A large-scale ridge remains the dominant story throughout the workweek. Mountain temperatures above the inversion (roughly 4500 ft) don't feel very December-like with overnight low temperatures mostly in the low 40s. A weakening shortwave disturbance embeds itself within the large-scale ridge of high pressure that has delivered days of dry weather. This disturbance fizzles as it moves onshore Thursday, but may bring a few sprinkles to the western Olympic Mountains. It will also bring some subtle cooling to mid-levels of the atmosphere, with freezing levels dropping to 7000-9000 ft across the region. It will also increase mid and high-level cloud cover over the Cascades as the day progresses.
The ridge will briefly rebuild late Thursday night through Friday as freezing levels rebound to 10,000-12,000 ft across the region.
Over the next 36 hours, low clouds/fog will stay locked in for deep valleys and especially along the eastern slopes of the Cascades and near the Cascade Passes. Winds above and below the inversion will be decoupled with generally WSW and light winds above the inversion and E flow below. Snoqualmie Pass remains the only location with moderate E winds at Pass level which are expected to decrease very slightly as time progresses.
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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Thursday
Mostly cloudy with a chance of a trace of rain. Mild.
Thursday
Night
Partly cloudy. Mild.
Thursday
Sunny in the morning with increasing mid-level clouds with scattered low clouds late in the day. Inversions and low cloud in some lowland valleys.
Thursday
Night
Partly cloudy. Inversions and low cloud in some lowland valleys.
Thursday
Sunny in the morning with increasing mid-level clouds with scattered low clouds late in the day.
Thursday
Night
Partly cloudy. Inversions and low cloud in some lowland valleys.
Thursday
Sunshine in the morning with increasing mid-level clouds as the day progresses. Inversions and low cloud in some lowland valleys.
Thursday
Night
Partly cloudy. Inversions and low cloud in some lowland valleys.
Thursday
Sunshine in the morning with increasing mid-level clouds as the day progresses. Persistent inversion with some low cloud at times through the mountain gap. Light ridgeline and E wind through the gap.
Thursday
Night
Partly cloudy. Persistent inversion with some low cloud at times through the mountain gap. Light ridgeline and E wind through the gap.
Thursday
Sunshine in the morning with increasing mid-level clouds as the day progresses. Persistent inversion with some low cloud at times through the mountain gap. Light ridgeline and E wind through the gap.
Thursday
Night
Partly cloudy. Persistent inversion and low cloud at times through the mountain gap. Mild at mid-elevations; cold at the Pass. Light ridgeline winds decoupled from moderate E wind through the gap.
Thursday
Sunny in the morning with increasing mid-level clouds in the afternoon. Strong inversion with low cloud in the valleys.
Thursday
Night
Mostly clear with low cloud. Strong inversion.
Thursday
Sunny in the morning with increasing mid-level clouds in the afternoon. Strong inversion with low cloud in the valleys.
Thursday
Night
Mostly clear with low cloud. Strong inversion.
Thursday
Sunny in the morning with increasing mid-level clouds in the afternoon. Strong inversion with low cloud in the valleys.
Thursday
Night
Mostly clear with low cloud. Strong inversion.
Thursday
Sunny in the morning with increasing mid-level clouds in the afternoon. Strong 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).