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 scattered flurries and a trace to 5 cm of new snow. Light southwest winds. Freezing level to 1600 metres with alpine temperatures of -5.Monday: A mix of sun and cloud. Light southwest winds. Freezing level to 1700 metres with alpine temperatures of -4.Tuesday: Mainly cloudy with isolated flurries and a trace of new snow. Light southwest winds. Freezing level to 1700 metres with alpine temperatures of - 4.
Avalanche Summary
Reports from Friday included observations of Size 1 wind slabs releasing with ski cutting in the Golden area. One report from the central Purcells detailed evidence of a natural avalanche cycle to Size 2 in steep terrain as a result of new snow and warm temperatures. Loose wet avalanches were observed running to Size 1 in steeper terrain below 1900 metres.Reports from Thursday showed loose wet avalanches releasing both naturally and with skier traffic in steep terrain from Size 1-2. Isolated natural Size 1-2 wind slab releases were observed occurring in steep, rocky terrain as a direct result of wind loading.On Tuesday there was a natural cornice fall near Golden that failed to release a slab on the slope below.
Snowpack Summary
Recent moderate to strong winds have been redistributing new snow into wind slabs on a range of aspects at higher elevations. Fragile new cornice growth also occurred along ridgelines over the course of the week. The new snow has buried melt-freeze crusts on all aspects below about 2200 metres and in the high alpine on solar aspects. Isolated surface hoar may be found below the new snow on shaded aspects at high elevations. Below the new snow interface, a number of storm snow and crust layers that formed over mid to late March appear to be well bonded. At higher elevations, the February crust/facet layer is now down around 130-150 cm and the deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack. These layers were active during a storm in mid-March and produced some very large avalanches. Occasional deep releases were also reported in late March and these deeply buried weaknesses remain a serious concern as warming temperatures continue to penetrate the snowpack at increasingly higher elevations.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.