Avalanche Forecast

Archived

Dec 30th, 2018–Dec 31st, 2018

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

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Dangerous conditions may exist where recent snowfall and winds have created thick slabs near and above treeline to watch out for. Though becoming less likely, very large avalanches can still be triggered in old snow layers near the ground, especially where the snowpack is shallow and variable.

Discussion

Snow and Avalanche Discussion:

The East Central Zone is like a tale of two worlds at the moment. This past storm brought 1.8” of water equivalent at the Sasse Ridge Snotel site in the Salmon la Sac drainage (which could equate to well over a foot of new snow at higher elevations), while further east Mission Ridge picked up just an inch or two of new snow, but with winds of over 100mph. This is likely creating a pronounced difference in avalanche danger. The big difference is how thick the wind slabs may be. We have very few recent observations from upper elevations in the western portion of the zone. This same area likely picked up substantial snowfall. If heading out to the Salmon la Sac, Teanaway, or Icicle Creek tomorrow be advised that forecast uncertainty is fairly high at the moment, especially for upper elevations.

We do know that on Washington Pass on the 30th, skiers were able to trigger widely propagating avalanches on a layer of buried surface hoar from December 28th. We also know that ski area snow safety teams on Snoqualmie Pass and Stevens Pass were able to trigger wind slabs big enough to injure, bury, or kill a person with control work. Did this layer of surface hoar survive above 6,000ft in parts of the East Central Zone? We do not know yet, but it may be there.

As always, thanks for sharing any and all observations with us!

Snowpack Discussion

Coming soon

Problems

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.

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.