The Bottom Line: Wind, snow, and rain will be just enough to build fresh slabs 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.
Snow and Avalanche Discussion
On Tuesday, the main concern for triggered avalanches is near and above treeline. In isolated locations, you could trigger avalanches in old snow. About 2.5 feet below the surface there's a change in hardness of snow layers. In some open, shaded, wind-sheltered areas you may find a thin layer of surface hoar at this interface. This weak layer is more prominent near Stevens Pass and the East Central zone. An observer reported collapsing and propagating test results on the surface hoar on Sunday near Pineapple Pass. Use snowpack tests to assess the weak layer. If you see cracks breaking through the snow or experience collapses, this is an indicator that you can trigger avalanches on the buried surface hoar.
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 create strong-over-weak layering even in sheltered terrain. 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: 2 - 2
Loose Wet
Small loose wet avalanches are possible on steep low elevation slopes in the afternoon. Snow will switch over to rain up to 4500ft by evening. Watch for rollerballs as indicators that you could soon see loose wet avalanches. Avoid terrain traps such as rocks, gullies or cliffs where these avalanches could be dangerous.
