The cold upper trough off our coastline will deliver its last hurrah over the next 24 hours. The trough continues to drift toward the Washington coastline while a well-defined comma cloud shows up in satellite imagery off the coast of NW Oregon. The combination of increased cold air aloft generating instability and the comma cloud drifting NNE along the coast today and tonight will bring a snowy pattern to the Pacific Northwest. While the water numbers aren't impressive, the cold air will squeeze out high snow-to-liquid ratios and, therefore, significant snowfall amounts. Snow bands brought 1-2" in the morning for parts of the Puget Sound lowlands and areas near or west of the Cascade Crest. Snow showers or bands of snowfall will increase from south to north throughout the day, becoming moderate to heavy for Mt Hood and the southern Cascade Volcanoes and light to moderate in the central Washington Cascades. Most snowfall further north should be generally light.
The heaviest snowfall for most areas is expected during the evening hours, with the comma cloud centered just west of the Olympic Peninsula. Ridgeline winds increase from the WSW at this time, helping the mountain enhancement. The upper low merges with troughing over British Columbia overnight as snowfall generally becomes light. Expect continued snow flurries under the trough for the west slopes of the Cascades on Thursday, while the southern Cascades volcanoes may continue some periods of very light snowfall as moisture from a system approaching California shunts northward. Expect partly cloudy skies to develop for most of the central and northern Cascades by the afternoon with light winds and 500-1000 ft snow/freezing levels.
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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Wednesday
Increasing light snow showers.
Wednesday
Night
Light upslope snowfall at times for the north slope of the Olympics.
Wednesday
Periods of light snowfall increasing in the afternoon.
Wednesday
Night
Light snow showers, potentially becoming moderate overnight (or locally heavy in convergence banding south of Mt Baker).
Wednesday
Periods of light to locally moderate snowfall.
Wednesday
Night
Moderate to locally heavy snowfall or snow showers. Convergence likely.
Wednesday
Moderate to heavy snow showers, heaviest at Mt St Helens. Moderate ridgeline winds at times during the afternoon.
Wednesday
Night
Moderate to heavy snow showers, heaviest at Mt St Helens, then gradually decreasing overnight. Moderate ridgeline winds at times during the evening hours.
Wednesday
Periods of light to potentially moderate snow. Light ridgeline and E wind at the Pass.
Wednesday
Night
Moderate to locally heavy snowfall with convergence possible at times. Light to occasionally moderate ridgeline and E wind at the Pass.
Wednesday
Periods of light to potentially moderate snow with a potential break during the mid-morning hours. Light ridgeline and E wind at the Pass.
Wednesday
Night
Heavy snowfall with convergence possible at times, decreasing overnight. Light to moderate ridgeline winds decrease overnight. Low level flow may briefly switch W during the evening before returning easterly overnight.
Wednesday
Periods of mostly light snow increasing during the afternoon hours.
Wednesday
Night
Light snow tapers or ends overnight.
Wednesday
Periods of light to occasionally moderate snow.
Wednesday
Night
Periods of moderate snow during the evening hours, then becoming light overnight.
Wednesday
Periods of light to snow potentially become moderate during the afternoon.
Wednesday
Night
Periods of moderate snow during the evening hours become light overnight.
Wednesday
Periods of increasing moderate to locally heavy snow. Increasing moderate ridgeline winds with strong gusts.
Wednesday
Night
Moderate to heavy snow becomes mostly light to occasionally moderate overnight. Decreasing moderate ridgeline winds with strong gusts.
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).