A short-lived but very intense cold outbreak will set daily cold temperature records in many lowland and mountain locations Friday through Saturday. The weather pattern creating the extreme cold is very cold upper low centered over Alberta capping a very strong surface high pressure over interior Canada. That high forced a modified arctic front into the Pacific Northwest, which pushed through from N to S Thursday afternoon and night. Strong Fraser outflow winds injected the cold air into the Puget Sound lowlands where temperatures hover in the single digits (Watcom County) into the 20s. The modified arctic fronthas stalled near the Oregon border.
Decreasing amounts of weak instability and moisture continue to feed moderate snow showers into northern Oregon which should become light by Friday afternoon. Further north Pressure gradients are already easterly, helping touch off some very light snow or snow flurries along the E slopes of the Cascades. The surface pressure increases east of the Cascades, while a low moves toward the Oregon Coast, driving increasing E gradients over the next 36 hours. E winds will become moderate at pass-level and ridgelines through the mountain gaps Friday night and may become strong at times on Saturday, particularly W of the mountain gaps. The aforementioned low should bring some mid-level or high cloud cover on Saturday and Saturday evening with increasing light to moderate snow for Oregon on Saturday. Further north, expect mostly sunny skies.
Clouds and snow flurries early, then becoming mostly sunny. Bitterly cold.
Friday
Night
Mostly clear with some high clouds. Bitterly cold.
Friday
Snow flurries or light snow tapering from north to south early in the day. Becoming partly cloudy. Bitterly cold.
Friday
Night
Mostly cloudy. Bitterly cold.
Friday
Low clouds and snow flurries early, then becoming partly to mostly sunny. Low clouds are possible in the mountain gap. Bitterly cold. Light ridgeline and W wind switching E at the Pass.
Friday
Night
Mostly clear with some high clouds. Bitterly cold. Increasing light to moderate ridgeline and E wind at the Pass.
Friday
Low clouds and very light snow are possible early, then becoming partly to mostly sunny. Low clouds are possible in the mountain gap. Bitterly cold. Light ridgeline and W wind switching E at the Pass and increasing during the afternoon.
Friday
Night
Mostly clear with some high clouds. Bitterly cold. Increasing moderate ridgeline and E wind at the Pass.
Friday
Snow flurries early, then becoming partly cloudy to mostly sunny. Extremely cold.
Friday
Night
Mostly clear. Extremely cold.
Friday
Light snow or snow flurries are possible through the morning hours, then becoming partly cloudy to mostly sunny. Bitterly cold.
Friday
Night
Mostly clear with thin high clouds. Bitterly cold.
Friday
Light snow or snow flurries are possible into the early afternoon hours, then becoming partly cloudy to mostly sunny. Bitterly cold.
Friday
Night
Increasing high and mid-level clouds. Bitterly cold.
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).