An upper trough offshore directs the third and strongest wave in a three-part atmospheric event across our region on Monday with the wave well defined in Satellite imagery. This wave will bring excessive amounts of precipitation to the region Monday and Monday night before precipitation tapers and becomes more showery late Monday night into Tuesday. Consistent very moist WSW will bring 1-6" of water equivalent to our mountains through the next 36 hours with the highest amounts at Paradise, followed by the west slopes of the central Washington Cascades. WSW ridgeline winds should be strong across the southern parts of the forecast region and the east slopes of the Cascades, but begin to ease late Monday night.
While temperatures have warmed significantly across the east slopes of the Cascades over the past 48 hours, there has been just enough cold air to maintain snow at Snoqualmie Pass and Stevens until 7 AM. Expect a mix or changeover to rain in the next few hours at Snoqualmie Pass and by noon at Stevens Pass. Once the east flow finally gets overwhelmed, expect snow levels to rise to 4500-8000 ft with cooler temperatures in the north and northeast Cascades maintaining additional heavy snowfall.
Rainfall should begin to ease and become more showery as the upper trough moves toward the coast Monday night, but showers may still produce significant amounts of moisture without a real cold front to cool and dry the air mass. Snow levels should begin to lower gradually.
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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Monday
Increasing heavy rain and high elevation snow.
Monday
Night
Moderate rain and high elevation snow becoming light and showery overnight.
Monday
Heavy rain and snow.
Monday
Night
Moderate rain and high elevation snow becoming light and showery overnight.
Monday
Very heavy rain and high elevation snow.
Monday
Night
Decreasing moderate rain and high elevation snow becoming more showery overnight.
Monday
Very heavy rain and high elevation snow. Strong ridgeline winds.
Monday
Night
Decreasing heavy rain and high elevations now. Decreasing moderate to strong ridgeline winds.
Monday
Heavy rain and snow with rising snow levels. Decreasing light E winds at the Pass.
Monday
Night
Heavy rain and high elevation snow becoming moderate. Light and variable winds at the Pass.
Monday
Very heavy rain and snow. Decreasing light E winds at the Pass.
Monday
Night
Decreasing heavy rain and snow. Light and variable winds at the Pass.
Monday
Heavy snow changing to rain and snow.
Monday
Night
Heavy rain and high elevation snow becoming moderate.
Monday
Heavy rain and snow with rising snow levels. Increasing ridgeline winds becoming strong.
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).