New storm problems combined with the persistent slab problem means tricky conditions for the weekend. Use a conservative approach if you are heading into the mountains.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain on Sunday
Weather Forecast
A weak ridge of high pressure will keep the interior dry for Friday night and Saturday morning. Light flurries are expected Saturday afternoon with moderate SW alpine winds. Freezing levels should be at around 1000m. On Sunday, another frontal system will cross the interior bringing 10-20mm of precipitation and moderate-strong SW alpine winds. Freezing levels may reach around 1500m. Lingering flurries are expected for Monday a ridge of high pressure begins to build over the interior.
Avalanche Summary
On Thursday, a natural cornice fall triggered a size 1 persistent slab which released on the mid-Dec layer. On Wednesday, explosives triggered a cornice but the cornice did not trigger a slab. On Tuesday, a natural size 2.5 persistent slab was reported. This released on the mid-Dec weak layer. Some natural avalanche activity is possible on Saturday in specific areas including fast sluffing from steep terrain and wind slabs in leeward alpine features. Small avalanches have the potential to step down and trigger a persistent slab which would greatly increase the consequences. Skier triggering is likely in wind loaded areas and steep terrain, and remains possible for persistent slabs where the tricky mid-December layer is still intact.
Snowpack Summary
The new snow has buried widespread 10-20mm surface hoar, a sun crust on steep sun exposed slopes, and/or wind affected surfaces in the alpine. The problematic mid-December surface hoar/crust layer is typically down 80-120cm below the new snow and remains sensitive to human triggering is some areas. The reactivity of this persistent weak layer appears to be quite variable but still has the potential for large, destructive avalanches in some areas. The layer appears to be the most reactive at and below treeline.
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.
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.
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.