The Bottom Line: A winter storm will impact the Snoqualmie Pass area Tuesday night and Wednesday morning with warming temperatures, rain, and snow. Avoid open slopes greater than 35 degrees where you find more than 8 inches of new snow, or the wind drifts new snow into thicker slabs.
Snow and Avalanche Danger
We are expecting significant precipitation with warming at Snoqualmie Pass Tuesday night and Wednesday morning. Avalanche danger should peak overnight during the warmest and stormiest periods. As precipitation ends and temperatures cool, the avalanche danger will slowly decrease.
The weak snowpack structure seen near Stevens Pass and the East-Central region does not seem to be as prevalent in the Snoqualmie area--that does not mean it is absent. We were able to find buried surface hoar near Cottonwood Lake/Roaring Ridge last week. An observation from Sunday also suggests an old weak snow layer may be present. This layer could exist 1-2 feet below the snow surface. If you experience any collapsing, or see larger avalanches, it’s time to dial it back and avoid avalanche terrain nearby.
Regional Synopsis
January 20, 2019
The recent weather pattern of lower accumulation storms (by NW standards) and longer stretches of calm weather should continue as we move into late January. Since January 17th, incremental snow accumulations punctuated with rising freezing levels favored the south and eastern parts of the region. Storm instabilities have risen with storms and gradually subsided.
A storm slab at Mt Baker.
New Snow Problems
Storms over the past week have brought a range of layers from rain crusts, to heavy moist snow, to stiff drifts, to light dry powder. Some storm days, like the 18-19th, saw reactive, but very short-lived avalanches caused by heavy precipitation and wind. Even the longer-lasting avalanche problems, wind slabs, haven't persisted form more than a few days. Where the recent snow is stressing underlying weak layers, more dangerous avalanche conditions have prevailed.
Surface hoar in the East Central zone
Old Snow Problems
Persistent weak layers (PWLs) have been a constant in the eastern zones of the Cascades this winter. As usual, they have been much less problematic at the Passes and west of the Cascade Crest. The latest PWL is a layer of surface hoar, buried around January 17th and found generally east of the Cascade Crest. Buried surface hoar is an active weak layer in the eastern zones and can be found to a limited extent on the eastern edge of the Stevens and Snoqualmie Pass zones. There few, if any, avalanches have been reported on the buried surface hoar. It may be most problematic in open, wind-sheltered terrain, especially well above the valley floor.
You are most likely to find other layers of old weak snow the further you move east from the Cascade crest. Here snowpacks are shallower, more variable, and generally weaker. In some locations, weak snow near the ground can still be found. These basal facets have hung around all season. Digging profiles and using snowpack tests is the best way to gain information about these old persistent weak layers. However, snowpack tests are just one piece of the puzzle. Your terrain decisions shouldn't hinge on any given test result. Because of the size of our forecast zones and the variability in the snowpack, it's important to make snow observations as you travel. We’ll keep watching these old layers, but let us know what you see while you are in the mountains.
Avalanche Problems
Storm Slabs
Above the rain-snowline, we expect upside-down storm snow caused by warming temperatures, heavy precipitation rates, and moderate winds. In these locations, you will be able to trigger avalanches on open slopes greater than 35 degrees. Avalanches will grow larger and be easier to trigger as you go up in elevation, or venture into areas where the wind drifted the new snow. Simple observations will help you identify if storm slabs formed overnight. Has the area received more than 8 inches of new snow? Do you see cracking? Can you feel stronger snow over weaker snow? Do you see signs of wind drifted snow? If you answer yes, storm slabs are nearby.
Aspects:
All aspects.
Elevations:
Alpine, Treeline.
Likelihood: Likely
Expected Size: 2 - 2
Loose Wet
Below the rain-snow line a wet snowpack will develop overnight. You may trigger loose wet avalanches on steep slopes greater than 35 degrees. Even small loose avalanches can be harmful if they carry you into trees, over cliffs, or bury you in a gully. As temperatures cool and the snowpack dries, wet avalanches will become more difficult to trigger.
