Another warm storm system starting Sunday night is expected to create dangerous avalanche conditions for Monday. The recent storm snow sits over a widespread weak layer which is expected to be increasing the reactivity and size potential of the new storm slabs.
Recent observations show older persistent weak layers in the snowpack may still be reactive and storm slab avalanches in motion have the potential to step down to deeper layers.
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. Ongoing mild temperatures may be causing the storm snow to settle into a more widespread slab.
Wet loose avalanches are possible at lower elevations where the recent precipitation has fallen as rain.
This MIN report from Saturday includes reports of numerous small storm slab avalanches but also shows an avalanche stepping down to an older weak layer.
On Saturday in the south of the region, numerous natural storm slabs up to size 2 were reported. In the north, a natural size 1.5 storm slab was reported and a size 1.5 storm slab was remotely triggered from 7 m away which was 30 cm thick. Around Bear Pass, a few natural wind slabs up to size 2 were observed. An assortment of natural loose dry activity was also reported.
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.
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 below a crust that had formed in mid-November which can be found below around 1200 m elevation.
Both the early-December surface hoar and mid-November crust have been dormant recently but could wake up with the new load from the storm snow, and avalanches have the potential to step down to these deeper layers.
A series of storm systems continues to impact the region with another wave of precipitation expected to arrive late Monday morning.
Sunday night
Precipitation 8-16 mm tapering off overnight, strong S-SW wind, freezing level around 1500 m with a potential inversion.
Monday
Precipitation 10-20 mm with potentially higher amounts for the SW of the region, strong SE-S wind, freezing level around 1500 falling to around 1000 m in the afternoon.
Tuesday
Mainly cloudy with a chance of isolated sunny breaks, moderate NE wind, freezing levels generally falling to around 500 m or lower.
Wednesday
Cloudy with light flurries, moderate SW wind, treeline high around -3 °C.
More details can be found in the Mountain Weather Forecast.
