An extended upper trough slowly rotates into the region from the SW on Sunday afternoon. The trough will bring gradual cooling and some light precipitation following a period of dry and mild conditions. The elongated band of associated clouds extends from Alaska to Arizona and maintains cloud cover across the Pacific Northwest. A surface low-pressure center formed off the mouth of the Columbia River adds to the circulation whereby SE winds enhance a loosely organized northward-moving band of precipitation along the east slopes of the Cascades. This band spreads throughout the Olympics and areas around I-90 southward during the evening hours, but precipitation may be slow to start as it enters a drier air mass. Moisture should extend northward to the Canadian border overnight. Snow levels generally start around 5000 ft and drop to around 4000 ft overnight. Low-level NE winds may develop for the east-central Cascades as a secondary low develops in the Columbia basin. This could enhance rain/snow for the northern Wenatchee Mountains late Sunday night into Monday morning.
The weakening upper trough drifts onshore on Monday with snow levels hovering around 3500 ft in most areas. More sustained light snowfall should focus on the northern Washington Cascades while showery conditions develop in southern portions of the Cascades as WSW ridgeline winds increase into the moderate range for areas around Mt. Hood.
A stronger closed low diving south from the Gulf of Alaska toward California takes much of the energy from the dissipating trough over our region. This leaves dissipating very light snow rain/snow shower activity as snow levels fall to near Snoqualmie Pass level Monday night and Tuesday morning. With increasing sunshine Tuesday afternoon, expect cumulus clouds and a chance of weak convective afternoon rain/snow showers.
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
Night
Light rain/snow at times.
Monday
Sunday
Night
Cloudy in the evening with periods of light rain and snow overnight.
Monday
Periods of light rain and snow.
Sunday
Night
Cloudy in the evening with periods of light rain and snow overnight.
Monday
Decreasing light to occasionally moderate rain and snow.
Sunday
Night
Increasing light rain/snow (moderate at Paradise). Moderate ridgeline winds near the Cascade Crest becoming light.
Monday
Decreasing light rain/snow (moderate at times for Paradise) becoming showery during the afternoon.
Sunday
Night
Cloudy in the evening, then increasing light rain/snow at times overnight. Decreasing light to moderate ridgeline winds. Light E wind becoming variable at the Pass.
Monday
Light to occasionally moderate rain and snow become showery late in the day. Light to occasionally moderate ridgeline and light W wind at the Pass.
Sunday
Night
Cloudy in the early evening, then increasing light rain/snow at times. Decreasing light to moderate ridgeline winds. Light E wind becoming variable at the Pass.
Monday
Light to occasionally moderate rain and snow become showery late in the day. Light to occasionally moderate ridgeline and light W wind at the Pass.
Sunday
Night
Cloudy in the evening with periods of light rain/snow overnight.
Monday
Periods of light rain and snow.
Sunday
Night
Periods of very light rain/snow in the evening, increasing overnight.
Monday
Periods of light rain and snow peak during the late morning hours.
Sunday
Night
Periods of light rain/snow.
Monday
Periods of light rain and snow becoming showery from late morning onwards.
Dry weather continues Tuesday night through early Wednesday evening with the upper-level flow splitting around the region. A cold upper-level low sliding down the British Columbia Coastline should bring increasing rain and snow to the region late Wednesday night through Thursday afternoon. Expect cool, showery conditions Thursday night and Friday with the trough over the region.
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).