We're entering a period of warming that may expose the weakness buried at the base of our snowpack. The possibility for large natural slab avalanches will increase with warmth and sun exposure.
Confidence
Moderate - Freezing levels are uncertain
Weather Forecast
Friday night: Cloudy with clear periods. Light to moderate northwest winds, increasing to strong in the alpine.Saturday: Cloudy with sunny periods. Light to moderate southwest winds, increasing to strong or extreme in the alpine. Winds increasing overnight. Alpine high temperatures around 0 or possibly higher with an above freezing layer around 2000 metres.Sunday: A mix of sun and cloud. Strong to extreme northwest winds easing over the day. Alpine temperatures falling from around 0 to -6 over the day as freezing levels decline from 2000 to 1000 metres.Monday: Mainly sunny. Light northwest winds. Alpine high temperatures around -9.
Avalanche Summary
Wednesday's avalanche observations were limited to small (size 1), thin slabs released with ski cutting. Looking forward into a period of forecast warming, our primary concern is the potential for a relapse into conditions observed a couple of weeks ago when warm temperatures stressed the deeper layers in the snowpack and produced several large (size 2) natural deep persistent slab avalanches.
Snowpack Summary
Strong winds have redistributed snow into thin winds slab in alpine lees and scoured windward areas. Strong northwest winds have had some of the most recent impact on snow redistribution, creating 'reverse loaded' pockets of wind slab.In more sheltered alpine terrain and between 1600-2000 m, 5-15 cm of recent snow overlies a layer of weak surface hoar and crusts (on solar aspects). Large variability in snow depths still exists in the region, ranging from almost no snow to nearly 200 cm in some areas. For average snowpack areas, expect to find weak and sugary faceted snow around 50 to 100 cm deep, which extend to the ground. This weak bottom half of the snowpack was the culprit in several large avalanches in the region in the past few weeks.
Problems
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.
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.