A reactive weak layer has reached its tipping point and is producing large human-triggered avalanches. The problem is especially prevalent at mid-elevations. Put your guard up around steep openings in the trees and keep it raised if you reach the alpine, where an even deeper snowpack weakness remains in play.
Recent snowfall has been activating a persistent weak layer 50-60 cm deep. In many cases, this interface has been even more reactive than the new snow interface above it. Additional flurries and ridgetop wind will keep this problem near its tipping point in the coming days.
Weak, faceted grains make up the basal snowpack. Where supportive to riders, a melt-freeze crust may be providing a bridging effect, making it more difficult to trigger deeper layers. Be especially suspicious of shallow, rocky, or cross-loaded areas with variable snow depths.
A widespread and reactive storm slab was reported around the region Friday and into Saturday. Skiers, vehicles, and explosives triggered slabs to size 2.5 with most crowns 30-50 cm deep, and many reported at treeline elevations 1700-2100 m (note poor visibility and no alpine observations). These avalanches were reported failing on the most recent interface or the early January crust/surface hoar layer about 10 cm lower. Wet loose avalanches were observed in below treeline terrain.
On Saturday and again on Sunday, explosives triggered a large (size 2.5) deep persistent slab avalanche (Saturday's report was from the western side of the Forecast region, which has received a bit more storm snow over the weekend, while Sunday's was in the Bonningtons). Both avalanches indicate our deep persistent layer remains reactive to large triggers at least.
Several more observations of natural and human-triggered persistent slab avalanches (size 1-2.5) were observed on Sunday and Monday in the Norns and the Bonningtons. Failure planes in all of these avalanches were identified as the early January layer discussed in our snowpack summary. 1700-2100 m was again the critical elevation band and east-to-northeast aspects figured prominently.
Please continue to share any observations or photos on the Mountain Information Network.
Upper elevation storm snow totals since January 12 vary from 20-35 cm. This overlies a rain crust at lower elevations where recent precipitation fell as rain with freezing levels exceeding 1900 m. It overlies a mix of wind-affected and sheltered low-density storm snow above about 1700 m.
At higher elevations, 50-60 cm of snow now sits on a layer of surface hoar from the start of January in wind-sheltered terrain, primarily around treeline. It has been identified as the failure plane in many recent avalanches in the region.
Another facet/crust layer is down 40-90 cm (and 2-10 cm thick at TL elevation). Below this, the mid-pack is settled and consolidated.
A concerning and weak layer of facets and crust 20-50 cm thick make up the basal snowpack. Treeline snow depths average 150-210 cm.
Tuesday night:
Mainly cloudy with isolated flurries. Alpine temperatures drop to a low of -7 °C. Ridge wind light to 20 km/h from the southwest. Freezing level at valley bottom.
Wednesday:
Mainly cloudy with sunny breaks and isolated flurries. Alpine temperatures reach a high of -4 °C. Mostly light ridge wind occasionally gusting to 35 km/h from the southwest. Freezing level rises to 1000 metres.
ThursdayA mix of sun and cloud with isolated flurries. Alpine temperatures reach a high of -3 °C. Ridge wind light to 25 km/h from the northwest. Freezing level rises to 1200 metres.
FridaySunny. Alpine temperatures reach a high of -6 °C. Ridge wind light to 15 km/h from the northwest. Freezing level rises to 500 metres.
More details can be found in the Mountain Weather Forecast.
