A cool upper trough slides gradually eastward on Thursday, initiating the start of a warming trend . The combination of NW flow aloft and low-level onshore W wind has a strong track record of trapping low-level moisture against the west slopes of the Cascades and that is what we're seeing once again. We're watching a weak convergence band west of Snoqualmie and some snow showers near Mt Hood, but both these areas of light snow showers should taper by mid-morning. Expect mostly cloudy skies west of the Cascade Crest with partial cloud cover to full sunshine further east. 5000 ft temperatures are off to a chilly start with upper 10s and low 20s widespread across the region. These temperatures should rise into the mid-30s by the afternoon as an upper-level ridge expands offshore. Winds should accelerate slightly through the mountain gaps and along the west slopes of the Cascades, with light to moderate ridgeline winds in exposed terrain.
This pattern continues overnight as the ridge gradually replaces the outgoing trough. Some models indicate another chance of weak convergence around the central Washington Cascades. Ridgeline winds gradually decrease overnight. Further clearing skies will lead to a cool night across the region.
Temperatures will moderate quickly. Expect a milder day on Friday with generally light winds and more sun than clouds as the previously discussed ridge moves into the region.
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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Thursday
More sun than clouds.
Thursday
Night
Mostly clear skies.
Thursday
A few snow flurries possible possible, otherwise, mostly cloudy skies.
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).