A system featuring multiple circulation centers moves onshore into the PNW region early Sunday morning. One low center sits off the Washington with another moving into NW Oregon. The deformation zone between these lows will produce the heaviest precipitation today for Mt. Hood, Mt. St. Helens, Mt. Adams, and to a lesser extent Mt. Rainier as the system moves over the area. Low-level E winds should decrease and switch W by late morning. This should decrease the spillover onto the E slopes of the Cascades and focus more precipitation on the west slopes of the Cascades with light precipitation shifting northward as the day progresses. Snow levels generally rise around 500 ft during the day with a messy rain/snow mix likely at Snoqualmie Pass level as warmer air moves in at low levels, but cooler air begins to move in aloft later in the day.
Temperatures cool further Monday night with westerly push of winds. W ridgeline winds could be moderate to strong near and E of the Cascades Crest, focusing light to moderate showers and convergence Monday evening on the west slopes of the Cascades with decreasing showers overnight as temperatures cool. Expect a convergence band to set up near or slightly north of Snoqualmie Pass, enhancing snow locally. Snow levels drop from around 3000 to 2500 ft overnight.
A "dirty ridge" moves over the area on Monday, with moisture spilling into the region ahead of a system sliding down the British Columbia coastline. Expect light rain and snow that should be steadiest from Mt Rainier northward. Temperatures will be seasonable.
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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Sunday
Light rain and snow spreading from the southern Olympics throughout the range during the morning hours and becoming showery later in the day.
Sunday
Night
Very light rain and snow showers focusing on the western slopes of the Olympics.
Sunday
Cloudy through mid-morning then periods of light rain or snow.
Sunday
Night
Decreasing light rain and snow showers.
Sunday
Light rain and snow developing during the morning hours and continuing in the afternoon.
Sunday
Night
Light to moderate rain and snow showers, heaviest in convergence (southern part) during the evening hours. Shower activity decreases overnight.
Sunday
Moderate rain and snow, heaviest south of Mt. Rainier.
Sunday
Night
Decreasing light rain and snow showers. Moderate to strong ridgeline winds near the Cascade Crest.
Sunday
Light rain and snow developing. Light ridgeline winds becoming moderate late in the day. Light pass-level winds start E, but quickly switch W.
Sunday
Night
Light to moderate snow showers and convergence in the evening with decreasing light showers overnight. Moderate ridgeline and light to moderate W wind at the Pass.
Sunday
Light rain and snow developing. Light ridgeline winds becoming moderate late in the day. Light pass-level winds start E, but quickly switch W.
Sunday
Night
Light to moderate snow showers and convergence in the evening with decreasing light showers overnight. Moderate to occasionally strong ridgeline and light to moderate W wind at the Pass.
Sunday
Very light snow at times.
Sunday
Night
Very light snow showers or flurries near the Cascade Crest. Moderate to occasionally strong ridgeline winds.
Sunday
Light rain or snow spreading northward.
Sunday
Night
Light snow showers. Moderate to occasionally strong ridgeline winds.
Sunday
Light rain and snow.
Sunday
Night
Very light snow showers or flurries near the Cascade Crest. Moderate to occasionally strong ridgeline winds.
Sunday
Increasing moderate to occasionally heavy rain, and snow. Increasing moderate to strong ridgeline winds.
Sunday
Night
Moderate rain and snow becoming light and showery overnight. Decreasing 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).