Triggering lingering storm slabs and fresh wind slabs is possible on Sunday, mainly from steep, north-facing alpine terrain.
Weather Forecast
SUNDAY: Periods of snow – around 10 cm. The freezing level is around 1400-1500 m. Winds are moderate from the W-SW. MONDAY: A mix of sun and cloud. The freezing level climbs to 1800-2000 m and winds ease to light. TUESDAY: Mainly sunny. The freezing level is around 2000 m and winds are light.
Avalanche Summary
On Friday there was one report of a very large (size 4) persistent slab on a large south-facing slope that was triggered by new snow loading. This avalanche likely failed on the late February interface. There were also numerous reports of natural storm slabs up to size 2.5, a few natural cornice falls, and several skier triggered slabs up to size 1.5. The majority of recent avalanches occurred on northerly aspects, but there were a few on south-facing alpine slopes as well.
Snowpack Summary
New storm slabs have developed above a mix of surfaces, including crusts on solar aspects, moist snow up to treeline, and dry snow or recent wind slabs in the alpine on shaded aspects. Cornices continue to be described as large and fragile. The late February persistent weak layer is an aspect dependant mix of surface hoar, facets and/or a thick crust down around 60 to 110 cm below the snow surface. Commercial operators continue to see hard sudden planar results in snowpack tests, which suggests that wide fracture propagations are possible. New storm loading above the buried persistent weak layer may increase the likelihood of triggering large avalanches. Expect loose wet avalanches and natural cornice falls during periods of strong solar radiation and/or daytime warming.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.