Avalanche danger will increase again on Monday with another dose of new snow. Loose dry avalanches have emerged as a new problem to manage.
Weather Forecast
Sunday night: Flurries and periods of snow bringing 10-15 cm of new snow, with only a trace in the north of the region. Winds light to moderate from the southwest.Monday: Flurries delivering another 10-15 cm of new snow. Winds light from the south. Alpine temperatures around -15, closer to -20 in the north.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 to moderate from the southwest. Alpine temperatures of -11.
Avalanche Summary
Reports from Saturday include observations of numerous size 1-1.5 loose dry avalanches running both naturally and under skier traffic. Observations of storm slab avalanches have also reported, but thus far have been limited to adjacent forecast regions in the Columbias. These should be taken into consideration in spite of geography as our new snow begins to undergo settlement.As for Monday'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 are also beginning to evolve into something more than a 'good skiing problem' and should not be underestimated in terrain where they have the potential to entrain significant mass.
Snowpack Summary
Around 20-40 cm of new snow over Friday and Saturday has now covered a range of different conditions that existed at our previous surface. In addition to wind slabs that existed on a variety of aspects in wind exposed terrain, the new snow also overlies a sun crust recently reported on steep sun-exposed slopes as well as surface hoar recently found growing on sheltered open slopes. Surface faceting was also recently reported as a result of last week's cold temperatures. The new snow's bond to these surfaces will likely be weak and touchy conditions can be expected as it continues to accumulate into storm slabs above them. A persistent weakness buried mid January is now down 40-80 cm and consists of buried surface hoar in sheltered areas, sun crust on south aspects, and/or widespread 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 persistent weakness buried mid-December may be found down 70-120 cm, but is generally considered dormant. This and deeper persistent weaknesses remain an isolated concern in shallow snowpack areas where they lie closer to the snow 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.