Rapid cooling occurred as a deep, cold upper trough approached our region Monday night. Temperatures dropped below freezing for most NWAC stations above 2000 ft by Tuesday morning with an accompanying transition from rain to snow showers with light to moderate WSW winds. A more organized disturbance will bring sustained light to moderate snow to much of the region through Tuesday morning. Convergence will likely enhance snowfall over the northern mountain loop with this enhancement dropping southward into Snohomish County during the afternoon as snowfall becomes showery in most other areas during the afternoon. With cold air aloft and some sun breaks heating the surface at times, expect a good chance for convective and locally intense snow or graupel showers along with an isolated thunderstorm.
Moderate winds will be gusting in the strong range through the mountain gaps and along the east slopes of the Cascades, but should gradually decrease Tuesday night as an upper low circulation (embedded within the trough) drops toward the mouth of the Columbia River in the early morning hours. This will lead to a decrease in snow shower activity in most areas during the evening hours with snow showers increasing across the southern Washington Cascades and Mt Hood areas through much of the day on Wednesday. Expect partially clearing skies with a few isolated snow showers for areas north of Mt Rainier.
Snow levels will fluctuate between 1000-3000 ft for the W slopes of the Cascades, bumping up slightly higher in the afternoons for the east slopes. However, expect temperatures to increase during breaks in the shower activity and drop quickly during snow squalls. Temperatures stay below seasonal norms.
Weather Forecast
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Tuesday
Light snow becoming showery from late morning onward.
Tuesday
Night
Light snow showers.
Tuesday
Moderate snow transitioning to showers by the afternoon.
Tuesday
Night
Decreasing light snow showers.
Tuesday
Moderate snow transitioning to showers by the afternoon. Convergence banding producing locally heavy snowfall.
Tuesday
Night
Decreasing light snow showers.
Tuesday
Moderate snow at Paradise, Mt St Helens, and Mt Adams becoming showery by the afternoon. Light snow at Crystal and White Pass becoming showery by the afternoon.
Tuesday
Night
Increasing light snow showers may become moderate in the southern part after midnight.
Tuesday
Light snow becoming showery by the afternoon. Moderate ridgeline and light W wind at the Pass.
Tuesday
Night
Decreasing light snow showers. Decreasing light ridgeline and W wind at the Pass.
Tuesday
Light snow becoming showery by the afternoon. Moderate ridgeline and light to occasionally moderate W wind at the Pass.
Tuesday
Night
Decreasing light snow showers. Moderate ridgeline winds become light and light W wind at the Pass.
Tuesday
Light snow showers. Moderate ridgeline winds.
Tuesday
Night
Light snow showers tapering off in the evening, then becoming partly cloudy.
Tuesday
Light snow showers. Moderate ridgeline winds.
Tuesday
Night
Light snow showers mainly in the western part with partial clearing further east. Moderate ridgeline winds becoming light.
Tuesday
Light snow showers. Moderate ridgeline winds.
Tuesday
Night
Light snow showers mainly in the western part with partial clearing further east.
Tuesday
Moderate snow through the morning hours, then light to occasionally moderate snow showers. Moderate to 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).