New snow and wind on Wednesday and into Thursday will continue to build wind slabs in open areas. Choose conservative, well-supported terrain and be aware of overhead hazards. Large avalanches have run long distances in recent days.
Confidence
Moderate - Forecast snowfall amounts are uncertain
Weather Forecast
TONIGHT: Mainly cloudy, light flurries. Accumulation 2-4 cm. Ridge wind strong, west. Temperature -10. Freezing level valley bottom.WEDNESDAY: Cloudy, flurries. Accumulation 4-8 cm. Ridge wind moderate to strong, west. Temperature -4. Freezing level 1200 m.THURSDAY: Cloudy, flurries. Accumulation 10-20 cm. Ridge wind light, northeast. Temperature -20. Freezing level valley bottom.FRIDAY: Sunny. Ridge wind light, north. Temperature -15. Freezing level valley bottom.
Avalanche Summary
On Monday ski cuts in the Castle area produced numerous small (size 1) wind slab results on freshly wind-loaded features.On Saturday in the neighboring Lizard Range, the east facing Mt. Corrigan slide path produced a very large natural avalanche estimated to be a size 4.0, which took out mature timber in the path as it overran the Flathead FSR south of Corbin. Reports are limited, but there was also an explosive triggered wind slab avalanche east of the divide on Saturday. There was likely natural wind slab activity in the northeast part of the region where the heaviest accumulations occurred.On Friday, a snowmobiler was partially buried by a size 2 avalanche in the Alexander Creek drainage. The avalanche occurred on a north aspect around 2000 m and failed on weak snow near the ground. A few small wind slabs were triggered by skiers in lee terrain on Thursday and Friday.
Snowpack Summary
An upslope storm has delivered lots of cold low density snow east of the divide since Saturday (20-40 cm). The distribution of new snow is highly variable, and so are the surface snow conditions. The western parts of the region in BC were relatively warm with less snow, while areas in Alberta and around the divide were much colder and windier which likely formed touchy wind slabs in open terrain.The lower snowpack in this region is weak, with a number of concerning weak layers. A layer buried mid-January is composed of surface hoar on sheltered aspects and a sun crust on solar aspects is now 60-80 cm deep. Another surface hoar layer from early-January is buried 70-90 cm deep. A widespread weak layer from mid-December composed of facets, crusts, and surface hoar is 100-150 cm deep. Finally, a rain crust with sugary facets buried in late-November is near the bottom of the snowpack. Although the snowpack structure is variable across the region, these persistent weak layers are generally widespread.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.