Recent snow and strong winds have produced touchy wind slabs. Watch for signs of avalanche activity and locally unstable snow, such as whumpfing and shooting cracks.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
TUESDAY: Mostly sunny with afternoon clouds, light westerly winds, alpine temperature rising to -15 C, freezing level below valley bottom.WEDNESDAY: Mostly sunny, moderate westerly winds, alpine temperature near -13 C, freezing level below valley bottom.THURSDAY: Mostly sunny, light easterly winds, alpine temperature -12 C, freezing level below valley bottom.
Avalanche Summary
Numerous large (size 2 to 2.5) slab avalanches were triggered naturally on Sunday at treeline and alpine elevations. The slabs were 20 to 40 cm deep on north to northeast aspects.Avalanche activity may slow down with a period of cold, clear weather that is expected to persist well into the week. However, the sun is packing more of a punch as the days get longer and it will warm the snow surface on solar aspects under clear skies. Watch out on steep sunny slopes due to the sun’s influence, lee features for reactive wind slabs, and be extra cautious near thin spots and shallow snow pack areas where triggering a deeper weak layer is more likely. There is a lower probability of triggering a deeper layer than in previous weeks, but the consequence would be very high if one was triggered.
Snowpack Summary
Around 50-80 cm of recent snow with strong winds have created wind slabs in direct lee features. This overlies a surface hoar layer and a melt-freeze crust buried mid-February up to 1800 m, and old wind slabs and a sun crust at higher elevations.Deeper in the snowpack, a widespread weak layer from mid-December composed of facets, crusts, and surface hoar is 120-160 cm deep. Also, a rain crust with sugary facets buried in late November is near the base of the snowpack. If triggered, they could produce large, destructive avalanches with high consequences.
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.