In the north of the region, persist weak layers are more likely to have endured recent storms. In these areas, the alpine Avalanche Danger may be High on Friday.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
On Friday expect 5-15cm on new snow, strong southwest winds and freezing levels at about 1000m. On Saturday and Sunday, the region will see another 5cm of new snow each day with freezing levels hovering around 800m. Winds for Saturday will be mainly moderate and southwesterly, switching to light and northerly on Sunday.
Avalanche Summary
In recent days, backcountry avalanche observations have been extremely limited, mostly due to stormy weather. That said, I'm sure there was a widespread natural avalanche cycle on Thursday in response to heavy storm loading. Forecast cooling should help gradually reduce natural avalanche activity. But, for the short term storm slabs will likely remain sensitive to light loads, while potential remains for very large persistent slab avalanche activity.
Snowpack Summary
On Wednesday night and Thursday morning, heavy snowfall and strong winds continued to form deep and dense storm slabs in high elevation terrain and cause substantial cornice growth. At treeline and below, rain fell and further saturated the snowpack. The snow line fluctuated a great deal during the storm which dropped up to 60mm of precipitation throughout the region.Where it still exists, the mid-January surface hoar layer is between 70 and 150 cm below the surface. The hope is the combination of heavy storm loading and warm temperatures has flushed out this weak layer in most areas, but that remains to be seen. In the wake of the storm, there will be a lot of uncertainty regarding the reactivity and distribution of this destructive persistent avalanche problem, especially at high elevations in the north of the region where continued reactivity at this interface has been observed. Additionally, the snowpack will require time to adjust to the stress of heavy storm loading. A very conservative approach to mountain travel is still required.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.