It’s a prequel to the infamous June Gloom — that dreary period of typical Northwest, in-between weather, with clouds that hang low and persist for weeks without providing any meaningful rain.
KNKX weather expert Cliff Mass says May actually marks the start of the "low cloud season" that dominates in June. And we should expect it to extend from now until mid-July.
“I’ve noticed a change outside," Mass said. "We have these clouds that are just hanging in."
And he expects them to stick around all weekend.
LOTS OF LOW CLOUDS
“We’ll start with low clouds on Saturday," he said. "Temperatures will only get up into the mid-60s because those clouds will keep us kind of cool. No rain at all."
On Sunday, a weak disturbance is coming through and may rev up the clouds a bit. Again, temperatures will be in the mid-60s. And Mass says it may be especially cloudy in the northern part of the region, from about Bellingham to Everett.
“But it’s going to be pretty much dry — except for a few showers maybe in the North Cascades,” Mass said.
He notes these clouds often burn out later in the day. They’re the kind that set up in the lowest 5,000 feet of the atmosphere, called stratus and stratocumulus.
And Mass says the main thing forming them might surprise you, because it’s also often the cause of extended periods of sunshine and blue skies.
“Amazingly enough, this is associated with high pressure," he said. "It’s there because of high pressure."
Mass says that high pressure can also bring in extended breaks from the low clouds in May. And that’s exactly what he’s expecting next week.
“We had an April that was a little bit cooler than normal and a little bit wetter than normal for most of the region," he said. "But things are going to change radically this week. A ridge of high pressure is building in and it’s going to stay in during the next week and it’s even going to amplify — giving us amazing temperatures by the time we get to next week."
Mass says as the week progresses, the ridge of high pressure will be getting stronger and extending northward. Another key ingredient for warmth should show up Wednesday and Thursday: offshore flow.
“And I expect Thursday, we’re going to warm up to the mid-70s, and potentially upper 70s, maybe even near 80 in some places away from the water, as we get to Friday,” Mass said. “So, amazing warmup. Not much precipitation.”
He says this is not unusual for May.
“Every May pretty much, we have this period of big warm up as we start getting some offshore flow,” Mass said.
Mass says we get these breaks in May because of those winds that blow the clouds toward the ocean, as well as some other disturbances.
“So, May is not such a bad month. We have the low clouds, but it gets broken up by periods like next week,” he says.
FEWER CLOUD BREAKS IN JUNE
As spring progresses, Mass says the high-pressure system that tends to come up from California amplifies. We’re getting more toward the summer, the jet stream is moving north and the high pressure tends to really hold in place, he says. And by June it’s firmly in place.
“And then we get a period where the high pressure is right off shore of us," Mass said. "And that produces low clouds.”
HOW HIGH PRESSURE CREATES LOW CLOUDS
Mass says high pressure does this in a few ways.
With higher pressure offshore, you get lower pressure inland. That tends to push the cool marine air and clouds in over Western Washington.
High pressure is associated with sinking air aloft. It produces warming aloft because sinking air tends to get compressed and warms. So, an inversion forms. This is an area of warming temperature with height that keeps the marine air in. It caps it off and prevents any kind of mixing out. And that’s good for the clouds. But it’s bad for precipitation.
“The worst you might get is a little bit of light drizzle if some of these marine clouds get deep, but you’re not going to get any real rain,” Mass said.
High pressure tends to produce northerly winds — winds from the north along the coast. That causes upwelling (cold water coming up from the depths), and that cold water tends to cool the atmosphere to saturation, which can produce a lot of clouds.
“So: high pressure, low clouds offshore. And it tends to push it right into us during this time of the year,” Mass said.
“And because of that, the low clouds tend to hold in place for much of the month of June and so we have a very cloudy, kind of gunky month and it’s given the name, ‘June Gloom.’”
To hear the full conversation, click on the "play" icon at the top of this post.
Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX 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 podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.