Wind slabs are expected to remain reactive in exposed high elevation terrain on Tuesday. It may still be possible to trigger a buried weak layer in some areas at and below treeline but the likelihood is decreasing with the cooler weather.
Recent strong winds from the SW through NW have formed reactive wind slabs in exposed terrain at higher elevations. These slabs are particularly reactive where they overlie buried surface hoar.
20-40 cm of settling snow sits over the January 30 surface hoar. This layer was responsible for several natural and human-triggered avalanches over the weekend. Treat open slopes at and below treeline with extra caution.
Dry and mainly sunny conditions are expected for the week except for a weak disturbance Tuesday night which may bring light flurries. A major warming event is current forecast to arrive late Wednesday which may persist for several days.
Monday Night: Lingering flurries in the north, partly cloudy in the south, light to moderate W wind, freezing level dropping to valley bottom.
Tuesday: A mix of sun and cloud with a chance of flurries overnight, light to moderate W wind, freezing levels reaching around 1400 m.
Wednesday: A mix of sun and cloud, moderate NW wind, freezing levels climbing to around 2000 m.
Thursday: Mainly sunny, light to moderate W wind, freezing levels reaching over 2500 m with an inversion.
On Sunday, several natural wind slabs and loose avalanches were reported. A skier remotely triggered a size 1.5 wind slab from 3 m away on an east aspect at 2500 m. Explosives triggered a number of slab avalanches size 1.5-2.5 in the deeper west part of the region where the slab thickness was typically 30-50 cm and up to 120 cm in wind loaded terrain.
This MIN post from the Dogtooth shows the type of open slopes where the buried surface hoar may be reactive at lower elevations.
On Saturday, several natural wind slabs up to size 1.5 were reported throughout the region. Ski cuts triggered a couple small slabs and some small loose avalanches in steep terrain. A skier remotely triggered a size 1.5 slab from 5 m away on a west aspect at 1900 m which failed on the late-January surface hoar down 55 cm in the deeper western part of the region. Explosives triggered a size 2 cornice which released a slab on the slope below.
A new sun crust is being reported on solar aspects into the alpine and a temperature crust on all aspects at lower elevations. Ongoing periods of strong wind from the southwest through northwest have formed reactive wind slabs in exposed terrain at higher elevations.
The January 30 interface is now typically down 20-40 cm and consists of a melt-freeze crust at lower elevations and on solar aspects extending into the alpine, surface hoar in sheltered areas at and below treeline, and small facets in some areas.
Several old surface hoar layers are now buried in the upper snowpack 40-70 cm deep. While these old layers appear to have gone dormant, there is still an isolated chance of avalanches stepping down to a deeper layer.
The early December crust/facet layer of concern sits around 80 cm deep in the eastern Purcells and shallow terrain, and up to 150 cm in the west. This layer produced numerous large avalanches in January but is now considered dormant. See this forecaster blog on how to manage this layer as it may come into play again later this season.
