Seasonally warm temperatures being observed in the Pacific Northwest Friday and Saturday will draw to a close on Sunday when an upper level trough is forecast to turn the upper level wind pattern more northwesterly over the region. As a result, cooler air sourced from the Gulf of Alaska will come in for a return to showers and lower snow levels.
A lowland snow event is not expected on either side of the Cascades, but some hills around Portland and Seattle may get some slush early Monday and Tuesday before temperatures moderate some midweek. Spokane, which is higher elevation than the Tri-Cities and Yakima, could see an inch (2 cm) or so of snow at some point next week but this isn’t guaranteed.
At the surface this starts with a cold front on Sunday bringing modest northwesterly winds in behind it. Highs are forecast to drop about 10°F (5°C) from Saturday to Monday and snow levels dip to well below pass level. Some light rain and overnight snow showers are possible in the Columbia Basin during this period. West of the Cascades, Portland and Seattle can expect around an inch (25 mm) of rain through early Wednesday with higher amounts on the coast.
National Weather Service offices throughout the region have issued winter storm watches for the Cascades through Monday afternoon. During this time, new snow accumulations above 1500 feet (500 m) elevation are forecast to range from around 12 inches (30 cm) down by Crater Lake to up to 30 inches (75 cm) in the Mount Baker area. Travel across Cascade passes will be difficult to impossible at times. Heavy snow is forecast to continue beyond Monday afternoon so that accumulations by midweek may be much higher.
Temperatures moderate in the lowlands midweek in between storm systems. Still, northwesterly flow coming off the Pacific Ocean will keep Seattle and Portland cool and showery with significant mountain snow continuing. The Columbia Basin will be insulated from this onshore cool flow by the Cascades allowing for temperatures to reach back into the mid- to upper-50s (12-15°C).
Models indicate another strong cold front sometime between Thursday and Saturday of next week. It’s still a ways out to determine specifics from this but it seems reasonable to expect that event to look similar to the Sunday-Tuesday event.
Snowpack in the Pacific Northwest has struggled to keep pace with average so far this season. Below average snowfall is typical of El Niño years when the jet stream commonly splits over the North Pacific, sending one plume of moisture toward Alaska and Yukon and the other toward California leaving the Northwest warm and dry.
This year has been a little different. With the exception of Western Washington and the Cascades north of Mount Adams, precipitation has been near to a little above average but temperatures have also been above average. As such, snowpack in Washington, the Idaho Panhandle, and British Columbia is severely lacking. Oregon has caught up some but the healthiest snowpack remains south of the Snake River in Idaho and the small sliver of Wyoming that is part of the Columbia River watershed.
The snow situation is likely to improve greatly over the next week but these gains still likely won’t be enough to completely eradicate the deficit currently being observed. Modeling favors below average temperatures continuing into the start of March but with decreased precipitation so that the mountains should still gain some snow but not at the rate we’re about to see this week.
