Temperatures may cool, but the snowpack is still warm and the sun will be out in full force on Sunday. There is no reason to expect a rapid drop in avalanche danger.
Weather Forecast
Sunday: Sunny. Moderate to strong west winds. Freezing level to 1400 metres with alpine temperatures around -7. Monday: A mix of sun and cloud. Light east winds. Freezing level to 1500 metres with alpine temperatures around -6. Tuesday: Cloudy with isolated flurries and a trace of new snow. Light south winds. Freezing level to 2000 metres with alpine temperatures around 0.
Avalanche Summary
Reports from Friday show a continuation of the recent and ongoing avalanche cycle, with several observations showing explosives triggered persistent slabs (and one storm slab) running from Size 2.5-3. Crown fractures generally ranged from 100-200 cm. On Thursday, storm slabs were observed releasing naturally to Size 2, with explosives control yielding deep persistent slabs to Size 3. Most crown fractures measured about one metre but one impressive storm slab featured a three metre fracture depth. North to northeast aspects were the main performers over both days.Reports from Wednesday showed widespread natural avalanches up to Size 3.0. Most of these were reported to be loose wet avalanches gouging and entraining mass as they travelled down the path. Looking forward, recently formed storm slabs in the alpine will likely be reactive to human triggering on Sunday. The heightened avalanche activity shown by the above reports should gradually decrease as cooling temperatures begin to penetrate the snowpack.
Snowpack Summary
Stormy weather on Saturday has been forming storm slabs at high alpine elevations while rain continues to saturate the snowpack at treeline and below. Below the new snow accumulations, recent heavy rain to mountain tops soaked the upper snowpack and caused rapid settlement of our recent storm snow. Below freezing temperatures since the rain event have refrozen the wet snow into a crust, which has been reported to be supportive above about 1700 metres. This crust will deteriorate at all elevations where rain is occurring before gradually reforming as temperatures cool again on Saturday night. Isolated basal facets still exist in shallow snowpack areas and can produce destructive full-depth avalanches. Possible triggers for these deeper weak layers include cornice falls and storm slab avalanches.
Problems
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.
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.