Avalanche Forecast
Issued: Dec 25th, 2019 10:05AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
Though becoming less likely, it is still possible to trigger a deep and wide avalanche where a slab rests over weak, older snow near the ground. Higher elevations, and slopes with a shallow snowpack that face the north half of the compass are the most concerning. Use extra caution before entering slopes steeper than 35 degrees in these areas.
Summary
Discussion
It has been a number of days now since the big storm event last weekend. Observers at Holden Village and in the Icicle Creek drainage reported a cycle of large, natural avalanches on the 20th and 21st. These avalanches were easily big enough kill a person (up to size D2.5). Some broke widely across the terrain, ran on weak old facets, and may have been triggered remotely. On the 23rd, Mission Ridge Ski Patrol got results with explosive work below the Microwave Tower. The slab was observed to have failed on a layer of facets over a crust, which coincides with the same layer of concern we have closer to the crest. Yes, the likelihood of triggering slides on weak layers near the ground are slowly tapering off, but they may still be possible. Persistent slabs bring uncertainty, but we do know that they are more likely above 6,000ft, and on northerly facing slopes. Watch this video I put together from my field day in the Icicle drainage on the 23rd for more info.
A refrozen melt freeze crust can be found up to around 5,500ft throughout much of the zone, making travel difficult at these lower elevations. It is also very unlikely you'll trigger avalanches at these lower elevations because of refrozen conditions.
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
Problems
Persistent Slabs
A layer of faceted snow that formed in late November is often found near the ground, resting on top of a stout crust. This layer can be found 2-3ft down from the surface, or about 1ft above the ground. It is still possible to trigger surprising avalanches from below, or on adjacent slopes because of it. Any avalanche that runs near the ground would be large. I would especially avoid thin rocky terrain, steep convexities, and unsupported slopes that face the north half of the compass. Check out a recent video I made from Icicle Creek a couple days ago that highlights some pertinent signs of instability.
Feel for weak snow near the ground with your probe. Listen for signs of instability such as whumphs, and look for shooting cracks. Dig down to identify this weak layer near the ground. Snowpack tests can help confirm the presence of this layer, but you will likely be able to identify it with a simple hand hardness test.
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: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 26th, 2019 10:05AM
