Parts of the northern Cascades were expecting up to 15 inches of snow from the first of the storm systems, which was moving ashore during the day on Monday. More than 2.5 inches of rain had already fallen in Seattle since Saturday, with more on the way.
At least two more rounds of significant precipitation are likely in parts of Washington and Oregon through the remainder of the week, with even heavier totals in British Columbia.
Precipitation, strong winds blasting both the Pacific Northwest and Northern California
The back-to-back storm systems signal the annual deluge brought about by the winter wet season. November, December and January are Seattle’s wettest months, accounting for nearly half of the city’s annual precipitation.
“Well, it’s raining … again,” wrote the National Weather Service in Seattle in its Monday morning forecast discussion.
Winter storm warnings were up across northern Washington state, with winter alerts stretching all the way through Oregon and even into the Sierra Nevada as a moist onshore flow dumps snow in the high elevations. Up to two feet is expected in the Northern California peaks above 5,000 feet through Monday night.
Wind advisories blanket the Northern California coastal plain into southwest Oregon, where gusts up to 50 mph are possible, with high wind warnings for the higher terrain inland.
Atop the summit of Mount Rainier in Washington, 65 to 90 inches of snow are likely to fall through Wednesday night. An avalanche warning was in effect Monday from the east slopes of the Cascades between Lake Chelan to Interstate 90, where heavy, wet snowfall will stress a less dense snowpack beneath.
Below 3,000 feet, the bulk of any precipitation will fall as rain, contributing to the risk of mudslides in areas with already saturated soil. The Weather Service in Seattle noted that “several minor mudslides were reported this weekend across the Olympic Peninsula.”
Additional systems on the way
The storm system will clear and head east by Tuesday, affecting Idaho and the Columbia River Basin Tuesday and midweek. Meanwhile, the next in a parade of storm systems will drench Vancouver Island overnight Tuesday into Wednesday before the moisture stream laps at coastal Washington, Oregon and Northern California.
Rainfall of 1 to 2.5 inches is probable near the coast, with double-digit snow totals expected at locations primarily above 6,000 feet.
Behind the system’s cold front, a few showers with hail or small ice pellets and an isolated flash of lightning can’t be ruled out. A drying trend is expected later in the week.
A third system may approach this weekend, with signs of an increasingly active stretch of soggy weather and gloom into the middle of the month.
What’s behind the stormy weather?
The busy weather pattern is thanks to the winter position of the jet stream, a river of swiftly moving air at high altitudes in the atmosphere. Storm systems form within southward buckles of the jet. During the winter, the jet stream slides southward toward the U.S.-Canada border, bringing with it an active “storm track.” The jet stream retreats back north during the summertime.
The Pacific jet stream is recovering from an influx of energy stemming from a pair of intense weather systems — a possible world-record-strong high-pressure system over Mongolia and the strongest nontropical low-pressure system to sweep through the northern Pacific. Those two storms modulated the jet stream, intensifying it and amplifying its waviness.
Think of a stone in a shallow babbling brook. If you drop a rock onto the riverbed, you may see a series of waves downstream. That’s sort of what happens when the jet stream is disturbed. If a strong pair of perturbations causes a big diversion north or south of the jet stream, through a deep layer of the atmosphere, then a series of waves — or additional storm systems — are probably downstream to the east.
Even after the energy from those disturbances has long since dissipated and dispersed, a stormy pattern is likely to continue in the Pacific Northwest for quite some time.