A deepening low pressure off the NW tip of the Olympic Peninsula will pass inland today. The track of the low-pressure system will cause a wide variance in weather outcomes from north-south across our area. There's high uncertainty with this system - this is an unusual track for a low-pressure system. The general idea is the heaviest precipitation will be on the north side of the low, affecting the Olympics and north Cascades on both sides of the Cascade crest with precipitation totals tapering further south. Areas further south will be in the warm sector of the storm and see high snow levels for a good portion of the day. SW winds will also be strong to extreme at ridgetop level at least in Washington State.
The frontal boundary will stay nearly stationary this morning with the heaviest precipitation from about I-90 and north through this morning. A trailing upper-level trough will dig offshore today and re-orient the front in a more south-north fashion as it passes through the area late this afternoon (4 pm). Heavy precipitation for all areas will transition quickly to post-frontal showers along the west slopes of the Cascades. Snow levels will plummet as much colder air rushes in behind the front. Showers should become more scattered by morning as snow levels settle around 1000' or lower across the area.
An upper-level ridge will amplify offshore tomorrow. There should be a few isolated showers and plenty of clouds along the west slopes of the Cascades and sunnier and drier conditions along the east slopes of the Cascades on Tuesday.
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
Moderate to heavy rain and snow.
Monday
Night
Light to occasionally moderate rain and snow showers becoming scattered showers overnight. Sharp cooling trend.
Monday
Moderate to heavy rain and snow. Slight warming trend.
Monday
Night
Light to occasionally moderate rain and snow showers becoming scattered showers overnight. Sharp cooling trend.
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).