An onslaught of devastating weather in the Pacific Northwest has broken records, forced evacuations and dazzled meteorologists.
The Pacific Northwest has a reputation for being rainy. But not like this.
December’s onslaught of devastating weather broke records, flooded rivers, forced evacuations, caused widespread power outages and dazzled meteorologists.
“This will be remembered as one of the most extreme weather months this region has ever seen,” Seattle-based meteorologist Michael Snyder told the X Show on Dec. 15.
Forecasters, including AccuWeather meteorologist Chad Merrill, place some of the blame on La Niña weather patterns. La Niña events are often responsible for frequent winter storms in the Pacific Northwest (La Niña events are also known to induce cold waves in other parts of the country).
This pattern, combined with a series of storms that dumped more rain than snow, led to historic flooding.
A USA TODAY analysis of 39 river gauges in Washington State shows just how remarkable December 2025 is.
The analysis tracked all instances since 2000 in which the gauge reached a height above the average daily flood level. December 2025 is the hottest month ever this century.
Analysis reveals records of flooding
A separate USA TODAY analysis analyzed more than 100 gauges in Washington and found 13 locations where historic ripple records could have been broken during the first two weeks of December. Most of these records are decades old, and some were broken by more than a foot. The U.S. Geological Survey in Washington is also tracking six more locations that appear to have reached record peaks.
In addition to the record-breaking locations, additional locations in Washington and Oregon reached flood levels.
La Niña combined with other factors to cause historic floods
Washington state climatologist Guillaume Mauger told USA TODAY in an email that there are two major reasons for the flooding.
- We held the Atmosphere River event for the second time in a row. This density can result in larger flooding.
- The snow line was very high, so there was more rain and less snow. When it rains, flooding also increases.
Seattle meteorologist Cliff Mass emphasized the first point, telling USA TODAY in an email: “The big problem was that there were multiple events closely spaced, and the first event caused the second flood.”
“Sustained strong high pressure in the Bering Sea has strengthened the jet stream. The blocking pattern in the Bering Sea has forced the jet stream out of East Asia and toward the northwestern United States. The strong jet stream promotes a temperature gradient that also helps promote strong winds amid heavy rain,” Merrill said.
Photos and maps show the effects of record flooding
Heavy rain and flooding caused multiple levees to fail in Western Washington this month.
About 18,000 acres were flooded in King County, where Seattle is located, on Dec. 11, according to data provided by Floodbase. About 1% of homes in the county are in FEMA flood hazard areas that require homeowner insurance, but First Street, a climate risk financial modeling organization, says about 6% of homes should be covered.
According to the U.S. Army Corps of Engineers, earthen levees can fail when high water levels from heavy rains compromise their structural integrity through three main mechanisms: water overflow, seepage (internal erosion), and slope instability. These breaches often lead to “bursts,” where parts of the levee collapse and flood surrounding land.
What is an atmospheric river?
Atmospheric rivers, also known as “rivers in the sky,” are a major contributor to extreme rain and snowfall in the West. When warm air from the tropics (lower latitudes) moves toward the poles in advance of a powerful storm front, a long narrow channel of moisture-laden air forms.
Atmospheric rivers function similarly to surface rivers, but they can carry even more water than the mighty Mississippi River. Water can travel thousands of miles.
What happened in December (shown in the map below) is that the stagnant weather pattern pumped large amounts of moisture from the tropics into the Pacific Northwest in the form of several atmospheric rivers. This pattern included a strong ridge in the Bering Sea, a trough of low pressure south of Alaska (L), and another weak high pressure ridge just west of California (H).
Perhaps the most well-known example of an atmospheric river in the world is the Pineapple Express. Atmospheric rivers are bands of water vapor that flow through Earth’s atmosphere and dramatically affect the amount of rain and snow that falls.
Pineapple Express and other atmospheric rivers are estimated to be responsible for up to 50% of the rain and snowfall on the U.S. West Coast.
Atmospheric river flooding prompts evacuations in Washington
An atmospheric river dumped a foot of rain, causing severe flooding in Washington state.
To learn more about how to analyze gauge data, check out our methodology on this GitHub page.
