Avalanche Forecast

High.

THURSDAY NIGHT: Cloudy, freezing level dropping from 1000m to below valley bottom. FRIDAY: Mostly sunny, moderate northwest winds, alpine temperature -10°c, freezing level below valley bottom. SATURDAY: Cloudy with light snowfall, accumulation 5 to 10cm, moderate southwest winds, alpine temperature -7°c, freezing level below valley bottom. SUNDAY: Cloudy with snowfall, accumulation 10 to 20cm, moderate southwest winds, alpine temperature -5°c, freezing level 1000m.

Many avalanches were triggered from skiers and explosives on Tuesday and Wednesday. Most of the avalanches were small to large (size 1 to 2) and released in the recent storm snow, 30 to 100cm deep. Many of the avalanches were stepping down to older layers. Three very large avalanches (size 2.5 to 3.5) were triggered in the region with slabs around 100 to 200cm thick and likely released on a weak layer near the base of the snowpack, as described in the snowpack discussion. Clearer skies on Friday may allow for better views of natural activity that occurred during the storm on Thursday.

About 3m of snow has accumulated over the past 10 days. Thursday’s storm brought 30 to 50cm more snow to the region. The snow fell with extreme (gusts to 180 km/h) southerly winds. Expect the new snow to be reactive to skier activity on Friday, especially in the immediate lees of terrain features. A weak layer of facets and surface hoar lies below all this storm snow. Recent avalanche activity is mostly running in the storm snow above this layer but there have been reports of avalanche stepping down deeper within the snowpack, possibly to this weak layer. At the base of the snowpack, weak and sugary facets exist below an early-season melt-freeze crust. This weak layer has been the culprit for sporadic, very large avalanches in alpine terrain in the past few weeks. The avalanches have occurred in areas where the ground roughness is very smooth, for example glaciers, firn, and shale/rock slab slopes. An avalanche could be triggered in this layer in areas with smooth ground roughness either where the snowpack is thin or with a very large trigger such as a cornice fall. Storm slab avalanches could also step down to this layer.