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How Northwest Mountains Make Their Own Summer Storms

Flickr via Compfight
Clouds forming around the crest of Mt Rainier on July 31, 2008.

Have you ever noticed that oftentimes on summer afternoons, big clouds seem to form right over the Cascades or the Olympics, almost like the mountains have their own weather systems? They do.

“It’s true,” says KPLU weather expert Cliff Mass.

“Many summer afternoons, we see puffy cumulus clouds starting to form over the upper slopes of the Cascades. And they increase in width and height during the afternoon and quite frequently you have a few showers there – even some lightning,” he said.  “And so that’s real, we see it over the Cascades, we see it over the Olympics.”

Mass, who is a professor of atmospheric sciences at the University of Washington, says the physical structure of mountains can create their own weather systems, caused by the way air flows around them.

“What happens is during a normal summer day, the slopes of the mountains heat up,” Mass says.

This causes the air to flow up the slopes on both the eastern and western sides of the Cascades or Olympics. And then the upward flow coming from both sides meets near the crest of the mountains.  

“And so you have a real strong current of upward motion over the mountains,” Mass says. Clouds form because as the air rises, it tends to cool and that causes saturation.

“But something else happens,” Mass says.

Often, the air is close to being unstable. So, when it’s pushed upward by the upslope flow, it keeps on going for a while and tends to form large, puffy cumulous clouds around the tops of the mountains.

“Cumulus clouds are associated with instability,” Mass says. “And if the instability is enough, you can get towering cumulonimbus with showers and lightning.”

In other words, the mountains seem to attract clouds that can bring thunderstorms and even lightning with them.

A Potential Hazard For Summer Hiking

This means people who head to the heights of local mountains can often find themselves hiking right into stormy weather, especially in the afternoon.

Mass says the dynamics at play can produce more than just a few showers.  It’s potentially dangerous.

“Because if there’s lightning forming, then that’s something you want to keep in mind if you’re up on the crest or on a ridge, which makes you quite vulnerable to being near or even right on a lightning strike,” he says.

How To Avoid Mountain Storms

Mass says there’s one pretty sure fire way to stay safe.

“The first thing you always can do is hike early,” he says, because the storms won’t start forming till early afternoon. “Start your hike at 7 o’clock in the morning and start coming down by the time you get to noon or 1 o’clock and you’ll generally be fine.”  

Other than that, Mass says use technology if you have it to check the outlook before you go and monitor the weather while you’re out.

“The National Weather Service is getting pretty good at forecasting these kind of thunderstorms developing,” Mass says. “And so look at the forecasts. If you see the cumulus forming and if you start seeing rain, you should come down off crest right away.”

He adds that many weather apps for smart phones are useful and can be taken with you on the trail.

“Often you have connectivity way up in the mountains. And you can get the weather service forecasts. And if you’re really weather oriented, you can look at the weather radar and you can see the cumulus develop – the showers associated with cumulus with that,” Mass says.  

So, go in the morning, keep your eyes on those forecasts, and if you’re really weather savvy, keep your eye on the radar.

The weekly KPLU feature ‘Weather with Cliff Mass’ airs every Friday at 9 a.m. immediately following ‘BirdNote’, and twice on Friday afternoons during ‘All Things Considered’. The feature is hosted by KPLU Environment Reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to a podcast of ‘Weather with Cliff Mass’ shows.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to