Avalanche Forecast
Regions: Cascades - North East.
Spooky and dangerous conditions exist where enough recent snow and wind have created thick slabs that sit over a layer of buried surface hoar. If venturing out into the mountains, beware of any steep, open slope. Careful navigation will be necessary for a safe day out.
Discussion
Snow and Avalanche Discussion:
On the 30th skiers were able to intentionally trigger avalanches in Portly Bowl. These occurred on East to Southeast aspects above 6,500ft. An observer reported a very large slide off Pt. 8183. On the 31st an observer in the Icicle Creek drainage found the snowpack to be very reactive. A supportable rain crust has been observed up to about 5,500ft. As soon as he got above 6,000ft, where the trees were more sparse, cracks began shooting out from his skis as he walked along the ridge. Avalanches were triggered from a distance while walking on this ridge. These were on average 18" deep, and easily up to 400ft wide. As the slabs slid downhill, light reflected off the shiny bedsurface, it was large, feathery, surface hoar. A widespread natural avalanche cycle had occurred during the tail end of the storm, but some slides may have released well after the snow stopped falling. We do not know much about the distribution of this layer across the ranges of the eastern slope. However, we need to treat our upper elevation terrain as guilty until proven innocent.Â
This past storm brought about 8" with 0.8" of water equivalent to Washington Pass, with moderate winds. This may have been just enough to create a slab over a layer of surface hoar from around Christmas. Other areas further east in the zone have been observed to be lacking a slab structure.Â
Snowpack Discussion
Coming soon.
Avalanche Problems
Persistent Slabs
In many parts of the zone, the latest storm has created slabs up to 16" thick that are resting on a layer of large, feathery surface hoar from Christmas. This low friction layer has been observed to be sitting on a firm, old snow surface above 5,500ft. It may be very easy to trigger avalanches on open slopes above this elevation, especially in places where the wind has helped to give cohesion to the snow near the surface. Avalanches may wrap widely around terrain features, and may initiate on low angle slopes. Steer clear of steep slopes if you suspect this layer has more than 8" of snow over it, as you may trigger slides from a distance and from below. Below mid 5,000ft range, a thick, supportable rain crust may help to inhibit any slides on this layer.
Very large avalanches remain possible within old, faceted layers near the ground. Deeper, persistent slabs are difficult to predict because you may not get direct signs of instability with these deeper layers. What we do know is that after a significant loading event such as this one, persistent slabs often become easier to trigger. The snowpack needs time to adjust to the new load.
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 - 2