Swain: ‘Super’ El Niño building off the coast may bring Seattle its first muggy, humid summer in a generation

Seattle doesn’t do humid. We brag about it. Dry summers, cool nights, sleep with the window cracked, no air conditioning required. It’s part of the deal in Washington. This summer may break the deal.

The Pacific off our coast is running hotter than it has in years. A strong, possibly historic El Niño is loading up in the tropical Pacific. Together they’re set to drop something on Seattle this summer most of us have never lived through: Muggy nights, sticky afternoons, and air that refuses to cool down after the sun goes off.

Climate scientist Daniel Swain, who writes the widely-read Weather West blog, is sounding the alarm.

“Something extraordinary could unfold in the global climate system later this year,” Swain wrote this month.

He’s not hedging on the strength of the signal, calling it “perhaps the strongest I’ve ever seen.”

Nearly 90% odds of a ‘Super’ El Niño by autumn

The National Oceanic and Atmospheric Administration said El Niño is already underway as of June. NOAA puts the odds of a “strong” event at nearly 90%, with better than 60% odds that it reaches “very strong” by autumn.

The European model ensemble goes further, explicitly predicting a record-breaking event by fall. Using a newer index built for a warming climate, 8 of 13 major modeling centers project an El Niño stronger than anything observed since 1982.

Forecasters have a nickname for the very strongest of these: A “Super” El Niño, and we’re staring at one.

Why Seattle will feel muggy this summer

A marine heatwave has been parked across the northeast Pacific since late winter, and the warm water now reaches all the way to the West Coast. Warm ocean dumps water vapor into the air, and water vapor is what makes summer feel awful.

Swain’s call for the West Coast is direct: “Warmer and more humid than usual along the immediate West Coast,” with that warm, moist influence “spilling inland through gaps in coastal terrain.”

And don’t expect much relief when you go to bed. “Nights will be considerably milder than usual,” Swain wrote.

We don’t see this much in Seattle. The advice for as long as anyone can remember has been simple: Open your windows at night, and it’ll cool off by morning. The trick only works because our nights cool off. A Super El Niño summer takes the trick away.

Los Angeles and San Diego could see “downright tropical dewpoints” by late summer, Swain said. Seattle won’t get tropical, but we’ll get the milder cousin of the same system, layered on top of a city that handles humid heat about as well as San Diego handles snow.

Why the muggy nights matter more than the hot days

Here’s the counterintuitive part.

Seattle’s truly scorching days, the 90-plus stretches, almost always require offshore flow: dry wind pushing down from Eastern Washington into the west side. That’s how the region baked in 2021. The hottest of the hot days are dry days, with low humidity, not the muggy ones.

So the humidity threat isn’t really about our peak temperatures. It’s about the warm, borderline-hot days, think upper 70s to mid 80s, when high humidity means the air never cools off at night. Those are the nights that wear you down. Those are the nights the open-window trick stops working.

There’s a fire-weather wrinkle too. In this El Niño setup, the odds increase for southerly flow, warm air pushing up from countries to our south. That’s the pattern that drives monsoon moisture and hot thunderstorms into Eastern Washington, a serious Washington wildfire concern. Southerly flow aloft also helps elevate humidity here on the west side, with relative humidity potentially climbing.

The lessons of the 2021 heat dome

Swain flagged elevated odds of “blocking” high-pressure ridges over the Pacific Northwest during El Niño summers, the kind of stubborn dome that bakes the region for days.

“The odds of major heatwaves and fire weather events may be especially high,” he wrote.

You already know what an extreme Seattle heat wave does here when we’re not ready.

On June 28, 2021, in Seattle, it hit 108 degrees, the peak of a heat dome that became the deadliest weather event in Washington’s recorded history. More than 100 people died. Peer-reviewed work later put the true toll closer to 160 over three weeks. The dead were disproportionately older, poorer, and without a way to cool down.

So how does 2021 fit in? That heat dome was deadly, but it was also bone dry. Seattle’s hottest days almost always are. This El Niño summer isn’t likely to beat 108 degrees. If we get extremely hot temps, it will almost certainly be a dry heat.

What’s different is the elevated chances of humid days and nights when it’s daytime highs in the mid-70s to mid-80s. It’s the kind of sticky air that keeps your house from ever cooling down, even after the sun goes down.

Why so many Seattle homes still have no air conditioning

In the early 1990s, only about 15% of Seattle area homes had air conditioning. You didn’t need it. Seattle was famous for being the major metro least likely to have a window unit.

That number has climbed steadily, and it jumped fast after 2021. But plenty of bedrooms in the region still depend on a cool night to be survivable, and a lot of them sit in what scientists call urban heat islands, where concrete holds the day’s warmth long after sundown.

A summer that refuses to cool down at night takes the one thing those homes were counting on. The people without air conditioning tend to be the same people who can least afford to install it or to leave.

Could this be a dud like the 2015-2016 El Niño?

Plenty of people around here remember the last “Super” El Niño in 2015 and 2016, and the wet winter it was supposed to deliver, but mostly didn’t. Fair point. A seasonal forecast in June is a tilt in the odds, not a guarantee.

Two things make this one different. The warm water is already off our coast right now. You don’t have to trust a model to see it. The buoys are reading it today, and Swain said the humidity signal for the immediate coast is among the more confident pieces of the whole outlook.

Seattle doesn’t do humid. This summer, we may not have a choice.

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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