Warming and sunshine will drive avalanche danger this week. Plan to start your day early and to avoid large sun-drenched slopes during the heat of the day.
Weather Forecast
MONDAY: Cloudy with a chance of flurries and sunny breaks in the afternoon. The freezing level climbs to 1600-1800 m and winds are moderate from the N-NE. TUESDAY: Mainly sunny. The freezing level is around 2000 m and winds remain light to moderate from the N-NE. WEDNESDAY: Mainly sunny. The freezing level spikes to 2300-2600 m and winds are light from the NE.
Avalanche Summary
On Saturday there was a report of a natural cornice fall that triggered a size 3 persistent slab on a steep, rocky, north-facing slope. There was also some minor loose wet sluffing to size 1.5 on steep sun-exposed slopes in the afternoon. On Friday, just outside the northern extent of the region, there was a 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.
Snowpack Summary
Light new snow covers a variety of old surfaces including crusts on solar aspects, moist snow up to treeline, and dry snow or fresh wind slabs in the alpine on shaded aspects. Approximately 40-50 cm of settled storm snow sits on a melt-freeze crust buried on Mar. 22. The late February persistent weak layer is an aspect dependant mix of surface hoar, facets and/or a thick crust down around 60-110 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. Cornices continue to be described as large and fragile. 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.
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.
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.