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.
The upper snowpack is tricky right now. Recent storm snow has been most reactive where it overlies surface hoar. Along with a handful of suspect layers in the upper snowpack, expect to find a more reactive slab on wind-affected slopes at higher elevations, and anywhere a slab forms 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.
SATURDAY NIGHT: Warm air aloft will maintain elevated freezing levels and prevent overnight crust recovery. Moderate to light west-northwest wind. Overnight low temperatures -5 C as freezing level drops below 800 m.
SUNDAY: The warm pattern continues with conditions similar to Saturday. Little overnight cooling is expected leading into another day of above 0 C temperatures as high as 2500 m. A possible inversion will produce fog and cooler temperatures in the valleys but above freezing temperatures between 1800 and 2500 m. Mostly sunny, light wind from the northwest.
MONDAY: Mix of sun and cloud. Decreasing light northwest wind, high temperatures dropping to -5 C, and freezing level dropping to valley bottom.
TUESDAY: Sunny. Light southwest wind. High temperature -6 C. Freezing level at valley bottom.
Another round of very large (size 2.5-4) deep persistent avalanches failed naturally between Thursday and Friday, with rapid loading and warming temperatures being the likely trigger. In the same timeframe, large (size 1.5-2.5) storm and wind slab avalanches failed naturally, these averaged 30-60 cm deep, and in some cases a weak layer of surface hoar was reported at the interface. An impressive report documents a size 4 avalanche in Rogers Pass here.
The snowpack is quite complex right now recently producing impressive avalanche activity - events of the past week are relevant through this WARM weekend. This past week numerous very large (size 2.5-3.5) deep persistent slab avalanches occurred 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. Throughout each recent snowfall, the upper snowpack has been reactive as well. There have been 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.
20-35 cm recent storm snow is settling fast, wind affected in higher elevation areas and moist on steep solar slopes and lower elevations (below 1500 m) in the afternoon. Crusts will quickly breakdown with warm temperatures on Sunday. The new snow covered a layer of surface hoar and was the interface responsible in many natural storm slab avalanches reported Jan 21.
The top 50-100 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 remain reactive with the warm temperatures.
The primary weak layer that remains a widespread concern across the Columbia Mountains 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.
