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 where there is more than 20 cm of new snow, on 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 100-150 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 15 to 30 cm range, winds moderate to strong wind from the southwest and veering west, treeline temperatures steady around -3 C with freezing level falling to around 500 m. Around 1000 to 1500m is may remain at or above freezing overnight (temperature inversion).
FRIDAY: Mix of sun & clouds, light to moderate wind from the northwest, treeline temperatures around -2 C and freezing level again approaching 1000 m.
SATURDAY: Warming temperatures overnight with freezing level rising from 1500 m Friday at sunset to 2500 m Saturday afternoon. Above freezing temperatures to all but the highest peaks is possible. With an inversion developing valley floors will be colder under the valley fog. Mostly sunny, light wind from the northwest
SUNDAY: Warming pattern continues with zero or warmer temperatures between 1500 to 2500 m. Increasing clouds, light wind from the west or northwest. At lower elevations, under the valley fog, expect below freezing temperatures.
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 deeply buried crust/facet layer that formed in early December. It is typically down 100-150 cm, but as deep as 200 cm in wind loaded terrain. 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.
