A dynamic and impactful atmospheric river impacts the Pacific Northwest through the weekend. The first wave that moved through Friday into Friday night brought some cooling. Moderate-intensity organized precipitation on the back of this wave should mostly slide east of the region by 10 AM. Overnight snow in some areas has now changed to back to rain except at upper elevations thanks to a brisk, very moist, and very mild SW flow taking aim at our region.
The main event of the atmospheric river reaches NW Washington in the late morning and afternoon hours, associated with a surface low tracking toward Heida Gwaii. Heavy precipitation reaches the Olympics by late morning, Mt Baker area mid-day, and the mountain loop by mid-afternoon. The heaviest precipitation expands throughout the region during the evening hours, continuing overnight with the highest precipitation rates shifting to the southern Cascades. Snow levels rise to 6000 ft in the north and 9000 ft by Mt Hood.
On Sunday, expect very heavy rain and highest-elevation snow for the southern and central Cascades, with some of the heaviest precipitation shifting back into the northern Cascades during the afternoon. Snow levels will lower to 5000-7500 ft, potentially causing significant snow accumulations at Washington Pass.
Mostly moderate SW winds will continue to buffet the region throughout the storm cycle.
Weather Forecast
Olympics
West North
West Central
West South
Stevens Pass
Snoqualmie Pass
East North
East Central
East South
Mt. Hood
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Saturday
Periods of light rain and higher-elevation snow in the morning, then becoming very heavy. Increasing moderate to strong ridgeline winds.
Saturday
Night
Decreasing moderate to heavy rain and higher elevation snow.,
Saturday
Periods of light rain and higher-elevation snow in the morning, then becoming very heavy. Increasing moderate to strong ridgeline winds.
Saturday
Night
Decreasing heavy rain and higher elevation snow. Moderate ridgeline winds become mostly light.
Saturday
Periods of light to moderate rain and higher-elevation snow become heavy late in the day. Moderate ridgeline winds.
Saturday
Night
Very heavy rain and higher elevation snow. Moderate ridgeline winds.
Saturday
Periods of light to moderate rain and snow. Moderate ridgeline winds.
Saturday
Night
Very heavy rain and higher elevation snow. Moderate ridgeline winds.
Saturday
Periods of light to moderate rain and higher elevation snow. Moderate ridgeline and light E wind at the Pass.
Saturday
Night
Very heavy rain and higher elevation snow. Moderate ridgeline and light E wind at the Pass might switch W at times overnight.
Saturday
Periods of light to moderate rain and higher elevation snow. Moderate ridgeline and light E wind at the Pass.
Saturday
Night
Very heavy rain and higher elevation snow. Moderate ridgeline and light E wind at the Pass might switch W at times overnight.
Saturday
Light rain and snow become moderate in the afternoon. Moderate ridgeline winds.
Saturday
Night
Very heavy rain and snow. Moderate ridgeline winds.
Saturday
Periods of mostly light rain, higher elevation snow, and wintry mix at times. Moderate ridgeline winds.
Saturday
Night
Heavy rain and higher elevation snow. Moderate ridgeline winds.
Saturday
Periods of mostly light rain, higher elevation snow, and wintry mix at times. Moderate ridgeline winds.
Saturday
Night
Heavy rain and higher elevation snow. Moderate ridgeline winds.
Saturday
Periods of light rain and higher elevation snow. Moderate to strong ridgeline winds.
Saturday
Night
Increasing very heavy rain and higher elevation snow. Strong ridgeline winds.
The NWAC program is administered by the USDA-Forest Service and operates from the National Weather Service Forecast Office in Seattle. NWAC services are made possible by important collaboration and support from a wide variety of federal, state and private cooperators.
The 5000’ temperature forecast does not imply a trend over the 12 hr period and only represents the max and min temperatures within a 12 hr period in the zone. The 6-hr snow level forecast, the forecast discussion, and weather forecast sections may add detail regarding temperature trends.
The snow level forecast represents the general snow level over a 6 hr time period. Freezing levels are forecast when precipitation is not expected.
*Easterly or offshore flow is highlighted with an asterisk when we expect relatively cool east winds in the major Cascade Passes. Easterly flow will often lead to temperature inversions and is a key variable for forecasting precipitation type in the Cascade Passes. Strong easterly flow events can affect terrain on a more regional scale.
Ridgeline winds are the average wind speed and direction over a 6 hr time period.
The wind forecast represents an elevation range instead of a single elevation slice. The elevation range overlaps with the near and above treeline elevation bands in the avalanche forecast and differs per zone.
Wind direction indicates the direction the wind originates or comes from on the 16-point compass rose.
Water Equivalent (WE) is the liquid water equivalent of all precipitation types; rain, snow, ice pellets, etc., forecast to the hundredth of an inch at specific locations. To use WE as a proxy for snowfall amounts, start with a snow to water ratio of 10:1 (10 inches of snow = 1 inch WE). Temperatures at or near freezing will generally have a lower ratio (heavy wet snow) and very cold temperatures can have a much higher ratio (dry fluffy snow).