Friday features the final day of a week-long early December high-pressure ridge. If we sound like a broken record this past week, you wouldn't be wrong. After brief cooling Thursday night, Friday features freezing levels rebounding to 10-12k ft. The inversion remains entrenched below 4500 ft with sub-freezing temperatures and a low cloud deck funneled through the lower mountain gaps.
However, change is afoot. A SW-NE oriented frontal boundary is bringing a stream of moisture into southern British Columbia on Friday. As the frontal boundary shifts SE, ridgeline winds increase throughout the day. They will be moderate to potentially strong (late in the day) for the Mt Baker area and light to moderate elsehwere.
Precipitation reaches NW Washington Friday evening as rain along with peak winds in the Mt Baker area. Precipitation intensifies and spreads into he central Cascades overnight. While it will arrive in most locations as rain, there is a slight chance that models underestimate the chance for freezing rain at the Passes as E flow persists at low levels.
The heaviest precipitation arrives with the cold front on Saturday morning with moderate ridgeline winds widespread. Snow levels plummet form 8-10k ft in the morning to 4500-6500 ft. Most areas of the west slopes of the Cascades can expect 1-2" of water equivalent with the heaviest likely in convergence bands develping in the northern mountain loop area.
Weather Forecast
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West Central
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Snoqualmie Pass
East North
East Central
East South
Mt. Hood
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Friday
Mostly cloudy with a chance of showers in the western Olympics. Ridgeline winds become moderate late in the day.
Friday
Night
Mild. Light rain develops in the evening and becomes moderate overnight. Moderate ridgeline winds.
Friday
Partly cloudy in the morning, becoming mostly cloudy in the afternoon. Increasing moderate ridgeline winds may become strong late in the day.
Friday
Night
Mild. Partly cloudy in the evening with light rain developing in the evening and becomes moderate to heavy overnight. Moderate to strong ridgeline winds peak in the evening hours.
Friday
Mild. Increasing high and mid-level clouds with some lower clouds arriving in the afternoon.
Friday
Night
Mild. Partly cloudy in the evening with light rain developing by midnight and becomes moderate overnight.
Friday
Mild. Increasing high and mid-level clouds with some lower clouds arriving in the afternoon.
Friday
Night
Mild. Partly cloudy in the evening with clouds lowering and thickening. Light rain arrives overnight.
Friday
Mild. Increasing high and mid-level clouds. Light ridgeline and E wind at the Pass.
Friday
Night
Mild above 4500 ft, cool with persistent low cloud below. High clouds lower and thicken in the evening with light rain developing overnight (20% chance of freezing rain). Increasing light to moderate ridgeline winds. Decreasing light E wind at the Pass.
Friday
Mild above 4500 ft, cold with persistent low cloud below. Increasing high and mid-level clouds. Increasing light ridgeline winds. Moderate E wind at the Pass.
Friday
Night
Mild above 4500 ft, cool with persistent low cloud below. High clouds lower and thicken in the evening with light rain developing overnight (20% chance of freezing rain). Increasing moderate ridgeline winds. Decreasing moderate E wind at the Pass becomes light.
Friday
Increasing mid and high cloud. Mild above an inversion and very cold air up to the Pass elevation.
Friday
Night
Light rain develops and becomes moderate in the early morning hours. Increasing moderate ridgeline winds becoming strong.
Friday
Increasing mid and high cloud. Mild above 4500 ft and cold below with some moderation in the afternoon.
Friday
Night
High clouds lower and thicken during the evening with light rain overnight. Increasing moderate ridgeline winds becoming strong.
Friday
Increasing mid and high cloud. Mild above 4500 ft and cold below with some moderation in the afternoon.
Friday
Night
High clouds lowering and thickening with a chance of light rain by morning. Increasing moderate ridgeline winds.
Friday
Increasing mid and high clouds. Ridgeline winds become moderate late in the day.
Friday
Night
Mild with high clouds lowering and thickening. Increasing 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).