Small weather inputs (sun, snow, or wind) could make buried persistent weaknesses more reactive to human triggering. Conservative terrain choices are essential.
Weather Forecast
Monday: Cloudy with flurries in the morning and sunny breaks in the afternoon. The freezing level is around 1500 m and ridge winds are light from the NW. Tuesday: Mainly sunny. The freezing level is around 1500 m and ridge winds are light from the W-SW. Wednesday: Mainly sunny. The freezing level is around 1600 m and ridge winds are light to moderate from the W-SW.
Avalanche Summary
Several size 1 loose wet and loose dry avalanches were reported on steep slopes on all aspects on Saturday. Specific wind loaded features were also reactive to skier testing, generally producing size 1-2 slabs. There was also a report of a size 1.5 snowmobile triggered slab avalanche from a treeline feature in the Southeast corner of the region on Saturday. Throughout last week there were reports of natural and human triggered slab avalanches up to size 3 each day, with a fairly widespread natural avalanche cycle observed on Thursday and Friday.
Snowpack Summary
Expect the surface to be a mix of dry snow on shady slopes, wind-affected snow in exposed terrain, and a sun crust/moist snow on solar aspects depending on the time of day. 60-90 cm of settled storm snow sits on a melt-freeze crust. In some areas there is a weak layer of surface hoar at the interface between the storm snow and the crust. The bond between the crust or surface hoar remains suspect, but there are also weaknesses reported within the top layers of the recent storm snow. Moderate winds have created areas of wind slab in the lee of west or southwest winds that may be close to a metre deep. Wind slabs in motion may trigger the persistent weak layer on the crust and result in wide propagations and very large avalanches. Surface hoar buried in early January now lies up to 200cm below the surface and has become less of a concern. Possible triggers for this deep and destructive layer include a large cornice fall, strong sunshine, or significant 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.