3:10 PM: Fixed a date error in the extended synopsis.
The closed low rotating into our nearshore waters on Wednesday is accompanied by a leading cold front and a surface circulation just off the Washington Coastline. A broad swath of moderate rain and snow extends ahead of the front, with peak precipitation in the southern and central Washington Cascades during the afternoon, then shifting to the northern half of the Washington Cascades into the evening and overnight. In advance of the front, temperatures are milder, with snow levels around 4000 ft. Winds are out of the SE, allowing for plenty of spillover onto the east slopes of the Cascades from the Wenatchee mountains northward. Shower activity and cooling fill in behind the frontal system, with W ridgeline winds lowering snow levels to around 2000 ft by Thursday morning.
The closed upper- and surface-low crosses the Cascades early Thursday, increasing onshore pressure gradients and driving moderate to locally strong winds through the mountain gaps and ridgelines of the east slopes of the Cascades. These winds also drive the mountain enhancement, allowing light to occasionally moderate snow showers to continue on the west slopes of the Cascades, decreasing slightly in the afternoon. Spillover to the lee slopes will be minimal, so expect more sunshine and milder temperatures further east.
Very light snow showers taper off Thursday night as a ridge of high pressure expands into the region on Friday. Freezing levels start the day around 3000 ft, but rise to 4500 ft in the north and above 7500 ft in the south by the end of the day.
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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Wednesday
Night
Light to moderate rain and snow early, tapering to isolated snow showers.
Thursday
Decreasing isolated snow showers in the northern and eastern Olympics, with most shower activity confined to the western slopes.
Wednesday
Night
Moderate rain and snow in the evening with light rain and snow showers overnight.
Thursday
Mostly cloudy with light snow showers.
Wednesday
Night
Moderate rain and snow. Moderate ridgeline winds possible in the evening.
Thursday
Decreasing light snow showers.
Wednesday
Night
Decreasing moderate rain and snow showers become light. Moderate ridgeline winds at times near the Cascade Crest.
Thursday
Mostly light rain and snow showers. Moderate ridgeline winds.
Wednesday
Night
Decreasing light to moderate rain and snow. Light to moderate ridgeline winds. Light E wind at the Pass switches W and increases overnight.
Thursday
Light to moderate rain and snow showers. Moderate ridgeline and light-to-moderate W wind at the Pass.
Wednesday
Night
Decreasing light to moderate rain and snow. Light to moderate ridgeline winds. Light E wind at the Pass switches W and increases overnight.
Thursday
Light to moderate rain and snow showers. Moderate ridgeline and W wind at the Pass.
Wednesday
Night
Light to occasionally moderate snow in the evening becomes showery in the early morning hours.
The ridge continues to expand through the weekend. This will bring increasing and above-normal warmth, along with dry weather, clear skies, and sunshine. By Saturday, freezing levels will be around 7,000-10,000 ft, rising to 9,000-11,000 ft by Sunday.
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).