Recently formed wind slabs are expected to remain touchy on Friday. Conservative decision making remains critical in wind affected terrain.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
A weak ridge of high pressure should dry things out for Thursday night and Friday morning before a weak storm pulse arrives on Friday evening. 2-5 cm of new snow is expected by Saturday morning with freezing levels below 1000m. Alpine winds are expected to be light to moderate from the southwest. Lingering flurries are expected for Saturday with afternoon freezing levels around 1000m and light alpine winds from the west. Light flurries and cool temperatures are currently forecast to continue on Sunday.
Avalanche Summary
On Wednesday, a natural size 1.5 wind slab released in a cross loaded gulley with a 40cm slab thickness. Explosives also triggered a size 2.5 deep persistent slab avalanche on a south aspect at 2000m in the southeast of the region. This released down 70cm on the early December crust layer. No new avalanches were reported on Tuesday. Wind slabs are expected to remain reactive to human-triggering on Friday at higher elevations.
Snowpack Summary
Recent strong to extreme winds from the south to west have redistributed the snow at treeline and above. Wind slabs have formed on lee slopes and windward slopes are being stripped away. Moist surface snow is being reported to around treeline elevation on Thursday and a crust is expected to form over the weekend as freezing levels fall. The mid and lower snowpack has been settling well with the warm temperatures and is expected to gain considerable strength with several days of cooling. A weak crust/facet layer from early-December is typically down over 1m. It has become difficult to trigger this layer but it is still reactive in snowpack tests suggesting that if you are able to trigger it, the layer is capable of wide propagations and large destructive avalanches. Recent explosive control triggered a few large avalanches on this layer in the southeast of the region. In the northwest of the region, there may still be reactive surface hoar layers in the upper snowpack.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.