The snowpack is complex and quite variable right now. Buried weak layers remain reactive and conservative terrain selection remains critical. Extra caution is required during the heat of the day, especially on sun exposed slopes and around cornices.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
A ridge of high pressure keeps the region dry Thursday and Friday before a weak storm system arrives Saturday. Thursday is expected to be mainly sunny with light alpine winds from the southwest to west. Freezing levels are expected to drop below valley bottom overnight and reach 1700-2000m on Thursday afternoon. Friday is expected to be mainly sunny in the morning with increasing cloudiness during the afternoon. Alpine winds are expected to be light from the southwest but may increase during the afternoon and freezing levels are forecast to reach 2000-2500m. There is still a lot of uncertainty as to how warm it will get on Friday. A weak storm front is currently forecast to reach the region Friday overnight bringing 5-15cm of snowfall on Saturday.
Avalanche Summary
On Tuesday, numerous natural and skier triggered avalanches were reported. Natural activity includes one size 1.5 storm slab from a northeast aspect around 1900m and lots of size 1-2 loose dry avalanches from steep and rocky sun-exposed terrain. A large cornice released naturally as well as a piece of glacier ice. A skier triggered a size 1.5 loose dry avalanche which ran 200m. Ski cutting triggered several size 1 wind slabs on east-southeast aspects at 2200m which were typically 10cm thick. One size 1.5 wind slab was ski cut on a northeast aspect at 2150m which was 20-30cm thick. Explosives triggered several large cornices as well as some slabs 20-35cm thick. On Monday, numerous size 1-2 natural and human-triggered avalanches were reported in the region. This included storm slabs, wind slabs, cornices, and persistent slabs releasing on the early February surface hoar layer.
Snowpack Summary
New surface hoar growth is being reported on all aspects and elevations up to 10mm and a sun crust has formed on steep south facing slopes. A weak layer of surface hoar and/or a sun crust can be found 15-60cm below the surface. This layer is reported to be increasing in reactivity in the deeper snowfall areas as the recent low density storm snow continues to settle into a cohesive slab. The surface hoar and/or crust layer from early February is now down 60-100cm and has been responsible for some very large avalanches recently. This layer is expected to become most reactive during the heat of the day, especially on sun exposed slopes. Large cornices remain a concern and will become weak with daytime warming.
Problems
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.
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.