Persistent weak layers are maintaining elevated avalanche danger as we head into prime skiing and riding weather. The likelihood of triggering avalanches may decrease but the consequences are very real. This is a time for cautious terrain selection and diligent travel practices.
Shifting winds have redistributed recent snow into wind slabs on a range of aspects at higher elevations. These should be on a positive track toward stabilizing, but remain cautious where wind transport appears recent and where upside-down snow quality is evident.
A a variable but often reactive pair of weak layers exist below our recent storm snow (30-50 cm) and a bit deeper (40-80cm). Depending on your elevation, aspect, and location in the region, these layers may present as either surface hoar (especially reactive) or a mix of facets and crust (not much better!) Wind slab releases may step down to one of these layers to create even larger avalanches. Keep your guard all the way up in the upper BTL.
WEDNESDAY NIGHT: Mainly clear. Moderate to strong southwest winds.
THURSDAY: Mainly sunny. Moderate to strong southwest winds, easing over the day. Alpine high temperatures around -5 to 0, possibly cooler at lower elevations under a mild temperature inversion.
FRIDAY: Mainly sunny. Light south or southwest winds. Alpine high temperatures around -5, possibly cooler at lower elevations under a mild temperature inversion.
SATURDAY: Becoming cloudy with isolated flurries and a trace of new snow, increasing a bit overnight. Light east winds. Aline high temperatures around -10.
No new avalanche reports have been included in limited MIN reports since the weekend, suggesting low observer numbers and/or a trend of our persistent slab avalanche problem toward a low probability/high consequence scenario. Explosives control on Tuesday in the thin snowpack area near Robson yielded many small dry loose and storm slab releases. One result out of about 17 reached size 2.5 (large).
As for the past weekend, a report from the Tumbler Ridge rail corridor on Saturday detailed evidence of a 24-48 hour-old widespread natural avalanche cycle producing avalanches up to size 3 (very large) This cycle appeared to be linked directly to recent southwest winds. Subsequent explosives control yielded smaller, thinner wind slab releases.
Pine Pass reports on Friday included natural, accidental, and remotely triggered avalanches up to size 2.5. That's a big slide to get tangled up in -- one party of three were caught and partially buried (one up to the neck) or fully buried (with a deployed airbag).
Many of the above avalanches ran on persistent weak layers currently found about 30-80 cm deep throughout the region.
Exposed higher elevations in the region have been significantly wind affected, with wind directions that have recently shifted from southwest to northwest and again back to southerly. As a result, new and recent wind slabs are suspected to exist across most aspects in the alpine and upper treeline. Wind slabs formed over the layers discussed below may be particularly reactive.
In more sheltered areas, 30-50 cm of snow from storms over last week has been settling well, with mid-storm interfaces largely healed up, however variable weak layers exist below this storm snow as well as about 20-30 cm deeper on an early December weak layer. Depending on aspect, elevation, and location in the region, these weak layers may appear as especially touchy surface hoar (think shaded, sheltered spots) or as a combination of crust and weak, faceted snow (think of areas that may have been exposed to sun or rain).
Reactivity at these interfaces has been variable, with the MacGregors, Pine Pass, Hasler and the Tumbler Ridge area highlighted in recent MIN reports. Where they are locally less evident (Renshaw) it is important to resist the temptation to consider them entirely absent.
A basal crust from November exists near the bottom of the snowpack in most areas. This crust may be a concern in shallow snowpack areas as well as in wind affected, thin-to-thick snowpack transition zones in the alpine.
Snow depths are in the 180-250 cm range around Pine Pass, the MacGregors, and McBride, closer to 120-150 cm in the Tumbler Ridge area. A tough rain crust may be found capping the snowpack in many areas below about 1400 metres.
