Warm temperatures and tricky conditions this weekend: The upper snowpack is full of thin layers, the mid-pack has a sugar/crust combo, and both may become active. It's a good time to approach the mountains cautiously, with your eyes open, and even to expect surprises.
New snow will likely form unstable slabs . 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.
THURSDAY NIGHT: Snowfall slows down with 5 cm overnight, bringing storm totals into the 20 or 40 cm range, moderate to strong wind from the west, treeline temperatures warming and steady around -4 C with freezing level reaching 1000 m.
FRIDAY: Mostly cloudy, light to moderate wind from the west, treeline temperatures around -2 C and freezing level again approaching 1000 m.
SATURDAY: Mostly sunny, light wind from the northwest, warming temperatures with a possible inversion developing. That means above freezing temperatures between 1500 and 2200 m but cooler temperatures in the valleys.
SUNDAY: Warming pattern continues -- at 1500 to 2300 m above freezing temperatures remained overnight. increasing clouds, light wind from the west or northwest, warming throughout the day except near treeline and low alpine elevations where it stayed warm overnight.
I take a long term view on the importance of recent avalanche activity -- events of the past week are relevant to the upcoming WARM weekend. This week 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, 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.
There were also numerous reports of smaller storm and wind slab avalanches reactive to human triggering. These smaller avalanches could step down to produce large avalanches in very rideable terrain..
The top 70 to 100 cm of snow of the snowpack is generally young and soft with a variety of little layers that may or may not be a problem. These include little surface hoar, thin crusts, and maybe even soft facets from early January. 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 with the upcoming warming.
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.
