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.
A series of layers in the upper snowpack continue to be reactive especially where influenced by wind. Expect to find a more reactive slab on wind-affected slopes at higher elevations, and anywhere the new snow bonds as a slab over crust or surface hoar layers.
Weak layers in the snowpack may quickly gain reactivity as the snow becomes moist from rising temperatures and strong solar input. Cornices may quickly loose strength under the direct sun.
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.
FRIDAY NIGHT: Partly cloudy with starry breaks, moderate wind from the northwest, treeline low around -9 C and freezing level dropping below 1000 m.
SATURDAY: Sunny and warm. Temperatures warming quickly in the early morning with freezing level rising to 2500 m Saturday afternoon. Possible inversion producing above freezing temperatures between 1500 and 2200 m and cooler temperatures in the valleys. 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.
MONDAY: Mix of sun and cloud. Decreasing light northwest wind, temperatures dropping to -8 C, and freezing level dropping to valley bottom.
The snowpack is quite complex right now recently producing impressive 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 size 1-2 storm and wind slab avalanches reactive to human triggering and failing naturally. (See this MIN from the Fingers) These smaller avalanches could step down to produce large avalanches in very rideable terrain.
20-40 cm recent storm snow is settling fast, wind affected in areas and has a thin surface crust on steep solar slopes. The new snow covered a layer of surface hoar or crust and is not expected to bond well in the short term. The top 70-100 cm of snow of the snowpack is generally soft with a variety of little layers that may or may not be a problem. Along with recent wind effect, upper snowpack layers 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 temperatures.
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.
