The snowpack is complex, tricky, and there are many unanswered questions. Three active weak layers remain sensitive to human triggering. The easy solution is to choose simple terrain free of overhead hazard as the snowpack adjusts.
Weather Forecast
The low that recently delivered significant wind, snow and rain to the interior is expected to weaken on Friday resulting in less frequent and less intense convective flurries. Saturday offers a brief lull in the action before another system takes aim at the interior Sunday.FRIDAY: Overcast, freezing level beginning at valley bottom rising to around 800 m in the afternoon, moderate southwest wind, 1 to 5 cm of snow possible.SATURDAY: Scattered cloud cover, freezing level holding near valley bottom, light southwest wind, 1 to 5 cm of snow possible. SUNDAY: Scattered cloud cover, freezing level holding near valley bottom, light to moderate southerly wind, 1 to 5 cm of snow possible.
Avalanche Summary
On Wednesday persistent slab avalanches were human triggered to size 2.0 on north, northeast, southeast and west facing slopes between 1450 m and 2200 m. These ran on the early January and mid-December Persistent Weak Layers (PWL). Natural avalanches to size 2.5 were reported from steep alpine features that face northwest and southeast. Wind slab avalanches to size 1.5 were human triggered near ridge top. Bear in mind that this activity occurred before the most recent storm.Tuesday was rather quiet. A size 1.0 human triggered avalanche was reported on a NE facing slope at 2100 m. The slab was 20 to 50 cm in depth suggesting it failed on the early January interface. Natural wind slabs to size 2.5 were reported from extreme north facing terrain at 2400 m.On Monday the early January interface continued to produce both natural and human triggered avalanches to size 2.5 on a variety of aspects above 1700 m. Crown depths were 30 to 75 cm and a size 1.5 avalanche was triggered by a skier on a north facing slope with a 22 to 32 degree incline.
Snowpack Summary
The current snowpack is complex, we now have three active Persistent Weak Layers (PWL) that we are monitoring. Wednesday and Thursday's highly convective and windy storm left 10 to 25 cm in its wake. This snow buried the mid-January PWL which is a crust and/or surface hoar interface. Prior to Wednesday's storm the crust was reportedly widespread, high elevation north is likely one of the few crust-free zones. The now buried surface hoar is 4 to 20 mm in size and was present to at least 2100 m, possibly higher.The second PWL is known as the early January interface, it is 40 to 80 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is present at all elevation bands. Recent snowpack tests have shown sudden results with moderate loads.The third PWL is the mid-December facet/surface hoar/crust combination that is now buried 70 to 150 cm deep and is most problematic at and below treeline. This interface is not thought to be present in the alpine.The November rain crust down 100 to 200 cm is thought to have gone 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.