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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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Numerous large human-triggered avalanches were reported Monday from this zone resulting several near misses. Recent avalanches are a clear sign of the dangerous avalanche conditions. Active wind transport was seen forming new wind slabs below ridges on New Year's Day. It remains wise to give avalanche terrain a wide berth and stay off of and out from under all slopes greater than 30 degrees.

Discussion

Avalanche and Snowpack Discussion

Monday was an active day in the Crystal backcountry. Avalanche were reported in Cement Basin, East Peak, Pickhandle Basin, Three-Way Pack, Sourdough Gap, Elk Pass, and Cash Run.  Many of these were human triggered avalanches, including some remote triggers. These avalanches were generally large and acted in unusual ways. They failed on lower angle slopes, propagated widely, crossed terrain features, and occurred low in the startzones. Observations like this scream buried surface hoar.  

12/31/18: A large avalanche that was remotely triggered in the Crystal backcountry. Photo: Jeremy Allyn. 

We have not received observations from other locations in the West-South zone. That doesn’t mean that buried surface hoar isn’t there. Areas such as White Pass and Chinook Pass are likely areas to encounter a similar, weak snowpack.

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.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.