The Bottom Line: You can trigger avalanches in the top 2 feet of the snowpack, especially where the wind formed slabs near and above treeline. Steer around wind drifted features and avoid steep roll-overs and convex terrain. Check the old snow surface for potential weak layers and if you see cracks in the snow or feel collapsing, stay off of similar slopes 35 degrees and steeper.
Snow and Avalanche Discussion
Over 16 inches of heavy snow fell since the 17th. The storm snow was primed for slab avalanches with classic strong over weak layering. Reported natural avalanche activity ended on the Saturday and was generally confined to the new snow in wind-drifted areas or on unsupported slopes. On Friday and Saturday, avalanche professionals triggered small avalanches in the new snow near and below treeline. Many were in wind affected terrain.
For Monday, expect calm conditions. The main concern for triggered avalanches is near and above treeline. In isolated locations, you could trigger avalanches below treeline on the old snow surface. About 1-2 feet below the surface there's a change in hardness of snow layers. In some open, wind-sheltered areas on the eastern edge of the zone you may find a thin layer of surface hoar at this interface. 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. If the sun comes out on Monday afternoon, expect small loose avalanches on sunny slopes.
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 for 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
On Saturday, gusty west winds formed slabs at all elevations, even open slopes below treeline. Expect cross-loading below ridges. Avoid pillows and drifts on leeward sides of ridges. Use your pole or probe to feel for wind-stiffened snow. Wind features may now be harder to see as covered by a thin blanket of snow. You may also be able to trigger avalanches in sheltered terrain where snow since the 17th is poorly bonded to the old snow surface. Use small test slopes and quick tests to check the bonding of the snow in the top 2 feet of the snowpack. Avoid steep roll-overs, convexities, and unsupported, rocky slopes. These trigger points are locations where you could start an avalanche more easily.
