A trough over the intermountain west continues to head eastward, allowing an offshore ridge to dominate our weather pattern this week. Flow around the ridge brings NNE winds aloft to the area with light ridgeline NW winds. The incoming ridge continues to trap a band of thinning low-level moisture in the Puget Sound region where some light showers linger through the morning hours from around Seattle southward. Most sprinkle/flurry activity will be limited to the westernmost portion of the Cascades. Expect increasing sun breaks along the west slopes of the Cascades on Monday with clear skies to the east of the Cascades. Temperatures started the day significantly cooler on Sunday with all NWAC stations dropping back below 32F. 5000 ft temperatures should rise to the upper 20s and low 30s throughout the region. Light to occasionally moderate Fraser River outflow will bring some cooler temperatures to Watcom County, but these winds should decrease as the day wears on.
The ridge shifts into the nearshore waters Monday night with skies continuing to clear along the west slopes of the Cascades. The low-level flow will switch easterly by Tuesday morning. A weak shortwave moving into central British Columbia pushes the ridge axis onshore over our region on Tuesday. Expect plenty of sunshine with high temperatures near normal and low temperatures below normal for mid-December.
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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Monday
Mostly sunny.
Monday
Night
Mostly clear.
Monday
Mostly sunny.
Monday
Night
Mostly clear.
Monday
Mostly cloudy with sprinkles/snow flurries in the morning, then becoming partly cloudy in the afternoon.
Monday
Night
Partly cloudy in the evening, then clearing skies overnight.
Monday
Mostly cloudy with sprinkles/snow flurries in the morning, then becoming partly cloudy in the afternoon.
Monday
Night
Partly cloudy in the evening, then clearing skies overnight.
Monday
Mostly cloudy. Light ridgeline and W wind at the Pass.
Monday
Night
Partly cloudy in the evening, then becoming mostly clear overnight. Light ridgeline and variable or E wind at the Pass.
Monday
Mostly cloudy with sprinkles/snow flurries in the morning. Light ridgeline and W wind at the Pass.
Monday
Night
Partly cloudy in the evening, then becoming mostly clear overnight. Light ridgeline and variable or E wind at the Pass.
Monday
Mostly sunny.
Monday
Night
Mostly clear.
Monday
Mostly sunny.
Monday
Night
Mostly clear.
Monday
Mostly sunny.
Monday
Night
Mostly clear.
Monday
Mostly cloudy along the west slopes of Mt. Hood with sunnier skies further east.
Monday
Night
Partly cloudy along the west slopes of Mt. Hood with clear skies further east.
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).