Ongoing stormy weather with a warming trend is creating dangerous avalanche conditions for Sunday. The storm snow sits over a widespread weak layer which is increasing the reactivity of the new storm slabs.
The recent storm snow sits over a weak layer that formed during the recent cold temperatures and is expected to be very reactive to triggering, especially in wind-loaded terrain. As temperatures rise on Sunday, the new snow is expected to quickly settle into a widespread storm slab.
Wet loose avalanches should be expected at lower elevations as freezing levels rise on Sunday and the snowfall transitions into rainfall.
An early report on Saturday from the north of the region included a natural size 1.5 storm slab that had occurred overnight and a size 1.5 storm slab that was remotely triggered from 7 m away and was 30 cm thick. Around Bear Pass, a few natural wind slabs up to size 2 were observed.
This MIN report from Friday shows a small wind slab avalanche with a relatively wide propagation given the thin slab and suggests a poor bond between the new snow and the old surface. Also on Friday in the south of the region, a natural size 1 wind slab was reported on a SW aspect. A skier-triggered size 1 slab was also reported which was only 10 cm thick but had failed on a weak layer of facets. With ongoing snowfall, these types of avalanches are expected to continue and increase in size and reactivity.
The new storm snow has buried a highly variable snow surface that had formed during the recent period of extended cold temperatures and outflow winds. This newly buried weak layer consists of widespread facets, patchy surface hoar in sheltered terrain, and heavily wind-affected surfaces in exposed terrain including wind-scoured and wind-pressed surfaces, old hard wind slabs, and sastrugi.
A layer of large surface hoar crystals, buried in early December, was found down 20 to 50 cm prior to the storm. This layer has previously produced small but remotely-triggered avalanches.
The lower snowpack is well consolidated above a crust that had formed in mid-November which can be found below around 1200 m elevation.
A series of storm systems will continue to impact the region. A warming trend is expected for Sunday with freezing levels climbing to around 1500 m elevation.
Saturday night
Snowfall 20-40 cm, strong to extreme SW wind, treeline low around -8 °C, rising by early morning.
Sunday
Snowfall 10-20 cm, strong SW wind, treeline high around 0 °C with a chance of a temperature inversion.
Sunday night and Monday
Snowfall 20-30 cm, moderate to strong S-SW wind, treeline high around 0 °C with a chance of a temperature inversion.
Tuesday
Mainly cloudy with a chance of sunny breaks, moderate to strong NE wind, treeline high around -4 °C.
More details can be found in the Mountain Weather Forecast.
