Intense sun exposure will maintain the potential for large destructive avalanches. Conservative terrain choices with low overhead exposure are essential.
Confidence
Moderate - Freezing levels are uncertain on Wednesday
Weather Forecast
TUESDAY and WEDNESDAY: Mainly sunny and dry with isolated valley cloud dissipating throughout the day. Freezing levels reaching 1500m and light variable winds. THURSDAY: Mainly sunny and dry with isolated valley cloud dissipating throughout the day. A temperature inversion is expected to result in above freezing temperatures well into alpine elevations Winds should remain generally light from the south.
Avalanche Summary
Reports from Sunday include more evidence of natural wind slab avalanches and cornice failures from Saturday, as well as ongoing natural cornice and wind slab activity up to Size 3.5. Human triggering was also reported with skier triggered storm and wind slab avalanches up to Size 1.5 and explosives triggering up to Size 3, a few of which failed on surface hoar down 60-70cm.
Snowpack Summary
10-20cm of new snow on Sunday night buried surface hoar and/or sun crust in most places. Weaknesses exist within 60-90 cm of settled recent storm snow, which is sitting on a crust buried on February 10th. 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. 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.