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Avalanche Forecast

Archived

Jan 1st, 2019–Jan 2nd, 2019

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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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.  

Snowpack Discussion

Happy New Year!

Thanks to all of you who volunteer, send observations, and support NWAC in various ways - we appreciate it.   

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.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.