Avalanche Forecast
Regions: Cascades - North East.
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.Â
Snowpack Discussion
Coming soon.
Avalanche Problems
Persistent Slabs
Moderate to strong winds have created stiffer slabs at upper elevations and in wind-exposed areas. At upper elevations, a layer of surface hoar that formed around Christmas may have persisted through the storm. Use extra caution above treeline, as these slabs may break surprisingly wide and wrap around terrain features. Slabs resting on surface hoar are known to run at lower angles. Be sure to distance yourself between terrain steeper than 30 degrees.
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 - 1