Pineapple Express hits the Pacific Northwest Monday as region reels from historic flooding

Another atmospheric river, rated AR-4, is expected to bring five to 10 more inches of rain to western Washington starting Monday.

Western Washington had a brief reprieve this weekend, but another powerful storm system is already taking aim at the region. A Pineapple Express, the nickname for atmospheric rivers originating near Hawaii, is forecast to arrive Monday and could push already swollen rivers back toward flood stage.

“By Sunday and especially Monday, we really ramp up the rain coverage again,” Max Tsaparis, a NewsNation meteorologist, told “Seattle’s Morning News” on KIRO Newsradio Friday morning. “We’re back to an atmospheric river level four for much of the coastal Washington region through Monday and Tuesday next week.”

The incoming system carries the same AR-4 rating as the atmospheric river that shattered flood records across the region this week. The Skagit River near Mount Vernon crested at 41.54 feet early Friday, surpassing the previous record of 41.18 feet set in November 1990. The Snohomish River crested at 34.1 feet Thursday, topping its 1990 record of 33.5 feet.

What is a Pineapple Express?

You’ve probably heard both terms thrown around this week, sometimes interchangeably. Here’s the difference: an atmospheric river is any long, narrow corridor of moisture that flows through the atmosphere, often stretching thousands of miles from the tropics to the Pacific Northwest. A Pineapple Express is a specific type of atmospheric river that originates near Hawaii.

“Atmospheric river and Pineapple Express are pretty much interchangeable; it just depends on where the moisture source originates,” Tsaparis explained. “But tropical moisture reaching the Pacific Northwest is a recipe for disaster, as we’re seeing right now.”

This week’s storm stretched even farther than Hawaii, pulling moisture from nearly as far as Asia. Next week’s system fits the classic Pineapple Express pattern, drawing warm, wet air directly from the Hawaiian islands.

Next week’s storm won’t be as severe, but rivers could flood again

The good news: next week’s Pineapple Express should not match the destruction caused by this week’s event. The key difference is duration.

“It won’t be as extreme because, unlike this week, where it was on and off pretty much since Sunday, the rain will be more confined to just a couple of days next week,” Tsaparis said.

Rainfall totals are expected to reach 5 to 10 inches in the hardest-hit areas, compared to the 15 to 20 inches that fell in parts of the Olympic and Cascade Mountains this week. Still, that is more than enough to cause problems for rivers that have barely begun to recede.

“The takeaway message is we could still see those rivers rise again,” Tsaparis said. “If you’re in a flood-prone region, heed those evacuation orders when they’re given, or even as a precaution, consider getting out before orders are issued.”

Warm temperatures continue to make flooding worse

One factor compounding the flood risk: it’s simply too warm. Record temperatures across the western U.S. have meant that precipitation is falling as rain rather than snow, even at higher elevations.

“The reason that’s making the rainfall situation worse is that if it were colder, we’d be talking about the foothills and above getting mostly snow,” Tsaparis said. “That snow would stay locked in, which would be good not just for skiers but also to hold that water until next spring before it flowed downhill.”

Instead, all that moisture races directly into rivers and streams.

“It’s all liquid rain for the most part, with the exception being the far peaks where we’re getting snowfall,” Tsaparis said. “Elsewhere, it just races downhill and turns into this historic flooding event.”

How the atmospheric river scale works

The AR scale, which rates atmospheric rivers from 1 to 5 based on their intensity and potential impact, is a relatively recent tool. Tsaparis said it has only been in use for the past couple of years, but it helps communicate risk to the public in a way similar to hurricane categories.

“Level 4 out of 5 is pretty intense,” he said.

The scale measures a combination of moisture content, wind speed, and how long the system will affect a given area. This week’s AR-4 caused catastrophic flooding in part because it lingered over the same region for days. Next week’s AR-4, while equally intense on paper, should move through more quickly.

What to expect this weekend and beyond

Saturday looks dry across most of western Washington, giving rivers a chance to recede and residents time to assess damage. Clouds return Sunday with scattered showers, followed by heavy rain on Monday and Tuesday.

After next week’s Pineapple Express passes through, the system is expected to shift southward.

“It will turn into Oregon’s problem, even into Northern California, as we get later into the workweek heading towards Christmas,” Tsaparis said.

But western Washington is unlikely to see a prolonged dry stretch anytime soon.

“This atmospheric river setup isn’t going anywhere,” Tsaparis said. “It’s just going to be a question of who gets the focus of that very heavy rainfall.”

Charlie Harger is the host of “Seattle’s Morning News” on KIRO Newsradio. You can read more of his stories and commentaries here. Follow Charlie on X and email him here. 

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