A special warning has been issued for the substantial warming and sun this week. Extra caution is required around and below cornices, and on steep sun exposed slopes during the heat of the day.
Weather Forecast
The ridge of high pressure is expected to persist until Friday night when a weak frontal system reaches the coast. Friday should be sunny with light alpine winds and freezing levels around 3000m. Cloudy conditions are expected for Saturday with moderate southwest winds in the alpine and freezing levels dropping to around 2000m. Light scattered flurries are possible on Saturday. A mix of sun and cloud is expected for Sunday with light to moderate southwest winds in the alpine and afternoon freezing levels around 2000m. A more substantial storm system is currently expected to arrive Sunday night.
Avalanche Summary
On Wednesday, a natural size 2 was reported on a south aspect at 2000m. Skiers triggered a size 1.5 avalanche on a south aspect at 2200m. These avalanches were 10-40cm deep with the most recent snow sliding on a crust. Loose wet avalanches up to size 2.5 and cornice releases up to size 3 were also reported. On Tuesday, a skier triggered a small wind slab in steep rocky terrain on a west aspect at 2300m and went for a ride. A skier also triggered a size 2 storm slab on a north aspect at 1800m which was 20cm thick. Explosives triggered several cornices up to size 2.5 and no subsequent slabs were reported. Widespread natural sluffing was also reported up to size 2.5. On Friday, continued warming, sun exposure, and limited overnight recovery means natural cornice releases and loose sluffing are expected to continue, and wet slab avalanches are possible.
Snowpack Summary
A moist snow surface is reported to be widespread to mountaintops except for shaded true north aspects above 2000m. Warm overnight temperatures means there was a limited refreeze of the snow surface. A weak crust was reported to have formed in some areas but is expected to quickly break down in the morning. Wind affected surfaces are reported in exposed alpine terrain in response to recently strong northerly winds. Large cornices are lingering and are expected to become very weak with the sustained warming this week. Below the snow surface, the mid snowpack is generally well-settled throughout with no notable persistent weaknesses.
Problems
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Wet Slabs
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.