Recently formed storm slabs are expected to remain reactive to human triggering on Wednesday. The region continues to deal with a tricky persistent slab problem and very large avalanches remain possible. Conservative terrain selection remains essential.
A persistent slab problem formed by a crust down 70-150 cm has created a low likelihood, high consequence scenario that is difficult to forecast. Avoid likely trigger spots such as steep, rocky slopes with a shallow or thin-to-thick snowpack. Smaller storm slabs or wind slabs have the potential to step down to a deeper weak layer.
Recently formed storm slabs are expected to remain reactive to human triggering on Wednesday, especially in wind loaded terrain at higher elevations. These slabs may overlie a weak interface which could result in the slabs remaining reactive for longer than normal.
A break in the weather is expected on Wednesday before the next system arrives overnight bringing snowfall and wind for Thursday.
Tuesday night: Isolated flurries 1-3 cm, light variable wind, treeline temperature around -12 °C.
Wednesday: A mix of sun and cloud with a chance of flurries in the morning, light NW wind, treeline high around -12 °C.
Thursday: Snowfall 4-8 cm, moderate to strong SW wind, treeline high around -10 °C.
Friday: Periods of light snowfall, moderate to strong SW wind, treeline high around -8 °C.
On Monday, visibility was limited but reports include isolated natural wind slabs up to size 1.5 and loose dry avalanches. Explosives and skiers were triggering slabs which were typically size 1-1.5 with a single size 2 reported. This MIN report from the south of the region shows an example of the instability in the storm snow and good decision making.
On Tuesday just east of the region in the Rockies near Invermere, a remotely triggered size 3.5 persistent slab avalanche was reported at 2300 m on a W aspect. This was triggered from the ridge and the failure started in the early-Dec layer down 120 cm but subsequently stepped down to the basal weakness. The resulting crown was 2-3 m deep and the avalanche ran about 1 km to the valley bottom. While this was not in the Purcells region, it provides an example of what might be possible with the weak layers deep in the snowpack.
On Wednesday, recently formed storm slabs are expected to remain reactive to human triggering. These new slabs may sit on a weak interface which may result in them persisting longer than normal. The early-Dec persistent slab problem remains a serious concern and the weight of the new snow may have increased the reactivity of these deeper layers. Smaller storm slab avalanches also have the potential to step down to deeper layers resulting in very large avalanches.
Most of the region received 30-40 cm of new snow from the recent storm. This new storm snow buried a variable snow surface which is potentially weak and includes widespread facets, wind affected surfaces, and/or surface hoar up to 3 mm in sheltered areas. The new snowfall was accompanied by moderate to strong winds at higher elevations and the new storm slabs are expected to be most reactive in wind loaded areas in the alpine.
The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 70-150 cm. Activity on this layer has been sporadic. It has generally been triggered on steep, rocky slopes, with a shallow or thin-to-thick snowpack. This layer has created a low likelihood, high consequence scenario which is best managed through conservative terrain choices and disciplined backcountry travel techniques.
