On Saturday, avalanches will become increasingly possible through the day as new snow accumulates and gets loaded on terrain from the wind. This will occur on North to east facing slopes at and above treeline. Additionally, take extra caution above treeline. On Thursday 12/19 a deep persistent slab problem showed life after a very large avalanche was triggered at Mt Bachelor via explosives on a deeply buried rain crust. Avoid cornices and smooth, pillowed snow surfaces on leeward slopes to minimize avalanche hazard.
Wind slab problem will increase through the day on Saturday as more snow falls. At all but the highest elevations of the Central Cascades, we are seeing a predominant surface crust from the rain event which occured Tues-Wed. This will be a good landmark for how much snow has accumulated in start zones. Moderate - strong winds on Saturday mean that wind slabs will be possible as snow accumulates.
On Thursday 12/19, Mt Bachelor Ski Patrol used explosives in their mitigation work and triggered a wind slab avalanche that stepped down to a deep persistent slab, on a weak layer below the 12/7 rain crust that we have been monitoring. We have not found any well developed facets below this crust, leading us to believe that it is unlikely this layer is weak enough to produce avalanches by itself. This means that there likely needs to be an abnormally large trigger (ie large windslab avalanche or cornice fall) in the isolated areas where this problem exists.
Whenever we have a Deep Slab, we have an increased amount of uncertainty to deal with. It can be difficult to assess the weak layer. The recommended strategy will be to avoid scenarios that could produce the big trigger: practice conservative decision-making on large, lee slopes, and steer clear of cornices.
On Friday I went out to the east side of North Sister with the intention of observing complex alpine terrain for signs of recent avalanches, and investigate the 12/7 rain crust in an above treeline snowpack that has seen minimal skier traffic. I didn't see any crowns nor get any test results that are indicative of avalanches. We will continue to monitor this problem, and as long as it's listed, extra caution is warranted, especially at upper elevation bands in high consequence terrain.
