A dome of extraordinarily cold air has settled over the Pacific Northwest - likely the coldest I have witnessed in six years as an NWAC forecaster. Temperatures bottomed out this morning at -27F at Mazama, -17 at Washington Pass, and remain below zero at a majority of NWAC stations in Washington State. Interestingly, while the cold pool spanning both sides of the Cascades is deep (~7000 ft thick), above that elevation Camp Muir is our warmest NWAC station right now at 26F. That's warmer than Seattle!
The cold temperatures will be a real factor with temperatures rising around 10F by the afternoon in most locations. That puts temperatures in the single digits. Moderate to strong E winds through the mountain gaps peak late morning and will result in wind chills in the -20 to -40F range in these windy locations. These conditions can produce frostbite on exposed skin.
An approaching warm front will produce an increasing chance of snow flurries by the afternoon. Accumulating light snow should start in most areas Thursday evening with moderate snow developing overnight from Mt. Rainier southward as the warm front lifts to the ENE. Mid-level winds switch to the WNW while E flow continues at low levels.
Moderate to heavy snow will spread throughout the region Friday morning. Where the forecast gets tricky is that freezing levels rise up to 6500-7000 ft in some areas west of the Cascade Crest, but are unlikely to dislodge entrenched cold air at low levels. This sets the stage for a significant icing even with a potential changeover to sleet for the west slopes of the Cascades during the day on Friday. The timing of a changeover remains highly uncertain. East of the Cascades, expect to continue through the day on Friday.
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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Thursday
Cloudy with very light snow developing. No accumulation expected. Cold.
Thursday
Night
Light to moderate snow with the potential for a mix or changeover to sleet or freezing rain.
Thursday
Cloudy with very light snow developing by the end of the day. No accumulation expected. Bitterly cold.
Thursday
Night
Increasing light snow. Bitterly cold.
Thursday
Cloudy with very light snow developing by the end of the day. No accumulation expected. Bitterly cold.
Thursday
Night
Increasing light snow. Bitterly cold.
Thursday
Cloudy with very light snow developing by the afternoon. No accumulation expected. Bitterly cold. Moderate to strong ridgeline SE wind near the Cascade Crest.
Thursday
Night
Light snow in the evening becoming moderate overnight and possibly heavy at Paradise. Moderate to strong ridgeline winds near the Cascade Crest.
Thursday
Cloudy with very light snow possibly developing during the afternoon. No accumulation expected. Bitterly cold with dangerous windchills. Moderate ridgeline and E wind at the Pass.
Thursday
Night
Light snow becoming moderate after midnight. Moderate ridgeline and E wind possibly becoming light to moderate overnight at the Pass.
Thursday
Cloudy with very light snow possibly developing during the afternoon. No accumulation expected. Bitterly cold with dangerous windchills. Moderate ridgeline and E wind at the Pass.
Thursday
Night
Light snow becoming moderate after midnight. Moderate ridgeline and E wind at the Pass. Bitterly cold with dangerous windchills.
Thursday
Cloudy with very light snow possibly developing by the end of the day. No accumulation expected. Bitterly cold with dangerous windchills.
Thursday
Night
Increasing light snow. Bitterly cold with dangerous windchills.
Thursday
Cloudy with very light snow possibly developing during the afternoon. No accumulation expected. Bitterly cold.
Thursday
Night
Increasing light snow. Bitterly cold.
Thursday
Cloudy with very light snow possibly developing by the afternoon. No accumulation expected. Bitterly cold.
Thursday
Night
Light snow becoming moderate after midnight. Bitterly cold.
Thursday
Cloudy with light snow developing. Cold.
Thursday
Night
Moderate to occasionally heavy snow, possibly mixing with sleet or freezing rain in the early morning hours.
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).