La Niña's historic fury mapped and measured
- - La Niña's historic fury mapped and measured
Janet Loehrke, Doyle Rice and Ignacio Calderon, USA TODAYDecember 19, 2025 at 4:30 AM
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Historic flooding due to persistent atmospheric rivers caused widespread flooding along the Pacific Northwest.
The Pacific Northwest has a rainy reputation. But not like this.
An onslaught of destructive weather in December has broken records, flooded rivers, forced evacuations, caused widespread power outages and dazzled meteorologists.
"This will go down as one of the craziest weather months you'll ever see in the region," said Seattle-based meteorologist Michael Snyder on X on Dec. 15.
Forecasters, including AccuWeather meteorologist Chad Merrill, lay part of the blame on the La Niña weather pattern, which often leads to more frequent winter storms in the Pacific Northwest, (La Niña also famously can steer cold weather elsewhere in the country).
That pattern, combined with a series of storms that dumped more rain than snow, has led to a historic spate of flooding.
More about La Niña: Learn about its impact on US weather
A USA TODAY analysis of 39 Washington state river gages shows how much December 2025 stands out.
The analysis tracked all the instances in which the gages reached a daily average height at or above flooding levels since the year 2000. December 2025 is the month that had the highest monthly tally, by far, this century.
Analysis reveals flooding records
Another USA TODAY analysis of over a hundred gages in Washington found 13 sites where the historic crest records were potentially broken in the first two weeks of December. Most of these records were decades old, and some were broken by over 1 foot. The Washington U.S. Geological Survey office is also tracking an additional six sites that appear to have reached record peaks.
Beyond the record-breaking sites, many more reached flooding levels in Washington and Oregon.
La Niña combines with other factors to cause historic flooding
There are two big reasons for the floods, Washington state climatologist Guillaume Mauger told USA TODAY via email:
We had two back-to-back atmospheric river events. When they are closely spaced like that, they can lead to bigger floods.
The snowline was very high, which meant that there was more rain and less snow. When there's more rain, there's more flooding.
Seattle meteorologist Cliff Mass emphasized the first point, telling USA TODAY via email that "the big issue was that we had multiple events that were closely spaced, so the first one set up the flooding for the second."
"The jet stream has been enhanced by a persistent strong high pressure in the Bering Sea. The jet stream has been beaming from east Asia into the Northwest U.S. due [to] the blocking pattern over the Bering Sea. The strong jet stream helps fuel a temperature gradient, which is helping to foster the strong winds within the heavy rain as well," Merrill said.
Photos, maps show effects of record flooding
Several levees have failed in western Washington this month due to the heavy rain and flooding.
In King County, where Seattle is located, almost 18,000 acres were flooded on Dec. 11, according to data provided by Floodbase. In that county, about 1% of homes are in FEMA's flood hazard area where homeowners need insurance, but according to First Street, a climate risk financial modeling organization, about 6% of homes should be included.
1 / 18Drone view shows heavy floods in Washington and OregonA drone view shows a person walking between baseball fields flooded by the Snoqualmie River, as an atmospheric river brings rain and flooding to the Pacific Northwest, in Fall City, Washington, U.S., December 9, 2025.
Earthen levees can fail when high water levels from heavy rain compromise their structural integrity through three main mechanisms: overtopping, seepage (internal erosion), or slope instability, according to the U.S. Army Corps of Engineers. These failures often lead to a "breach," where a part of the embankment gives way and allows water to flood the surrounding land.
What is an atmospheric river?
Sometimes called "rivers in the sky," atmospheric rivers are a major factor in extreme rain and snowfall in the West. Long, narrow corridors of air heavy with moisture form when warmer air from the tropics (lower latitudes) moves toward the poles ahead of powerful storm fronts.
Atmospheric rivers function much like rivers on the surface but can carry even more water than the mighty Mississippi River. The water can travel for thousands of miles.
What happened in December (shown in the map, below) is that a stuck weather pattern helped pump copious amounts of moisture from the tropics to the Pacific Northwest in the form of several atmospheric rivers. That pattern included a strong ridge over the Bering Sea, a trough of low pressure south of Alaska (L), and another weak high-pressure ridge (H) just west of California.
Perhaps the most well-known example of an atmospheric river in the world is the Pineapple Express. An atmospheric river is a band of water vapor that flows through the Earth's atmosphere and dramatically impacts the amount of rain and snow that falls.
The Pineapple Express and other atmospheric rivers are estimated to be responsible for up to 50% of the rain and snowfall on the US West Coast.
For a full breakdown of how we analyzed the gage data, you can read our methodology on this GitHub page.
This article originally appeared on USA TODAY: La Niña fuels historic flooding in Washington, Oregon. See maps, data.
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