Saturday's weather puts all the ingredients in place for a natural avalanche cycle to generate large, destructive avalanches. Extreme caution is required for any travel in avalanche terrain.
Weather Forecast
Friday night: Wet flurries bringing approximately 10 cm of new snow to higher elevations. Rain below about 1700 metres. Strong to extreme southwest winds. Saturday: Periods of snow and rain bringing 15-30cm of new snow to higher elevations. Rain below about 1800 metres. Strong to extreme southwest winds. Freezing level to 2200 metres with alpine temperatures of 0 to +1. Sunday: Sunny with cloudy periods. Moderate west winds. Freezing level to 1500 metres with alpine temperatures around -6. Monday: A mix of sun and cloud Light southeast winds. Freezing level to 1700 metres with alpine temperatures around -5.
Avalanche Summary
Reports from Thursday show a continuation of the recent avalanche cycle, with storm slabs releasing naturally to Size 2 and explosives controlled deep persistent slabs to Size 3. North to northeast aspects have been the most active. Most crown fractures have measured about one metre but one impressive storm slab featured a three metre fracture depth.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. The heightened avalanche activity shown by the above reports should be expected to persist on Saturday.
Snowpack Summary
5-10 cm of new snow fell over Thursday night. Below it, recent heavy rain to mountain tops has 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. The existence of this crust may not have much of a stabilizing effect on the snowpack, especially as freezing levels rise again on Friday through Saturday. 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, sustained warming, or storm slab avalanches.
Problems
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.
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.
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.