @MatthewBrownAP

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https://www.local10.com/tech/2022/07/07/yellowstone-flooding-reveals-forecast-flaws-as-climate-warms/ Yellowstone floods show forecast errors as climate warms

Sarah Y. Kim

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