A capping crust may hold deep persistent slab problems at bay on Tuesday. Expect a different set of hazards at alpine elevations and be aware of the potential consequences of releasing a heavy trigger.
Weather Forecast
Tuesday: Cloudy with scattered flurries bringing up to 5 cm of new snow, with light rain climbing to about 1600 metres. Light south winds. Freezing level rising to 2000 metres with alpine temperatures around 0. Wednesday: Cloudy with isolated flurries and a trace of new snow, light rain below about 1600 metres. Strong southwest winds. Freezing level to 2000 metres with alpine temperatures around 0. Thursday: A mix of sun and cloud. Moderate to strong southwest winds. Freezing level to 1700 metres with alpine temperatures around -2.
Avalanche Summary
No new avalanches were reported on Sunday. but reports from Saturday included observations of one naturally triggered Size 3 deep persistent slab as well as numerous natural storm slab releases occurring on northeast to northwest aspects just west of Fernie. Reports from Friday showed 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.Looking forward, recently formed storm slabs at alpine elevations may remain reactive to natural and human triggering on Tuesday, especially as temperatures rise. While recent strong crust recovery has been tempering the likelihood of persistent slab activity, the rise in freezing levels and forecast rain may prompt renewed persistent slab activity.
Snowpack Summary
Stormy weather on Saturday formed fresh storm slabs at high alpine elevations while rain once again saturated the snowpack at lower alpine elevations and below. Lower temperatures have since formed a roughly 5-10 cm thick crust at the rain soaked surface, which tends to break down with daytime warming at lower elevations and on solar aspects. Isolated basal facets still exist in shallow snowpack areas and still carry the potential to produce destructive full-depth avalanches. Possible triggers for these deeper weak layers include cornice falls and storm slab avalanches, which may trigger naturally with daytime warming.
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.