Sundays storm could drop 30 cm of snow with strong winds, onto a highly complex snowpack. Three weak layers are 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 15-30 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 800 m.TUESDAY: Partly cloudy with intermittent snowfall, accumulation 1-3 cm, light to moderate southwesterly winds, alpine temperature near -10 C, freezing level near 600 m.
Avalanche Summary
On Friday, there were several reports of small to large (size 2 to 3) persistent slab avalanches, triggered naturally and by skiers and snowmobilers. They were about 90 cm deep on northerly aspects at all elevations. Expect similar avalanches to release on all aspects with new load.On Thursday, avalanche activity was reported as storm and persistent slabs. Storm slabs were running to size 2 on southerly and northerly aspects in the alpine and tree line. There were two persistent slab avalanches reported that were both skier triggered 80cm deep between 1100 and 1400m on north aspects. One of them was remotely triggered (from a distance) from 60m away and was a size 2.5. The other was triggered by a ski cut and was a size 1, yet still 80cm deep. On Wednesday, a small natural avalanche cycle was reported from steep ridge features. Fast and loose sluffing was also reported.
Snowpack Summary
The current snowpack is complex, we now have three active weak layers that we are monitoring. 15-40 cm of storm snow and another 10-30 cm of forecasted snow for Sunday sits on a newly formed crust and/or surface hoar layer (mid-January). Prior to the storm the crust was reportedly widespread; high elevation north is likely one of the few crust-free zones. The mid-January surface hoar is up to 10 mm and reported at all elevation bands. 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 30 to 60 cm below the surface. It is composed of surface hoar on sheltered slopes and sun crust on steep solar aspects and found at all elevation bands. Snowpack tests show sudden fracture characters with moderate loads and high propagation potential, as well as whumpfs and cracking. Another weak layer buried mid-December consisting of a facet/surface hoar/crust combination is buried 40 to 100 cm deep. It is most problematic at and below tree line.A rain crust buried in November is 100 to 150 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.