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. Deep persistent avalanches may also become possible.
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. Partly cloudy conditions are expected for Saturday with moderate southwest winds in the alpine and freezing levels dropping to around 2000m. Partly cloudy conditions are expected for Sunday with light to moderate southwest winds in the alpine and afternoon freezing levels around 2000m. A weak storm system is currently expected to arrive Sunday night.
Avalanche Summary
On Wednesday, a skier triggered a size 1.5 avalanche on a south aspect at 2200m which was 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 2.5 were also reported. On Tuesday, widespread natural loose wet sluffing was reported on solar aspects up to size 2.5. A natural size 2 wind slab avalanche was also reported in large open icefield area caused by down flowing wind. Natural cornice failures up to size 2.5 were also observed. 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. As the warm temperatures continue, deep persistent weak layers have the potential to become reactive and produce very large slab avalanches.
Snowpack Summary
A moist snow surface is reported to be widespread to mountaintops except for shaded true north aspects at the highest elevations. 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, there are two layers of concern in the northern half of the region. Professionals are tracking a melt freeze crust down around 40cm and a thick layer of facets down 120cm. As the snowpack continues to heat up this week, these layers have the potential to wake-up and produce very large avalanches.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.