The gradually weakening, broad, vertically stacked low-pressure system continues to wobble over the Pacific Northwest. Within the broader low, two more discreet low-pressure centers can be identified, with the southernmost affecting the Pacific Northwest by rotating into instability snow showers on Wednesday afternoon. Partial sunshine in the morning, combined with cold air aloft, creates the convection, while the offshore low provides the moisture and a bit of dynamic support.
The low center moves into NW Oregon Wednesday evening, rotating light to moderate snow showers into the eastern Olympics and southern Washington Cascades through the night. Mt St Helens stands to receive the heaviest amounts of precipitation with 0.75" of water equivalent expected overnight.
The light upper and low-level east flow, which develops Wednesday evening, will focus slightly more on the partly to mostly cloudy skies and snow shower activity on the east slopes of the Cascades on Thursday morning, expanding to areas further west as the light winds shift back westerly. Most shower activity should be light overall, but expect locally more intense convective precipitation and graupel in the afternoon hours.
Light snow shower activity tapers Thursday night and Friday with a gradient from mostly sunny skies in the north to partly cloudy skies in the south as the upper low morphs into a trough and heads for the Intermountain West. This leaves a weak ridge over our region.
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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Wednesday
Night
Mostly light snow develops for portions of the northern and eastern slopes of the Olympics.
Thursday
Partly cloudy.
Wednesday
Night
A few snow showers in the evening, becoming partly cloudy overnight.
Thursday
Partly cloudy in the morning, with mostly cloudy skies and isolated light snow showers in the afternoon.
Wednesday
Night
A few snow showers in the evening, becoming mostly cloudy with isolated flurries overnight.
Thursday
Partly cloudy in the morning, with mostly cloudy skies and isolated light snow showers in the afternoon.
Wednesday
Night
Increasing light to occasionally moderate snow, potentially heavy at times for Mt St Helens.
Thursday
Decreasing mostly light scattered snow showers.
Wednesday
Night
Light snow showers at times in the evening decrease to isolated flurries overnight. Light ridgeline and E winds at the Pass.
Thursday
Partly cloudy with scattered snow flurries in the morning, becoming mostly cloudy with scattered light snow showers in the afternoon. Light ridgeline winds. Light E wind switches W by the afternoon.
Wednesday
Night
Light snow showers at times in the evening decrease to isolated flurries overnight. Light ridgeline and E winds at the Pass.
Thursday
Partly cloudy with scattered snow flurries in the morning, becoming mostly cloudy with scattered light snow showers in the afternoon. Light ridgeline winds. Light E wind switches W by the afternoon.
Wednesday
Night
Light snow showers mainly in the evening hours. Cold.
Thursday
Partly cloudy with isolated snows howers in the afternoon.
Wednesday
Night
Light snow showers mainly in the evening hours.
Thursday
Partly to mostly cloudy with isolated to scattered snows howers in the afternoon.
Wednesday
Night
Increasing light snow.
Thursday
Periods of light snow in the morning becoming showery in the afternoon.
The ridge is likely to hang on Friday night, but another broad, deep low drops south from the Gulf of Alaska, well offshore. Snow showers start moving into the Olympics on Saturday, but don't spread inland until Sunday as a low-pressure center swings northward offshore. Temperatures remain below seasonal norms Friday night and early Saturday, but warm with a shift to S flow.
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).