With all the bright sun, blue skies and warm temperatures that have dominated Puget Sound weather recently, it’s a little ironic to see showers in the forecast for Saturday. This weekend is the one anyone planning a major outdoor event in the Northwest typically aims for, says KNKX weather expert Cliff Mass.
“The day that you really should bet on having dry conditions — the driest day of the year — is Monday, the 29th,” Mass said. He says the days on either side of that date also tend to be sunny and mostly rain-free.
This is based on the climatological record of rainfall measured at Seattle-Tacoma International Airport. The average works out to a tiny fraction of an inch, which most people wouldn’t even notice.
“We typically get four thousandths of an inch of rain,” Mass said. “Most 29ths we get no rain at all.”
It works out that on only about 7 percent of those days (July 29) does any rain fall at all.
DRY ALL WEEKEND – EXCEPT SATURDAY
Friday will fit that picture, with dry, sunny skies and warm temperatures that should reach the mid-80s. “Very, very nice,” Mass said.
But then a weak front comes in Saturday morning. And temperatures will drop into the lower 70s.
“Sometime around 3 to 5 o'clock in the morning, the rain will sweep in from the coast, we’ll get a few sprinkles here – maybe a convergence zone north of Seattle that’ll give a little heavier rain,” Mass said.
(The convergence zone is a narrow band of heavy rainfall that forms after currents of moist air from the west are split by the Olympic Mountains, and then come back together again. The zone typically sets up somewhere from Seattle northward to Everett.)
“And rain will hold in maybe till the middle of the afternoon on the western slopes of the Cascades, where they’ll get maybe a tenth of an inch of rain,” Mass said.
But that rain event should be through sometime later on Saturday. Then it’s back to the dry, sunny weather that’s more typical for this time of year.
“Sunday and Monday should be really nice,” he said, with temperatures climbing back to the mid-70s Sunday and “maybe even lower 80s as we get into Monday and Tuesday.”
Later in the week, another front is expected to bring rain back to the region on Thursday, as August begins.
PEAK FIRE SEASON ARRIVES
The atypical wet weather should be a blessing for firefighters, who face the peak of the wildfire season in August.
Already, several grassland fires and many forest fires are burning in Washington. A burst of lightning Tuesday initiated most of them.
“There were over 3,000 lightning strikes over Eastern Washington,” Mass said. “Nothing over the western side of the Cascades or Puget Sound, but they had massive lightning Tuesday.”
Mass says a fairly dry July left grasses and other light material on the ground primed to catch a spark and grow quickly into major fire events. Most were between 100–500 acres as of Friday. Several blazes have kept burning around Yakima and in the Okanogan Wenatchee National Forest.
QUICK-BURNING 'CHEAT GRASS'
“A lot of the fires are grass fires. And one thing that maybe is not advertised enough is that we’ve had a massive invasion of what’s called ‘cheat grass’ — extremely flammable, non-native grasses have spread all over Eastern Washington. And they’ve pushed out the bunch grass, the native grasses that didn’t burn as much,” Mass said. “So we have this heavy load of grasses that burn with any kind of initiation — either from humans or from lightning.”
This is particularly extreme in flatland areas of Eastern Washington, but it also can combine with other kinds of underbrush and leftover slash on the forest floor, creating an ideal mix of fuels to get a blaze going quickly.
The Left Hand Fire that started Tuesday on the slopes of the Cascades in a steep rocky drainage is a good example of how those kinds of materials can initiate a fire at lower elevations that quickly travel upslope, making them challenging to contain.
Mass says winds that typically happen this time of year also stoke and spread the fires.
EASTERLIES STOKE FIRE RISK
“Wind is an issue on the eastern slopes of the Cascades going down into the Columbia Basin," Mass said. "Around here, the winds pick up typically sometime in the late afternoon and evening and we get some very strong winds on the eastern slopes of the mountains.
“That’s one of the reasons all the wind turbines are there. But those winds can help the fires rev up and push them very quickly down wind.”
Mass says winds Friday could stoke fires, as is typical for this time of year. But the weather should be on the side of the firefighters in a couple of days, at least for a little while.
“As we get into the weekend, the front will move through, the relative humilities will increase and we expect the winds to really die down over the weekend. So I think there’ll be an opportunity to knock these fires down,” Mass said.
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.
