Sunday's storm will bring snow and strong winds, loading a highly complex snowpack. Three weak layers will be reactive to natural and human triggers. The easy solution is to choose simple terrain free of overhead hazard as the snowpack adjusts.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
SUNDAY: Cloudy with snowfall, accumulation 5-15 cm, strong to extreme southwesterly winds, alpine temperature -8 C, freezing level near 900 m.MONDAY: Mostly cloudy with light snowfall, accumulation 2-5 cm, light to moderate southwesterly winds, alpine temperature near -9 C, freezing level near 900 m.TUESDAY: Partly cloudy with intermittent snowfall, accumulation 1-3 cm, light to moderate southwesterly winds, alpine temperature near -9 C, freezing level near 800 m.
Avalanche Summary
On Friday, there were several reports of small to large (size 1 to 3) storm slab, wind slab, and persistent slab avalanches, triggered naturally and by skiers. Similar avalanches were reported on Wednesday and Thursday. These avalanches have been releasing on all three persistent weak layers described in the Snowpack Discussion. Expect similar avalanches to release on all aspects and at all elevations with new load.
Snowpack Summary
The current snowpack is complex, we now have three active weak layers that we are monitoring.30-60 cm of storm snow and new forecasted snow for Sunday sits on a newly formed crust and/or surface hoar layer (mid-January). The crust is reportedly widespread, except for possibly at high elevations on north aspects. The mid-January surface hoar is 5 to 20 mm in size and was reported at all elevations. The new snow is falling with strong southwesterly winds, producing wind slabs in lee features. Deeper in the snowpack, the early-January persistent weak layer is 50 to 90 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is found to at least 2100 m, possibly higher. Snowpack tests show sudden fracture characters with moderate loads and high propagation potential, and signs of instability such as whumpfs and cracking. Another weak layer buried mid-December consisting of a facet/surface hoar/crust combination is buried 90 to 150 cm deep. It is most problematic at and below tree line.A rain crust buried in November is 100 to 200 cm deep and is likely dormant for the time being.
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.