Rain will bring loose wet avalanche problems to the fore on Monday. Thick storm slabs may continue to build at high alpine elevations.
Confidence
Moderate - Freezing levels are uncertain
Weather Forecast
Sunday night: Heavy rain, 40-60mm accumulation. Heavy snow accumulation possible at high alpine elevations. Strong to extreme south winds. Monday: Cloudy with continuing heavy rain below about 1600 metres. 40-50mm accumulation. 30-40 cm of new snow possible at high alpine elevations. Strong to extreme southwest winds. Freezing level to 2000 metres with alpine high temperatures around 0 to +1.Tuesday: Cloudy with continuing flurries bringing 10-20 cm of new snow. Moderate southwest winds. Freezing level to 900 metres with alpine high temperatures of -3.Wednesday: Mainly cloudy with isolated flurries and a trace of new snow. Light west winds. Freezing level to 800 metres with alpine temperatures of -4.
Avalanche Summary
Reports from Saturday included one MIN submission detailing a near miss incident involving several members of a ski touring party on Hollyburn Mountain. Two skiers were partially buried and one was fully buried in a Size 1.5 storm slab that released from steeper pitch below treeline. The remaining party members performed a rapid companion rescue and no injuries were sustained by anyone involved. Other MIN reports from the region over the past two days have shown ski cutting easily producing storm slab results at the depth of each day's accumulations.Looking forward, areas that see rain falling on dry snow are expected to undergo a natural avalanche cycle over Sunday night into Monday. Areas that experience precipitation falling as snow will see rapid formation of touchy new storm slabs over the same time period.
Snowpack Summary
Heavy rainfall is set to soak the upper snowpack at most elevations over Sunday night and Monday. High alpine elevations in areas like Sky Pilot and the Chehalis Range may see thick new storm slabs develop over the same time period. Below these variable surface conditions, storm snow totals from the past week have reached 110-180 cm and strong southerly ridgetop winds during the storm have been redistibuting each day's snow into storm slabs at all elevations. The touchiest conditions will be likely to exist in exposed, higher elevation terrain, however heavy rain may also destabilize more recent storm slabs at lower elevations and will certainly promote loose wet avalanches in steeper terrain. The crust that exists beneath all our recent snow accumulations is likely to have now formed a solid bond to the overlying snow.
Problems
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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.