Avalanche Forecast
Regions: Cascades - North East.
Though becoming less likely, it is still possible to trigger a deep and wide avalanche where a thick slab rests over weak, older snow near the ground. Places with a shallow snowpack, and steep slopes 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.
Discussion
Since the 21st, the snowpack at the Washington Pass weather station has settled down from 62" to  48" deep. It has been a number of days now since the big event increased our snowpack depth from 23" to 62" deep in less than 48 hours. That whopping load of snow caused a widespread, large avalanche cycle between the 20th and 21st. Large slides were observed to have run in Silver Star Creek, Delancey Ridge, 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 21st, 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, starting as low as 4,900ft 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 pack is substantially deeper at the top of Washington Pass than further east towards Vasiliki Ridge. 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 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.
Avalanche Problems
Persistent Slabs
A layer of faceted snow that formed in late November may be found near the ground, underneath a thin crust, and sometimes resting on top of a stout crust. This layer can be found 2-3ft down from the surface, or about 1ft above the ground. You can trigger dangerous and surprising avalanches from below, or on adjacent slopes because of it. An avalanche that runs near the ground 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. Check out a recent video I made from adjacent East Central Zone 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 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 - 1