Marine air surges in with cooler temps, but wildfire risk remains
The mercury hit a record-breaking 95 degrees at Sea-Tac Airport Wednesday, causing a road to buckle in Seattle’s SoDo neighborhood in a dramatic display of what heat can do to infrastructure.
But where there was heat, there were none of the wildfires some folks often associate with warmer temperatures. In a somewhat ironic twist, the risk of wildfires in the region increased Friday as a push of cooler marine air surged in off the ocean, bringing the low clouds locals more often associate with June weather, sometimes known as "June gloom."
“This morning was a real shock,” said KNKX weather expert Cliff Mass of Friday’s conditions. “Clouds everywhere, temperatures much cooler and winds coming from the west."
A COOLER WEEK AHEAD
Mass says Friday will be mostly to partly cloudy, with temperatures getting up to only about 70 degrees. The weekend will be warmer, with no precipitation and morning clouds that burn off as temps reach the mid-70s, which is still above normal for this time of year.
Monday’s conditions should be similar, with highs around 75. But on Tuesday, Mass says a weather system will approach, bringing with it clouds, a chance of showers and cooler temps that drop into the 60s.
“I don’t see any heat waves, at least through the end of the week,” Mass said.
MORE THAN HEAT NEEDED FOR WILDFIRES
Mass says heat is not the main story when it comes to Northwest wildfires. It plays a role in drying out the ground, grasses and underbrush that fuel wildfires. But the more important ingredient is wind, especially downslope wind, which is dry and gets warmer and drier as it descends mountainsides.
“When you have these strong downslope winds, that helps in a few ways,” Mass said.
“As the air sinks, it dries. The relative humidities tend to be low. Also, the winds tend to stoke the fire. Fires expand rapidly, they move rapidly, embers get pushed ahead when you have strong winds.”
Mass says most of the big wildfires in the Northwest, especially on the west side of the mountains, have been associated with strong downslope winds.
“And this week, we may have gotten up to the low- to mid-90s, but that day was one in which we didn’t have strong winds at all,” Mass said.
He says there also wasn't anything to initiate a blaze.
“We had relatively clear skies and no lightning at all that day,” he said of the heat spike Northwesterners experienced Wednesday.
“So temperature alone is not enough, we need some other ingredients to get the big wildfires here,” Mass says.
DOWNSLOPE WINDS STOKE FIRES 'LIKE A HAIR DRYER'
Further evidence can be gleaned from fire events in the past year. In March, western Washington had an unusual spate of early wildfires. Mass says they were stoked by an unusual weather situation, after a wet winter that left plenty of grasses ready to be dried out.
“High pressure in eastern Washington, lower pressure over the west, and we had extraordinarily unusual and powerful easterly winds that came from eastern Washington, descended on the Cascades, and as they descended the air warmed as it compressed and it dried. So we had this extremely dry air and strong winds,” Mass said.
The wind hit lush fuels leftover from the moist winter.
“These strong downslope winds are like a hair dryer,” he says. “It dries out grasses within hours.”
In those conditions, a tiny spark from a dragging chain or the embers of a cigarette butt can be enough to ignite a small bit of grass that quickly explodes into an inferno.
These conditions can occur even in cool temperatures, as was the case here in March. Another good example, Mass says, is California’s fatal Camp Fire last year that started near the retirement community of Paradise, obliterating the town, causing billions of dollars of damage and killing 86 people.
“They had strong downslope winds on the Sierra Nevada, but the temperatures were in the upper 40s to around 50,” Mass said. “Temperatures don’t have to be warm to get major fires that are blown by the wind.”
He says Friday’s winds caused by the cooldown are now creating conditions that bring a greater risk of wildfire than the record heat of last week.
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, anda popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, viaiTunes or Google Play.