An atmospheric river arrives on Monday with a warm front lifting northward. The front brings increasing moderate SSW winds along with plentiful moisture. Warm air efficiently displaces any cold air west of the Cascade Crest with snow levels quickly rising to 5000-6000 ft in these areas. The situation becomes much more interesting near and east of the Cascade Crest where cold air remains entrenched with strong E gradients streaming that cold air through the mountain gaps. Expect mostly snow for Snoqualmie and White passes today with a possible transition to sleet or freezing rain late Monday through the evening hours. Stevens Pass likely remains mostly snow although some sleet or freezing rain may mix in at times; expect 4-8 with this storm.
A cold frontal passage in the early morning hours on Tuesday may briefly switch low-level flow westerly, enabling elevations below 4000 ft to change to rain before E flow returns by late morning. WSW behind the front shifts SW by the afternoon with light snow showers cutting off from south to north as the day progresses. Snow levels should settle around 3000-4000 ft for most of the Cascades. However, some mixed precipitation remains possible as E flow returns. Areas south of I-90 can expect some sunshine with a break between storms as the next system spins offshore.
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
Periods of moderate rain and high-elevation snow. Moderate ridgeline winds.
Monday
Night
Moderate to heavy rain and high-elevation snow. Moderate to strong ridgeline winds.
Monday
Increasing rain and high-elevation snow becoming heavy by mid-morning. Moderate ridgeline winds.
Monday
Night
Heavy rain and high-elevation snow. Moderate to strong ridgeline winds.
Monday
Increasing rain and high-elevation snow becoming moderate to occasionally heavy by mid-morning. Light to moderate ridgeline winds.
Monday
Night
Moderate to occasionally heavy rain and high-elevation snow. Moderate ridgeline winds.
Monday
Increasing rain and high-elevation snow become heavy for the volcanoes and moderate for Crystal and Paradise. Increasing light to moderate ridgeline winds.
Monday
Night
Heavy rain and high elevation snow for Paradise; moderate for Crystal and White Passes.
Monday
Light snow becoming moderate by the afternoon. Light to occasionally moderate ridgeline and E wind at the Pass.
Monday
Night
Moderate snow may mix with freezing rain or sleet at times. Light to occasionally moderate ridgeline and E wind at the Pass.
Monday
Light snow becomes moderate by the afternoon and mixes with freezing rain or sleet late in the day. Moderate ridgeline and E wind at the Pass.
Monday
Night
Moderate snow or mixed precipitation. Moderate ridgeline and E wind at the Pass decrease in the early morning hours.
Monday
Light snow develops by late morning. Light ridgeline winds.
Monday
Night
Light to moderate snow. Light ridgeline winds.
Monday
Increasing light snow. Moderate ridgeline winds.
Monday
Night
Light to occasionally moderate snow. Moderate ridgeline winds.
Monday
Light to moderate snow mixing with sleet or freezing rain in the afternoon. Light to moderate ridgeline winds.
Monday
Night
Light to moderate mixed precipitation. Light to moderate ridgeline winds.
Monday
Moderate snow, mixed precipitation, and rain. Rising snow levels. Moderate to occasionally strong ridgeline winds.
Monday
Night
Moderate snow, mixed precipitation, and rain changing back to snow in the early morning hours. 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).