Avalanche Forecast

Archived

Dec 30th, 2018–Dec 31st, 2018

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 unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

.

Dangerous conditions exist where recent snowfall and wind have created slabs that overlay buried surface hoar. 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:

This past storm brought about 8” of snow with 0.8” of water equivalent to Washington Pass and rain up to about 5,500ft during the storm. We are left with fresh, cold snow and some deep wind drifts. An observer on Washington Pass noted two avalanches on the 30th, both were skier triggered. These were on East to Southeast aspects at 6500ft and 7,000ft respectively. One was roughly 500ft wide and split around a terrain feature. Details are still filtering in, but it appears that a layer of buried surface hoar was likely preserved above pass level in the area. Observers on the other side of the mountains (near Mt Baker Ski Area) reported widely propagating avalanches above 5500ft. These were suspected to have failed on a layer of buried surface hoar from December 28th up to 2 feet deep with crowns of over 1,000ft wide.

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.