Strong NW flow continues with the jetstream squarely over the Pacific Northwest through Monday, gradually weakening over the time period.
A final major upper-level disturbance in NW flow is bringing significant snowfall to the Washington Cascades above 3000-3500 ft on Saturday along with very strong to extreme winds for Mt. Hood, White Pass, and Mission Ridge. A low and associated surface cold front should pass through the Cascades late Saturday afternoon, bringing cooling and a changeover to all snow at Snoqualmie Pass level during the early evening hours. Expect light snow and snow showers across a wide swath of the west slopes of the Cascades, but the heaviest accumulations will focus on strong convergence bands from the southern Mountain Loop to just south of Snoqualmie Pass. These bands will be moving and re-forming, making the timing of intense snowfall difficult to predict. We expect around 10-15" of additional snowfall in the mountains around Barlow Pass, Stevens Pass, and Snoqualmie Pass by the end of the day Sunday.
Strong winds should continue to gradually ease, but the major E-W passes across the Cascades, the Cascade Crest, the east slopes of the Cascades, and Mt. Hood will continue to experience decreasing moderate and in some cases strong winds. As temperatures continue to cool (with 1000-2000 ft snow levels), wind chills may be in the single digits and teens.
Models indicate lingering convergence focuses on Snoqualmie Pass Sunday night, where an additional 6" of snow may be possible. Snow shower activity should be very light on Monday with increasing sunshine east of the Cascade crest.
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
Night
Partly to mostly cloudy with snow flurries possible.
Sunday
Mostly cloudy with light snow showers.
Saturday
Night
Light snow or snow showers.
Sunday
Mostly cloudy with light snow showers.
Saturday
Night
Moderate snow, heaviest in strong convergence bands. Gradually decreasing moderate to strong ridgeline winds.
Sunday
A mix of light snow showers and heavy snow in convergence bands.
Saturday
Night
Light snow showers, heaviest at Paradise. Decreasing moderate to strong ridgeline winds in most locations. Very strong ridgeline winds continue, but gradually decreasing near White Pass.
Sunday
Light snow showers heaviest at Paradise and White Pass. Decreasing moderate ridgeline winds in most areas. Decreasing strong winds near White Pass.
Saturday
Night
Moderate snow, locally heavy in convergence bands. Moderate to strong ridgeline winds. Decreasing light to moderate W winds at the Pass.
Sunday
A mix of light snow showers and heavy snow in convergence bands. Decreasing moderate ridgeline winds. Moderate W winds at the Pass becoming light.
Saturday
Night
Moderate snow, locally heavy in convergence bands. Moderate to strong ridgeline winds. Decreasing moderate W winds at the Pass.
Sunday
A mix of light snow showers and heavy snow in convergence bands. Decreasing moderate ridgeline winds. Moderate W winds at the Pass becoming light to moderate.
Saturday
Night
Light snow showers. Moderate to strong ridgeline winds.
Sunday
Light snow showers.
Saturday
Night
Moderate snow showers western part, lighter further east. Strong ridgeline winds.
Sunday
A mix of light snow showers and heavy snow in convergence bands closer to the Cascade crest with scattered light snow showers further east. Strong ridgeline winds decreasing into the moderate range.
Saturday
Night
Light snow showers. Moderate to strong ridgeline winds.
Sunday
Light snow showers. Decreasing moderate ridgeline winds with strong gusts.
Saturday
Night
Light rain and snow showers. Strong to very strong ridgeline winds with extreme gusts.
A broad, stationary upper-level low pressure dominates south-central Canada. A weakening ridge of high pressure remains off the west coast of North America. The Pacific Northwest remains under NW flow between the large-scale features, but that flow should be less vigorous than over the weekend. The flow pattern should keep clouds and light snowfall banked up against the Cascades with weak convergence bands likely incrementally adding snow. Temperatures will be below normal with below-freezing temperatures at night in the lowlands. Westward migration of the arctic low (and associated modified arctic airmass) appears to hold off until later in the week.
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).