Avalanche Forecast
Regions: Cascades - South West.
Happy Holidays! While taking your new toys out in the mountains, keep in mind that avalanches are possible to trigger at middle and upper elevations, where the snow is drier and deeper. In the Crystal region, very large and destructive avalanches have been observed and require an added layer of caution.
Discussion
The snowpack and resulting avalanche hazard change by location and elevation around the West South region. You may experience locally lower avalanche danger in areas that did not receive the massive water numbers of Crystal and Paradise (7.45†and 6.57†respectively). Throughout the zone, wind loading and newly formed cornices have been reported. Look for evidence of recent avalanches, whumpfing, and shooting cracks as red flag indicators of instability. At lower elevations, there just isn’t enough soft snow to form an avalanche hazard. Instead, expect difficult travel conditions with refreezing snow, exposed objects, and open creeks.Â
In the Crystal area, reports of recent large avalanches continue to be reported, letting us know that instability is clearly present.Â
One example of the many natural avalanches from the storm Jeremy Allyn observed on Tuesday in the Lakes Basin area of Crystal. Photo: Jeremy Allyn 12/24/19
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
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Avalanche Problems
Persistent Slabs
A persistent weak layer within the snowpack in this zone was the cause of several very large (D3) natural and explosive triggered avalanches in the Crystal region over the last few days. The danger rating in our zone has been on a downward trend because naturally triggered avalanches have slowed down or stopped, but the threat of avalanches due to this persistent weak layer remains, specifically in the Crystal area.
The nature and existence of the persistent weak layer differs across the zone, but the effect is the same. A large load now sits atop this weak layer, and in some areas, slopes have not yet reached their tipping point. Determining which slopes are about to reach their tipping point is a very difficult task. If you trigger an avalanche, it could be 3-6ft deep, break on slopes as low as 30 degrees, and wrap around features in the terrain. While snowpack tests can confirm this layer’s presence, they can not prove its absence. Don’t rely on a snowpit to interpret stability. Avoid traveling on steep slopes above 5800’ on W-N-E aspects.
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: Possible
Expected Size: 2 - 2