An atmospheric river arrives with a warm front moving into the area on Tuesday. A shield of rain and snow extends from Victoria, BC to Seattle, and to areas of the Cascades south of I-90 at 7 AM as it lifts NE. This will bring light precipitation with snow levels around 3500-4500 ft west of the Cascades and at valley bottoms east of the Cascades and through the mountain gaps. Precipitation rapidly ramps up into the moderate to heavy range throughout the central and southern Cascades this morning, and particularly the Olympics. The precipitation also reaches the northern Cascades by the late morning hours (a few hours ahead of yesterday's forecast). SSW flow will bring significant warming, initially to the Olympics and southern Cascades during the afternoon hours with snow levels rising to near the Cascade Crest in these areas late in the day. The timing of the warmup remains an area of significant uncertainty.
Moderate to heavy rain, snow, and freezing rain continue as warm air rides over the cold air entrenched east of the Cascades and through the mountain gaps. Expect around 50% of the precipitation falling around Snoqualmie Pass, Stevens Pass, and areas further east to be in the form of freezing rain Tuesday night. That means rain will fall in a layer above these freezing temperatures.
A cold front moves through early Wednesday morning, shifting winds from SSW to W and bringing an end to the steady precipitation. Temperatures gradually cool with snow levels lowering to 3500-5500 ft in the afternoon after topping out around 7500-8000 ft Tuesday night.
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
Moderate to heavy rain and snow (heaviest southern part). Rapidly rising snow levels. Increasing moderate ridgeline winds.
Tuesday
Night
Very heavy rain. Strong ridgeline winds.
Tuesday
Moderate to heavy rain and snow develop in the late morning hours. Rising snow levels. Increasing moderate ridgeline winds.
Tuesday
Night
Heavy rain and high-elevation snow. Strong ridgeline winds.
Tuesday
Moderate to heavy rain and snow develop in the late morning hours. Rising snow levels. Increasing moderate ridgeline winds.
Tuesday
Night
Heavy rain and high-elevation snow. Moderate to strong ridgeline winds.
Tuesday
Heavy rain and snow. Rising snow levels. Increasing moderate ridgeline winds.
Tuesday
Night
Heavy rain and high-elevation snow. Moderate ridgeline winds with strong gusts.
Tuesday
Moderate snow develops in the late morning hours. Rising snow levels. Increasing light to occasionally moderate ridgeline and E winds at the Pass.
Tuesday
Night
Moderate to heavy snow changing to freezing rain, then rain. Moderate ridgeline and light to moderate E wind at the Pass.
Tuesday
Moderate snow develops in the mid-morning hours and likely mixes with freezing rain late in the day. Rising snow levels. Increasing moderate ridgeline and E winds at the Pass.
Tuesday
Night
Moderate to heavy snow changing to freezing rain, then rain. Significant freezing rain is expected. Moderate ridgeline and light to moderate E wind at the Pass.
Tuesday
Light to occasionally moderate snow develops by late morning. Rising snow levels.
Tuesday
Night
Heavy snow possibly mixes with freezing rain or rain overnight. Moderate ridgeline winds with strong gusts.
Tuesday
Moderate snow develops mid-morning. Moderate ridgeline winds develop as well by late morning.
Tuesday
Night
Tuesday
Moderate snow possibly mixing with freezing rain or rain in the afternoon. Moderate ridgeline winds.
Tuesday
Night
Moderate snow changing to rain and freezing rain. Moderate ridgeline winds with strong gusts.
Tuesday
Heavy rain and snow. Rapidly rising snow levels. Moderate ridgeline winds with strong gusts.
Tuesday
Night
Heavy rain and high-elevation snow. Strong 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).