A major winter storm is brewing for the Pacific Northwest. A deep upper-level low sits over northern British Columbia. A surface low forms as it rotates around the parent low and drops toward the Pacific Northwest coastline. Moisture over-running a warm front just offshore has come in a touch early with very light snow beginning across most of the region by mid-morning. Light snow increases throughout the day with 1-3" of snow expected by the end of the day in most areas. Cold air is in place across the region with below zero readings at Washington Pass while most temperatures are in the single digits, teens, and low 20s. Temperatures should moderate slightly during the day, but it will remain quite cold. SSW winds aloft should increase in the light to moderate range on Tuesday. Easterly winds at low levels continue to increase through the mountain gaps.
The frontal system reaches the coast in the evening spread heavy snow and strong winds into the Olympics. The heavy snowfall spreads throughout the Cascades overnight with the strongest winds likely with strong E gradients targeting the major Cascade passes.
Snowfall, winds, and easterly pressure gradients all ease Wednesday morning, although heavy snow lingers through much of the day at Mt. Hood. Snow levels stay quite low throughout the event for the entire region, but rise to around 2500 ft by Wednesday in the southern Cascades.
Snowfall totals throughout the region should be in the 1-2.5 ft range with the highest amounts likely for the Washington Volcanoes and southeast Olympic mountains.
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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Tuesday
Increasing light snow.
Tuesday
Night
Heavy snow and strong ridgeline winds.
Tuesday
Light snow developing and increasing in the afternoon. Increasing light to moderate ridgeline winds with strong gusts late in the day.
Tuesday
Night
Moderate snow quickly becoming very heavy. Strong ridgeline winds.
Tuesday
Light snow developing and increasing in the afternoon.
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).