While the weather has been tame across the U.S. so far this holiday weekend, low pressure organizing in northwestern British Columbia will swoop southward, bringing heavy rain and mountain snow to the Northwest. Already, moisture is streaming into the Northwest with spits of rain falling from Interstate 5 to the coast.
As upper-level energy rotates into western Canada, low pressure will strengthen and dig southeastward late tonight into Sunday. The Pacific flow will keep the coast and northern Cascades warm enough for rain with the heaviest downpours expected by early Monday. Meanwhile, snow will overspread the Northern Rockies today into tonight.
The low will strengthen even more as it slides north of Seattle on Monday. The counterclockwise flow around the low will send cold, Canadian air rushing in behind it. The rain will change to heavy, wet snow in the northern Cascades where 4 to 8 inches are possible Monday. The rain could even mix with wet snow into downtown Seattle on Monday but no accumulation is expected.
Ahead of the storm, Winter Storm Watches are in effect across the Northern Rockies and northern Washington. While 1 to 2 feet will accumulate in the highest mountain peaks, 2 to 8 inches will stick in the valleys. The snow will slow travel along Interstates 15 and 90. Be sure to carry a winter weather survival kit when traveling in the Northern Rockies later this weekend.
Pacific moisture will ring out heavy rain for the Northwest Coast, including the Olympics and Cascades where temperatures will be above freezing. Four to 7 inches of rain will lead to flooding on rivers, poor drainage and urban locations. If you come across a flooded roadway, it`s best to "Turn Around, Don`t Drown." Water is usually deeper than it appears.
Flood Watches are in effect in western Washington, including Seattle.
The rain will help ease the mounting rainfall deficit in the Northwest. Both Seattle and Portland, Ore., are 4 to 5 inches below average since the start of the fall.
The cold front will swoop into the northern Great Basin, northern Rockies and Northern Plains by early Tuesday spreading a swath of snow from northern California into the Dakotas.
Meanwhile, in the front`s wake, much colder-than-average temperatures will greet the Northwest. In Seattle, for instance, instead of highs in the upper 40s, the mercury will only climb just a degree or two above freezing with overnight lows in the 20s. The cold temperatures at night could cause any standing water to freeze, creating slippery sidewalks, parking lots, and overpasses.
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