The upper snowpack will gain strength overnight, but watch for breaks in cloud cover to undermine slab and cornice stability on Sunday. Spring sunshine packs a punch.
Weather Forecast
Sunday: Mainly cloudy with isolated flurries and a trace of new snow. LIght west winds. Freezing level to 1700 metres with alpine temperatures of -4.Monday: A mix of sun and cloud. Light southwest winds. Freezing level to 1700 metres with alpine temperatures of -5.Tuesday: A mix of sun and cloud with isolated flurries and a trace of new snow. Light southwest winds. Freezing level to 1600 metres with alpine temperatures of -5.
Avalanche Summary
Reports from Friday showed storm slabs releasing naturally from Size 2-2.5 on north aspects in the Rogers Pass area. Smaller wind slabs (Size 1-1.5) released naturally and with ski cutting on west aspects northeast of Revelstoke. Numerous loose wet releases up to Size 2 occurred over the previous 24 hours.Reports from Thursday included observations of two storm slab releases, Sizes 1 and 2.5. The smaller of these was skier triggered and the larger ran naturally. They both occurred on north aspects at 2200 metres and above. Loose wet avalanches were observed running naturally and with ski cutting from Size 1-2.5 in various areas in the region.Reports from Wednesday included details of one natural Size 3.5 deep persistent slab release that occurred on a northeast aspect in the alpine, just northeast of Revelstoke. It was triggered by a smaller storm slab that released above it due to solar warming. Natural cornice falls up to size 3.5 were reported on Tuesday.
Snowpack Summary
Roughly 30-40 cm of new snow has formed fresh storm slabs on the surface after a series of storms over the end of the week. These new storm slabs have developed above a mix of old surfaces that include melt-freeze crusts at treeline and below and on solar aspects in the alpine. In addition to storm slab formation, the new snow has also contributed to fragile new cornice growth. Below the new snow, a number of storm snow and crust layers that formed over mid to late March appear to be well bonded. The February weak layers are now down about 170-220 cm and the deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack. These deep weak layers produced large avalanches with cornice falls and other heavy triggers in late March and early April. They remain a concern and may be more likely to fail on southerly aspects during periods of strong solar radiation.
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.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.