A warm storm is approaching for the weekend that is likely to drop rain onto an already weak snowpack.
Summary
Confidence
Fair - Intensity of incoming weather systems is uncertain on Saturday
Weather Forecast
A succession of storms progressively get stronger and warmer through the weekend. Friday: Light rain or snow becoming heavier in the afternoon. Daytime amounts around 3-5 mm, with a further 6-10 mm likely overnight. Freezing levels around 2200 m. Ridgetop winds 60-80 km/h from the southwest. Saturday: Cloudy in the morning, rain in the afternoon. A futher 5 mm looks like the best current estimate. Freezing levels around 2000 m, rising to 2500 m in the afternoon. Strong southwesterly winds. Sunday: More rain for all but the highest areas. Sunday's system looks punchier with 15-25 mm expected. Freezing levels around 2500 m. Strong southwesterly winds.
Avalanche Summary
Numerous natural and human-triggered avalanches were reported to have run on a variety of aspects and elevations during and after the most recent storm, up to and including Tuesday 20th. By Wednesday 21st most avalanche activity had subsided, although small pockets of wind slab could still be triggered in wind loaded areas. Most of the avalanches ran on a layer of buried surface hoar from mid-January, with crown depths between 40-60 cm, although a couple of deeper releases occurred on the mid-December surface hoar/crust layer.
Snowpack Summary
Between 30 and 50 cm of new snow fell during the most recent storm over the weekend. It was initially distributed by strong winds into wind slab deposits, and in many places has consolidated into a denser slab with warm alpine temperatures. A weak layer that comprises feathery surface hoar crystals up to 15 mm lies directly below the recent storm snow and may sit above a crust, particularly on south-facing slopes. This "mid-January" weak layer is widespread and has been the main focus for most of the recent avalanche activity. It has also been consistently producing easy "pop" type failures in snowpack tests.The still problematic mid-December surface hoar/crust layer is now buried anywhere from 60 to 110cm below the surface and continues to produce clean shears at and below treeline. Avalanches were reported to have stepped down to this layer during and after the most recent storm cycle. Although it has become less likely to trigger, I'd continue to treat this layer with respect and suspicion, particularly in the 1700-1900 m elevation band. A release on this layer would have very high consequences.
jfloyer,
Avalanche Canada
