A strong frontal system crosses the region from NW to SE on Tuesday through the evening hours, bringing significant snow accumulations to the entire region. The moisture is rapidly spreading across the region early Tuesday, arriving on strong W winds aloft, that will translate into moderate to locally strong WSW ridgeline winds ramping up alongside heavy snowfall as the front approaches. Snow levels start around 2000 ft across the region, but will rise to 2500 ft in the northern Cascades, 3000 ft in the central Cascades, and 3500-4000 ft in the southern Cascades as slightly warmer air moves in briefly during the heaviest precipitation.
Post-frontal showers gradually replace the steadier precipitation during the evening hours, but a powerful westerly push behind the front will form strong convergence and heavy snowfall in the mountain loop. This should drop southward overnight, settling over the highway 2 corridor. Some models show 12-18" of snow Tuesday night for Stevens Pass. This convergence is unlikely to reach as far south as Snoqualmie Pass, but expect mountain enhancement to squeeze out plenty of snow.
Snow ratios increase on Wednesday as snow levels drop to 1000-2000 ft. A secondary disturbance should arrive with the upper-level trough in the afternoon, shifting moderate ridgeline winds from WSW to SSW, and bringing in another more organized band of snowfall. Snowfall remains heavy, despite the lower water numbers.
Areas near and west of the Cascade Crest can expect 2-3 ft of snow above 3500 ft, with 1-2 ft amounts likely east of the Cascade Crest over the next 36 hours.
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
Heavy snow. Moderate ridgeline winds.
Tuesday
Night
Mostly light to occasionally moderate snow showers.
Tuesday
Very heavy snow. Moderate ridgeline winds, strong gusts.
Tuesday
Night
Decreasing moderate to occionally heavy snow snow showers. Moderate ridgeline winds.
Tuesday
Very heavy snow, mixing with rain below 3000 ft in the afternoon. Increasing moderate to strong ridgeline winds.
Tuesday
Night
Heavy snow showers in the evening decreases in to the moderate range, but remains locally very heavy in areas of convergence south of Barlow Pass. Moderate ridgeline winds.
Tuesday
Heavy snow, mixing with rain below 3000 ft in the afternoon. Moderate ridgeline winds.
Tuesday
Night
Very heavy rain and snow in the evening transitions to light to moderate snow overnight. Moderate ridgeline winds, strong gusts.
Tuesday
Heavy snow, mixing with rain below 3000 ft in the afternoon. Moderate ridgeline and mostly light W wind at the Pass.
Tuesday
Night
Heavy snow and very heavy convergence. Moderate ridgeline and light to moderate W wind at the Pass.
Tuesday
Heavy snow, mixing with rain below 3000 ft in the afternoon. Moderate ridgeline and mostly light W wind at the Pass.
Tuesday
Night
Decreasing heavy snow (mixing with rain below 3000 ft in the evening) becomes light to moderate and showery overnight. Moderate ridgeline and light to moderate W wind at the Pass.
Tuesday
Heavy snow. Moderate ridgeline winds, strong gusts.
Tuesday
Night
Decreasing light to occasionally moderate snow showers. Moderate ridgeline winds.
Tuesday
Heavy snow mixing with rain below 3000 ft. Moderate ridgeline winds, strong gusts.
Tuesday
Night
Moderate rain and snow become light overnight as snow levels lower, but remains heavy in areas of convergence spillover near Highway 2. Decreasing moderate to strong ridgeline winds.
Tuesday
Light snow early becomes heavy in the afternoon, mixing with rain below 3000 ft. Moderate ridgeline winds, strong gusts.
Tuesday
Night
Heavy rain and snow becomes light overnight as snow levels lower. Moderate ridgeline winds, strong gusts.
Tuesday
Light to moderate snow develops in the morning and becomes heavy, mixing with rain below 3500 ft as snow levels rise. Strong ridgeline winds, extreme gusts.
Tuesday
Night
Very heavy rain and snow in the evening, gradually decreasing overnight as snow levels drop. Decreasing strong winds, extreme gusts.
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).