The Bottom Line: Wind and new snow will be just enough to build fresh slabs, stress old weak layers, and create dangerous conditions by late Tuesday afternoon. Steer around wind drifted features and avoid steep roll-overs and convex terrain. Check the top 2 feet of the snowpack for a thin weak layer, and if you see cracks in the snow or feel collapsing, stay off of similar slopes 30 degrees and steeper.
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
Wind Slabs
Expect a dynamic day with avalanches becoming easier to trigger in wind loaded areas by afternoon. Steer around fresh drifts on leeward slopes below ridges. Avoid steep roll-overs and convex features. A warming trend may also create strong-over-weak layering even in sheltered terrain with rain possible to 4500ft. While it will be difficult to trigger avalanches in the morning, natural avalanches may occur Tuesday night with the heaviest snow.
Aspects:
All aspects.
Elevations:
Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
Use extra caution where you expect dangerous persistent slabs avalanches. Avoid areas where drifting snow is stressing underlying weak layers. Expose only one person at a time to avalanche terrain. Stop and regroup in safer areas, well away from where avalanches could start or run.
There is a lot of uncertainty about exactly where you can trigger persistent slab avalanches and they will not be widespread. This depends on the distribution of the weak layer. Digging down about 2 feet into the snow, you can find a thin weak layer of surface hoar, buried January 17th. You're most likely to find it in wind-sheltered, shaded terrain, well above the valley floor, possibly up to ridges, and east of the Cascade Crest. Observers have reported signs of instability on thie surface hoar near Yodelin, Lichtenberg Mountain, Smithbrook, and Jove Peak areas. Small and large snowpack test can help you identify this weak layer and gather information. Lack of evidence in one profile or snowpack test shouldn't be a reason to travel in more consequential terrain.
