New snow will create dangerous avalanche conditions, especially since small avalanches could trigger much larger deep persistent slab avalanches. Conservative route selection is strongly recommended.
New snow will likely form unstable slabs in certain types of terrain. The most suspect areas would be parts of the region that receive more than 20 cm of new snow, wind-affected slopes at higher elevations, and anywhere the new snow is burying crust or surface hoar layers.
The early December crust can be found down 100-200 cm below the surface, with weak sugary crystals (facets) on top of it. Large avalanches have been observed at all elevations including some unusual areas such as the middle of slide paths, open trees, and burns. Although the likelihood of triggering an avalanche is decreasing, there is ongoing concern for smaller avalanches stepping down to this layer.
WEDNESDAY NIGHT: Cloudy with flurries bringing 5-10 cm of snow, moderate wind from the southwest, treeline temperatures around -10 C.
THURSDAY: Snowfall continues with another with 10-15 cm, strong wind from the southwest, treeline temperatures warming to -6 C with freezing level reaching 1000 m.
FRIDAY: Mostly cloudy, light wind from the west, treeline temperatures around -4 C.
SATURDAY: Mostly sunny, light wind from the northwest, warming temperatures with a possible inversion developing above freezing temperatures between 1600 and 2200 m.
We are coming out of a period of notable avalanche activity. On Monday and Tuesday we saw numerous very large (size 2.5-3.5) deep persistent slab avalanches occurring in both the Selkirks and Monashees. These avalanches failed on the early December facet/crust layer which was typically 100-150 cm deep. One notable pattern is that many of these avalanches released at relatively low elevations (1700 to 2000 m) in open clearings, burns, and slide paths. In many cases, they were triggered by smaller avalanches starting at higher elevations.
In addition to the cycle of very large avalanches, there were also numerous reports of smaller storm and wind slab avalanches reactive to human triggering.
On Thursday, we will likely see reactive slabs form with the new snow and the looming possibility that smaller avalanches could step down to produce very large avalanches in some atypical types of terrain.
Expect 15 to 30 cm of new snow on Thursday. This incoming snow is burying recently formed sun crust and suface hoar layers, which means we could see thin and reactive slabs. Some complex layers have formed in the top 60 cm of the snowpack over the past week, including several crusts and surface hoar layers in isolated areas. While none of these upper layers are necessarily going to remain a long-lasting problem, there is a fair bit of uncertainty about their spatial distribution and whether they will be reactive in the upcoming week.
The primary weak layer that remains a widespread concern across the Columbias is a 100-200 cm deep crust/facet layer that formed in early December. There has been regular avalanche activity on this layer at all elevations for over a month, and we expect to see avalanche activity continue on this layer.
