New snow and wind are adding to a tricky snowpack with deeply buried weak layers that have produced several recent large avalanches.Special Avalanche Warning in effect for the interior ranges. Copy this address to view details:http://bit.ly/2nSOUyX
Weather Forecast
FRIDAY: Mix of sun and cloud. Ridge wind light, northeast. Temperature -15. Freezing level valley bottom.SATURDAY: Mostly sunny. Ridge wind light to moderate, northwest. Temperature -7. Freezing level valley bottom.SUNDAY: Mix of sun and cloud. Ridge wind light, east. Temperature -12. Freezing level valley bottom.
Avalanche Summary
Several large and very large, destructive avalanches have been reported in the past week. These avalanches are failing on weak layers deep in the snowpack and running to valley bottoms. On Wednesday naturally triggered, deep persistent slab avalanches up to size 2.5 were reported in the Alexander Creek Drainage. The suspected failure layers were mid-December or late-November layers near the base of the snowpack. Also avalanche control in the Line Creek area produced several size 2.5 and two size 3 avalanches that also failed on weak layers near the base of the snowpack. 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. Last 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
Wednesday storm has delivered another 15-30 cm of snow bring storm snow totals to 40-80 cm over the past week. The distribution of this new snow is highly variable. 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.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
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.
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.