A major winter storm remains on track to ramp up from W to E on Christmas day as it moves into our region with increasing widespread rain and snow.
Light snowfall commences during the morning hours with snow levels around 2500 ft west of the Cascade Crest and 500-1000 ft lower snow levels from the passes eastward with increasing low-level E flow. Expect moderate snow to develop by the end of the day in most areas, but the Olympics ramp up into the moderate to heavy range in the mid-morning hours, followed by Mt Baker by late morning. These areas will also experience moderate to strong ridgeline winds as SSW flow buffets the region.
The most intense precipitation with this seasonally cool atmospheric river arrives Wednesday night with the passage of the initial frontal system. Expect very heavy rain and snow along with a subtle increase in snow levels relative to Wednesday. Moderate to SSW winds continue to ramp up with moderate SE flow at low-levels.
A secondary developing moves into SW British Columbia early Thursday as heavy rain and snow continue across the region. Winds wrapping around this low have a "poisonous tail" meaning you can expect a period of strong to extreme ridgeline winds for areas favored by WSW flow. Once this bout of wind passes, rain and snow becomes showery and snow levels should linger around 3500 ft. This switch from E to W flow during the late morning hours should result in a jump in snow levels for the E slopes of the Cascades. Uncertainty remains, but Snoqualmie Pass proper likely changes to rain from early Sunday onwards.
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
Light rain and snow develops in the morning, quickly becoming moderate to heavy. Increasing moderate to occasionally strong ridgeline winds.
Wednesday
Night
Heavy rain and snow. Moderate to occasionally strong ridgeline winds.
Wednesday
Light snow develops in the mid-morning hours and becomes heavy by the afternoon. Increasing moderate to strong ridgeline winds.
Wednesday
Night
Very heavy rain and snow. Strong moderate ridgeline winds with strong gusts.
Wednesday
Light rain and snow develop in the mid-morning hours and become moderate in the afternoon. Increasing moderate ridgeline winds.
Wednesday
Night
Heavy rain and snow. Moderate ridgeline winds.
Wednesday
Light rain and snow develop in the mid-morning hours and become moderate to heavy in the afternoon. Increasing moderate ridgeline winds.
Wednesday
Night
Heavy rain and snow. Increasing moderate ridgeline winds.
Wednesday
Light snow develops and become moderate by the end of the day. Light to moderate ridgeline and E wind at the Pass.
Wednesday
Night
Heavy snow. Incrasing moderate ridgeline and light to moderate E wind at the Pass.
Wednesday
Light snow develops and become moderate by the end of the day. Light to moderate ridgeline and E wind at the Pass.
Wednesday
Night
Heavy snow possibly mixing with or changing to rain below 3500 ft overnight. Increasing moderate ridgeline and E wind at the Pass.
Wednesday
Cloudy in the morning with light snow developing before noon. Increasing light snow in the afternoon. Moderate ridgeline winds.
Wednesday
Night
Increasing moderate to heavy snow. Moderate ridgeline winds.
Wednesday
Light snow develops by around noon, then increases in the afternoon. Increasing moderate ridgeline winds.
Wednesday
Night
Moderate to heavy snow, possibly mixing with rain below 3500 ft overnight. Moderate ridgeline winds.
Wednesday
Light snow develops by around noon, then increases in the afternoon. Increasing light ridgeline winds.
Wednesday
Night
Moderate to heavy snow, possibly mixing with rain below 3500 ft overnight. Moderate ridgeline winds.
Wednesday
Light snow at times in the morning, then becomes moderate in the afternoon. Increasing moderate 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).