Avalanche Forecast
Regions: Cascades - North East.
Happy Holidays! You can trigger a deep and dangerous avalanche where a recently formed thick slab rests over weak, older snow near the ground. Be most cautious in areas with a shallow snowpack, and on steep slopes that face the north half of the compass.
Discussion
Happy Holidays from all of us at NWAC! The recent atmospheric river event left Washington Pass with 3 feet of snow and 3.2" of water equivalent between the 19th and 21st. Observers reported a cycle of large slab avalanches in Hairpin Valley, Silver Star creek, the Delancy Ridge area, on a northwest aspect of Silver Star Mountain, and a northeast aspect of Hinkhouse peak. Some of these were "wall to wall" with very wide crowns. On the 20th, observers watched a natural avalanche, run at noon on a north aspect of Peak 6460ft near Silver Star Creek.
On the 22nd, an observer near Vasiliki Ridge reported multiple collapses on a weak layer buried in late November that covered areas as wide as 30 feet.
On the 23rd, observers reported no signs of instability during their travels on slopes from west to northeast aspects near the Hwy 20 hairpin.Â
The snowpack is substantially deeper at the top of Washington Pass than further east towards Vasiliki Ridge. Yes, the likelihood of triggering slides on this deeper weak layer is slowly tapering away, but yes, they are still possible. Persistent slabs are more likely further east in the zone where the snowpack is more shallow.Â
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
The snowpack still needs time to adjust to it's hefty new load of snow from the recent storm. A layer of faceted snow that formed in late November is often found underneath a thin crust, and sometimes resting on top of a stout crust. You can trigger dangerous and surprising avalanches on this layer from below, or on adjacent slopes. Any triggered avalanche could break widely across the terrain and would be large. Put a wide buffer of space between you and any slopes steeper than 35 degrees. I would especially avoid thin rocky terrain, steep convexities, and unsupported slopes that face the north half of the compass.
Feel for weak snow near the ground (or 2.5 to 3.5ft down) 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 them 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: All elevations.
Likelihood: Possible
Expected Size: 1 - 2