Reactive wind slabs may build on Thursday with new snow, strong wind and warmer temperatures.
Be mindful of the lingering deep persistent slab problem. Caution on shallow snowpack areas and thin to thick steep rocky zones.
Up to 10 cm of new snow and strong wind will possibly build new and reactive wind slabs at upper elevations. They could be poorly bonded to the underlying snow surfaces, like a surface hoar/crust combo, so be extra cautious in leeward terrain and convex rolls.
The early December crust can be found 90-160 cm deep. A deep persistent slab problem has created a low likelihood, high consequence scenario that is difficult to forecast. There was an increase in avalanche reports on this layer earlier this week. This is a complex problem so our best advice is to avoid likely trigger spots such as steep, rocky unsupportive slopes with a shallow or thin to thick snowpack.
Thursday Night: Increasing cloud cover later in the evening with isolated flurries. Ridgetop wind light from the southwest and alpine temperatures near -10.
Thursday: Cloudy with new snow 5-10 cm. Moderate to strong ridgetop wind from the southwest. Alpine temperatures warming to near -3 and freezing levels valley bottom.
Friday: Mix of sun and cloud. Ridgetop wind light from the West. Alpine temperatures near -5 and freezing levels 1100 m.
Saturday: Sunny with some clouds. Alpine temperatures near -4 and freezing levels 1200 m.
On Wednesday, explosive cornice control triggered isolated wind slabs up to size 2 on the slopes below. These did not pull deeper slabs.
Over the last week, the early December persistent weak layer has produced a few large natural avalanches (2-3) in the center of the region. Explosive controls have also produced a few size 2 avalanches that failed on the same persistent weak layer, one being remotely triggered. These occurred on thin to thick features on northeast-facing slopes from between 1600 m and 2000 m, where the layer was buried 100 cm + deep. Although avalanche activity has tapper off, it remains a concern across the region.
The recent storm snow has been redistributed on alpine lee slopes and open areas at treeline forming reactive wind slabs on leeward slopes. Around 1900 m and below, a crust is now capping the dense 10-30 cm of snow which has settled significantly with the past mild temperatures. The crust varies from thin/breakable to thick/supportive, according to the aspects and elevation. Below the recent snow, a layer of feathery surface hoar and melt-freeze crusts are found, which seems to be still reactive in isolated areas.
The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 90-160 cm. This layer was reactive last weekend producing many large size 3 avalanches from all aspects and elevations. Activity has tapered, however, warming, cornice fall, and human triggers from steep, rocky slopes, with a shallow or thin-to-thick snowpack may wake it up again.
