Avalanche Forecast
Regions: Cascades - North East.
A buried weak layer of surface hoar is responsible for several recent avalanches near Washington Pass. You can trigger large avalanches on this layer that break in surprising ways. Cautious route finding is essential to navigate these dangerous avalanche conditions.
Discussion
Snowpack and Avalanche Discussion
Reports of triggered and natural avalanches failing on a layer of buried surface hoar have us very concerned about the snowpack east of the Cascade Crest. In the past two days, several avalanches were even triggered remotely from flat ridgelines, adjacent slopes, or from below. These avalanches broke 1-2ft deep with impressive propagation. It will still be easy to trigger an avalanche on this layer Wednesday. Â
Snowpack Discussion
Happy New Year!
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December of 2018 was fun (from a forecasting perspective) with three pronounced avalanche cycles, a couple different persistent weak layers, some rain events, and a flurry of human-triggered avalanches to ring in the New Year. Most importantly, it seems that we made it through the last days of 2018 without anyone getting seriously hurt by an avalanche.
The deep (Dec 9) layer responsible for many of the avalanches early in the month no longer seems to be a problem in the western zones. That said, it is still possible to trigger an avalanche on its counterpart (or basal facets) in the eastern areas.
A widespread layer of surface hoar formed around Christmas. Late December storms preserved this layer in areas above the rain line and we have numerous (more than a dozen) reports of people triggering avalanches on it in the last three days. At least 4 people were caught and carried during this period, but so far we have no reports of serious injury. Most of these avalanches were soft slabs, D1-D2+, but there were several harder wind slabs in the mix.
It appears that the layer is most reactive and/or prevalent in the Crystal Mountain backcountry and in the mountains around Leavenworth and west of Mazama.
Surface Hoar can be an especially tricky and persistent weak layer. Read more about it here.
Avalanche Problems
Persistent Slabs
The latest storm created slabs 1-2ft thick that are now resting on a layer of feathery surface hoar that formed around Christmas time. This non-cohesive layer has been observed to be sitting on a firm, old snow surface above 5500ft. It may be very easy to trigger avalanches on open slopes above this elevation. Be especially careful of wind loaded areas where stiffer slabs rest atop this layer. Avalanches may break widely around terrain features, may initiate on low angle slopes, and may be triggered from a distance. Below the 5500ft range, a thick, supportable rain crust at the snow surface may reduce the likelihood of slides on this layer.
In addition, very large avalanches remain possible within old, faceted layers near the ground. Deeper, persistent slabs are difficult to predict because they may not offer direct signs of instability. Regardless, after a significant loading event such as this recent holiday storm, persistent slabs often become easier to trigger. The snowpack needs time to adjust to the new load.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 2