The axis of a ridge shifts across the Pacific Northwest on Sunday, bringing dry weather across the region. Ridgeline winds shift from W to S, enabling a gradual increase in mid-level cloud cover to move into the region as the day progresses. Freezing levels remain unusually high for mid-December (in the 8500-11000 ft range). Most mountain locations will experience temperatures in the mid-30s up to around 50F. However, strong (15-20F) inversions are present along the east slopes of the Cascades and through the mountain gaps and will be present much of the day. In these locations, temperatures will struggle to get above freezing below 4000 ft. Moderate E flow should continue through the lower mountain gaps (Snoqualmie Pass) but is likely to stay light for the higher passes. A mid-level cloud deck should increase in coverage with mostly cloudy skies throughout the region by the afternoon.
S or SSW ridgeline winds increase into the light to moderate range Sunday night as easterly low-level flow continues Sunday night. Skies become mostly cloudy across the region.
On Monday, mostly cloudy skies continue with a subtle decrease in freezing level. Some light rain or high-elevation snow arrives from late morning onwards from Mt Hood to areas south of I-90 and the Olympics. In areas exposed to the cold air pool, this precipitation may be a wintry mix.
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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Sunday
Partly cloudy in the morning, becoming mostly cloudy by the afternoon. Light ridgeline winds. Unseasonably mild.
Sunday
Night
Mostly cloudy. Light to occasionally moderate ridgeline winds. Unseasonably mild.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by early afternoon. Light ridgeline winds. Unseasonably mild.
Sunday
Night
Mostly cloudy. Light to occasionally moderate ridgeline winds. Unseasonably mild.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by early afternoon. Light ridgeline winds. Unseasonably mild.
Sunday
Night
Mostly cloudy. Light to occasionally moderate ridgeline winds. Unseasonably mild.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by the afternoon. Light ridgeline winds. Unseasonably mild.
Sunday
Night
Mostly cloudy. Light to occasionally moderate ridgeline winds. Unseasonably mild.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by the afternoon. Light ridgeline winds. Increasing light to potentially moderate E winds at the Pass. Unseasonably mild at ridgelines, but below or near freezing at the Pass.
Sunday
Night
Mostly cloudy. Light to occasionally moderate ridgeline winds. Light to moderate E winds at the Pass. Unseasonably mild at ridgelines, but below or near freezing at the Pass.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by the afternoon. Light ridgeline winds. Moderate E winds at the Pass. Unseasonably mild at ridgelines, but below or near freezing at the Pass.
Sunday
Night
Mostly cloudy. Light to occasionally moderate ridgeline winds. Moderate E winds at the Pass. Unseasonably mild at ridgelines, but below or near freezing at the Pass.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by late afternoon. Light ridgeline winds. Unseasonably mild at ridgelines, but well below freezing at pass level.
Sunday
Night
Mostly cloudy. Light ridgeline winds. Mild near ridgelines with a strong low-level inversion.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by early afternoon. Light ridgeline winds. Unseasonably mild at ridgelines, but near or below freezing below 4000 ft.
Sunday
Night
Mostly cloudy. Light to occasionally moderate ridgeline winds. Mild near ridgelines with a low-level inversion.
Sunday
Partly cloudy in the morning, becoming mostly cloudy by early afternoon. Light ridgeline winds. Unseasonably mild at ridgelines, but chilly below 4000 ft.
Sunday
Night
Mostly cloudy. Increasing light ridgeline winds. Mild nea ridgelines with a low-level inversion.
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).