Atmospheric rivers drench the Pacific Northwest, ending drought but raising flood risk

After months of record dryness, the Pacific Northwest is getting drenched. A series of atmospheric rivers — long, narrow corridors of warm, moisture-laden air streaming up from the subtropics — began hitting Washington and Oregon this week, with forecasters expecting the rain to continue into the weekend.

The timing is significant. Washington had just come off its fourth consecutive summer of severe drought. Earlier this month, the U.S. Drought Monitor found more than 94% of the state under 'abnormally dry' conditions, with more than 61% suffering from 'moderate' drought or worse, according to the Seattle Times. This October was drier and sunnier than normal — the state received around half its typical precipitation.

The current system dropped nearly an inch of rain on Seattle on Tuesday alone, according to National Weather Service meteorologist Jeff Michalski. After a brief pause, another inch is expected between Friday and Saturday. Along Washington's coast, the rain has arrived alongside rough surf and flood concerns; farther inland, strong winds caused power outages.

What exactly is an atmospheric river? Think of it as a river in the sky: an elongated, river-like band of concentrated water vapor, sometimes thousands of miles long, that flows from the tropics or subtropics toward higher latitudes. When this moisture-saturated air hits the coast and is forced upward by mountains, it releases its water as heavy rain or snow. The Pacific Northwest sees a dozen or more of these systems each year during the October–March wet season. You may know them by an older nickname — the 'Pineapple Express' — though that term has largely fallen out of use among scientists.

We're kind of a one-trick pony when it comes to flooding.— Guillaume Mauger, Washington State Climatologist

Mauger's point is that most of Western Washington's major storms — and nearly all of its flood risk — trace back to atmospheric rivers. That makes the arrival of this week's systems a relief and a risk at the same time.

The danger compounds when atmospheric rivers collide with other conditions. When they arrive during king tides — the highest tides of the year in Puget Sound — low-lying areas can flood fast. That happened in 2022, when the combination inundated Seattle's South Park neighborhood, flooding homes and forcing evacuations, the Seattle Times reported. When back-to-back systems arrive in quick succession, they can overwhelm waterways and erode flood-control infrastructure like levees. That scenario played out last year in what the Seattle Times described as one of Washington's most devastating natural disasters, damaging or destroying thousands of homes and racking up hundreds of millions of dollars in damage.

Fresh burn scars from this fire season — described by the Seattle Times as one of the most devastating in state history — add another layer of risk. When heavy rain falls on land stripped of vegetation by wildfire, the roots that normally hold soil in place are gone, making landslides and mudslides far more likely. The remote town of Stehekin suffered serious damage during December floods after water rushed down a burn scar left from the previous year's fires, hitting houses, roads, and the town's water treatment plant.

Climate change is reshaping how these systems behave, though the picture is not yet fully clear. There is mixed data on whether warming will make atmospheric rivers more frequent or simply more intense, according to Mauger. Some meteorologists believe global warming could slightly reduce how often they occur. However, a 2018 NASA study cited by the Seattle Times found that when they do arrive, they could be as much as 25% wider and longer — dramatically increasing the wind and rain they deliver.

One trend is clearer: Washington's precipitation is falling increasingly as rain rather than snow. That means more water arrives during wet seasons when it cannot easily be captured or stored, and less is available as snowpack during dry summer months when it is needed most. Atmospheric rivers, which originate in the tropics and have historically brought rain rather than snow except at high elevations, could worsen that imbalance — though Mauger said probably not by much. A bigger concern is that heavy rain falling on existing snowpack can melt it rapidly, worsening both floods and the long-term drought cycle.

For now, the region's parched soils are drinking in the moisture. But with more systems forecast through the weekend, and burn scars fresh across the landscape, emergency managers and residents in flood-prone areas are watching the rain gauges closely.

Why it matters — Atmospheric rivers are the Pacific Northwest's primary source of both drought relief and flood risk, and understanding how climate change may make them wider and wetter is essential for communities still recovering from last year's disasters and facing fresh burn scars across the landscape.

Reported by ucsd.edu, cw3e.ucsd.edu, seattletimes.com, koin.com