A strong system with periods of intense snowfall and high winds will increase avalanche danger on Tuesday. Stick to low-angle, simple terrain and avoid areas with overhead hazard. Large avalanches may run long distances.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
TUESDAY: Snow. Accumulation 10-20 cm. Ridge wind moderate to strong, southwest. Temperature -10. Freezing level valley bottom.WEDNESDAY: Mostly cloudy, light flurries. Accumulation trace. Ridge wind light, west. Temperature -10. Freezing level valley bottom.THURSDAY: Mix of sun and cloud. Ridge wind light, south. Temperature -12. Freezing level valley bottom.
Avalanche Summary
Monday skiers remotely triggered (from a distance) a size 3.5 from 40 m away on a ridgeline adjacent to a south-facing slope at 2600 m.On Sunday explosive control work north of Revelstoke produced storm slab results from size 1.5-2.5 and two size 3 deep slab failures (120-170 cm deep) on a east aspect at 2400 m.A very large, widespread avalanche cycle was observed on Thursday, with a few events continuing on Friday and Saturday. Slopes of all aspects and elevations ran full path and reached valley floor. In some cases, mature timber was smashed. These failed on all of the persistent weak layers discussed in the snowpack section. Avalanches involving only the recent storm snow were also very large. Explosives continued to produce very large persistent slabs on Saturday.
Snowpack Summary
Strong north to north-westerly winds have shifted 25-70 cm recent storm snow into wind slabs on lee slopes. Alpine and open treeline areas have seen extensive wind-affect, while a sun crust exists on solar aspects and a rain crust can be found below 1600 m.Below the snow surface, several persistent weak layers make up a troublesome snowpack. In the top 1.5-2.5 m of the snowpack, two surface hoar layers buried in January can be found. Expect to find one or other of these on all aspects and elevations.Deeper in the snowpack (around 200 cm deep) is a facet/crust/surface hoar layer from December, most prevalent at and below treeline.Near the base of the snowpack is a crust/facet combo, most likely to be triggered from thin spots in the alpine.All of these layers have produced large avalanches recently. The wide distribution and ongoing reactivity of these layers suggests that avoidance through choosing simple terrain is the best strategy.
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.
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.