This weekend features a significant and complex storm system sandwiched between high-pressure ridges.
The SW-NE-oriented frontal boundary sliding SE from British Columbia has plenty of warm air and moisture to tap into. It generally moves into a very mild air mass, with the exception of low-level cold air trapped east of the Cascade Crest. Rain reaches the Olympics and Mt Baker areas in the late evening hours and spreads across northern Washington overnight. Ridgeline winds in these areas will become moderate to strong as the moisture plume arrives from the SW.
The front moves through during the morning hours with moderate to heavy precipitation along with widespread moderate ridgeline winds. Snow levels plummet from around 9000 ft in the morning to 4500-6500 ft by the afternoon. Convergence forming in the mountain loop shifts south toward Stevens by the end of the day, then fluctuates between Stevens and Snoqualmie Passes overnight. A secondary frontal boundary leads an upper trough that arrives Saturday evening with a further drop in snow levels to around 3000 ft. We expect most of the mid and low-elevation snowfall to arrive with this second feature which should favor the west slopes of the Cascades. Expect moderate to heavy snow through mid-morning on Sunday before the upper low slides eastward. The west slopes of the Cascades can expect 6-18" of snow before all is said and done.
Some low-elevation areas east of the Cascade Crest and even the passes could experience a wintry mix Friday night and Saturday. This is more likely in the northern passes as low-level cold air is predicted to moderate more significantly in areas south of highway 2 as the precipitation arrives.
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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Friday
Night
Mild. Light rain develops in the evening and becomes moderate overnight. Moderate ridgeline winds.
Saturday
Rain transitions to light rain and snow showers by mid-morning. Light to moderate ridgeline winds.
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.
Saturday
Heavy rain transitions to moderate rain and snow. Strong winds quickly drop into the light to moderate range.
Friday
Night
Mild. Partly cloudy in the evening with light rain developing by midnight and becomes moderate overnight.
Saturday
Heavy rain changing to rain and snow with convergence banding. Moderate ridgeline winds.
Friday
Night
Mild. Partly cloudy in the evening with clouds lowering and thickening. Light rain arrives overnight.
Saturday
Moderate to heavy rain at Paradise in the morning changing to moderate rain and snow. Light to moderate rain changing to rain and snow at Crystal and White Pass. Moderate ridgeline winds.
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 (30% chance of freezing rain). Increasing light to moderate ridgeline winds. Decreasing light E wind at the Pass.
Saturday
Moderate to heavy rain becoming rain and snow. Convergence is likely late in the day. Moderate ridgeline winds peak in the morning. Light E winds at the Pass switch W by mid-morning.
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.
Saturday
Moderate to heavy rain becoming rain and snow. Moderate ridgeline winds with strong gusts. Light to moderate E winds at the Pass switch W by mid-morning.
Friday
Night
Light rain develops and becomes moderate in the early morning hours. Increasing moderate ridgeline winds becoming strong.
Saturday
Moderate to heavy rain, snow, and possibly freezing rain tapering to showers as freezing levels lower.
Friday
Night
High clouds lower and thicken during the evening with light rain and possibly freezing rain overnight. Increasing moderate ridgeline winds becoming strong.
Saturday
Moderate rain, snow, and possibly freezing rain tapering to showers as freezing levels lower.
Friday
Night
High clouds lowering and thickening with a chance of light rain by morning. Increasing moderate ridgeline winds.
Saturday
Moderate rain, snow, and possibly freezing rain tapering to showers as freezing levels lower.
Friday
Night
Mild with high clouds lowering and thickening. Increasing moderate ridgeline winds with strong gusts.
Saturday
Increasing moderate rain and snow may become heavy at times in the afternoon. Strong ridgeline winds.
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).