Welcome to a week of firsts, Western Washington.
The first arrival of arctic air this season will usher in a rather cold and soggy week in the lowlands and the first accumulating snowfall in the mountains. The first freeze of the season and the subsequent end of growing season is expected to arrive this week, too (more on that later).
A cold front brushing over Canada will drag down the coast and into Washington Tuesday, taking the mercury down the thermometer with it.
At the same time, moisture from the Pacific Ocean will drift inland as low pressure and a pool of cool air from up north begin their work down on the surface.
So, what exactly does all that mean?
It's a recipe for wind, rain and mountain snow — and temperatures trailing the cold front are expected to be some of the coldest across the region so far this fall, the National Weather Service said.
In the lowlands, widespread rain — a half-inch to three quarters of an inch, the weather service said — will fall Tuesday afternoon through Wednesday.
Below-average highs will hug 50 degrees in many locations across the region, while overnight lows sink into the 40s and even lower in higher elevations by early Wednesday.
A second system arriving Thursday is expected to bring a reinforcing shot of cold air through the early part of the weekend.
High temperatures in the lowlands will remain in the upper 40s, "staying quite a bit below average" and ushering in the region's first freeze in some areas, said Samantha Borth, a meteorologist with the weather service in Seattle.
As steady rain soaks the ground in the lowlands on Tuesday, snow elevation is expected to plummet from around 2,500 feet to as low as 1,500 feet (and possibly lower for areas near the Canadian border) by Wednesday morning, "and that's going to be impactful to our passes, naturally," Borth said.
Snowfall will be wet and heavy in all the mountain passes through Wednesday, with totals "potentially exceeding a foot in most areas," the weather service said.
The weather service on Monday issued a winter storm watch for up to 15 inches of snow Tuesday afternoon through Wednesday afternoon in the Cascades of Whatcom, Skagit, Snohomish and King counties, including Snoqualmie and Stevens passes. Some areas in the Cascades may exceed 20 inches of snowfall.
There was also a winter storm watch in effect for up to 11 inches of snow through Tuesday night in the Olympics.
If you plan to drive across the passes, you should plan on slippery road conditions, especially Tuesday night, the weather service advised.
"The vast majority of mountain pass closures in winter are due to unprepared drivers going too fast, not chaining up then spinning out and blocking the road," WSDOT said. "It can take quite awhile to clear things in the snow up there. So don't be one of those drivers."
WSDOT crews were in the process of closing Highway 542 to Artist Point for the season on Monday.
"We typically close the road when the first winter storm arrives," WSDOT said. The highway will reopen in the spring, the department said.
