Climate change primed Washington state for historic flooding

Earlier this month, a long, warm plume of airborne water made its way from the subtropical Pacific Ocean to the West Coast of the United States. Since the so-called atmospheric river As it made its way north, it absorbed moisture from above-average sea surface temperatures.

When the river of air spilled onto the shore on December 8, unleashed a torrential torrent of water across the Pacific Northwest. it didn't stop for most of the week. Now a second atmospheric river is flowing through Washington state, and a third is likely to appear later this week.

On Wednesday, Washington Gov. Bob Ferguson declared a state of emergency and government officials gave the command “go now” evacuation alerts up to 100,000 people. Thousands of residents in western Canada were also ordered to evacuate. Parts of western Washington state and rivers throughout the region received up to 18 inches of rain last week. burst their banks. At least 30 major roads were impassable or closed due to damage. 250 water rescues.

“The flood levels we're seeing are potentially historic,” Ferguson said at a press conference on Thursday.

However, they are not completely unexpected. A study published earlier this year found that atmospheric rivers have become slightly wetter, larger and more frequent since 1980. These changes are consistent with basic climate change physics: a warmer atmosphere can hold more moisture. Meteorologists say these dynamics increase precipitation from small squalls to large atmospheric rivers, contributing to thorn flash floods in many parts of the country this year.

Beyond this, there are other climate conditions that are fueling the chaos in Washington. Much of the western United States was unusually warm this time of year, resulting in little snow cover in mountain ranges from British Columbia to California. Above-average warmth, not only in the area where the atmospheric river came ashore but also along its route across the Pacific Ocean, meant that the rain that fell on Washington was heavy and warm. It's warm, said Chris Gloninger, a senior climate scientist at Woods Hole Group“would not have been statistically possible without the anthropogenic influence of climate change.”

The lack of snow cover made the rains even more deadly. Deep and frozen snowpack can absorb rain and freeze it where it lands, noted Daniel Swain, a climate scientist at the California State University of Agriculture and Natural Resources, in live broadcast event on Friday. But unusually light snow that had accumulated in the mountains was easily washed away by the atmospheric river last week, further worsening already severe flooding.

“If you get some snow, but it's not very significant, if you get a really big, long, warm rain, you can actually melt the entire snowpack in one go,” Swain said. “This is probably exactly what we saw during the flooding in the Pacific Northwest.”

Chad Magby, a project manager for an industrial insulation contracting firm who lives in Index, Wash., began watching the river rise Monday from his cabin along the North Fork River. Magby and his wife Jasmine (related to this author's cousin) are accustomed to flooding in the Index, where heavy rainfall in 2006 and 2015 caused the river to overflow its banks.

But this time everything was different. The area was still recovering from the 2022 Bolt Creek Fire, which left a burn scar, scorched trees and loose soil that made thousands of acres above the region's main highway more prone to landslides. So after last week's rains, standing water and debris blocked roads outside the city in all directions. Magby and the 150 or so people living in Index couldn't get out even if they tried. Chad and Jasmine sat and watched the river rise, hoping it would stop before it reached the several dozen houses on their dead-end road.

“What was different about it was the feeling of being trapped,” Magby said. “There was no way to leave.”

Before the atmospheric rivers collapsed, the western US was in various stages of drought. The abnormal drought of the rest of the year hit the region again when the rains finally arrived. Washington released drought declaration for the third year in a row, and conditions throughout the Colorado River Basin were consistent drought criteriaand parts of the upper basin are experiencing “extreme” and “exceptional” drought. At Index, the normally fast-flowing North Fork River was so low this summer that parts of it appeared to be crossed on foot, Magby said. Six months later the river had risen almost to the height of his house.

Index and other parts of Western Washington are likely to see at least another 8 inches of rain in the coming days as more storms hit the state. By the end of the year, Washington's annual rainfall totals could look close to normal or even slightly above average, despite the drought.

“We had so much rain in just one event, and that's why it's probably an average season on paper,” Gloninger said. “But when you peel back the layers, if it only happens over a couple of days rather than over a year, it's extremely troublesome.”