Big variability in the Purcells with 25 cm to over a metre of new snow in some areas! Keep watching for signs of slab formation and reactivity in the new snow and take extra caution if the sun pokes out on Tuesday.
Weather Forecast
Tuesday: Mainly cloudy with isolated flurries and a trace of new snow. Winds light from the southwest. Alpine temperatures around -15.Wednesday: Cloudy with sunny periods. Winds light from the southwest. Alpine temperatures of -10.Thursday: Periods of snow delivering 15-20 cm of new snow. Winds moderate to strong from the southwest. Alpine temperatures of -4.
Avalanche Summary
Reports from Saturday and Sunday have been hampered somewhat by stormy weather, but include observations of numerous size 1-1.5 loose dry avalanches running both naturally and under skier traffic. Explosives control in the north of the region on Sunday yielded several size 1.5-2.5 storm slab results. Observations of natural and human triggered storm slab avalanches have also been reported, but thus far have been focused in adjacent forecast regions in the Columbias. These should be taken into consideration in spite of geography as our new snow begins taking on slab properties.As for Tuesday's outlook, fresh storm slabs are expected to be sensitive to human triggering, especially where they overlie weak surfaces and in wind-exposed areas where slab properties will develop more quickly. With this in mind, the potential for storm slab avalanches to 'step down' to deeper weaknesses in shallow snowpack areas should not be overlooked.As noted above, Loose snow avalanches have also begun to evolve into a genuine hazard and should not be underestimated in terrain where they have the potential to entrain significant mass.
Snowpack Summary
Up to 30 cm of new, low density snow fell again on Sunday night, bringing new snow totals from the past few days to around 40-90 cm. There has been a distinct southern focus throughout the storm, with the northernmost part of the region only receiving about 25 cm of new snow. Our new snow lies over older surfaces that include wind slabs on a variety of aspects in wind exposed terrain, sun crust on steep sun-exposed slopes, faceted snow, as well as surface hoar on sheltered open slopes. The new snow's bond to these surfaces will likely be weak and touchy conditions should be expected at this interface as well as at mid-storm interfaces within the new snow. A persistent weakness buried mid January is now down 40-100 cm and consists of buried surface hoar in sheltered areas, sun crust on south aspects, and/or faceted old snow. It has generally stabilized but may be sensitive to triggering in isolated areas where surface hoar is preserved. Another surface hoar/facet weakness buried mid-December found 70-130 cm is generally considered dormant. This and deeper persistent weaknesses remain a concern isolated to shallow snowpack areas where they lie closer to the surface.
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.
Loose Dry
Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
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.