Wind slabs, loose avalanches, and weak cornices are all expected Sunday.
There is a lot of uncertainty about how buried weak layers are going to react to the sun and warming. Use a conservative approach to terrain selection and watch for signs of instability.
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.
Cornices may become weak with sun and warming.
Up to 50 cm of relatively unconsolidated snow sits over the January 30 surface hoar. With warming and sun this weekend, this snow may settle into a reactive slab. There is a lot of uncertainty regarding the timing and spatial extent of this scenario but it is expected to be most likely in the western parts of the region which received more recent snowfall. Treat open slopes at and below treeline with extra caution.
Loose avalanches should be expected on steep sun exposed slopes during the heat of the afternoon.
A ridge of high pressure brings dry and sunny conditions for Sunday with a layer of warm air expected to reach the alpine and a temperature inversion in the valleys.
Saturday Night: Mainly clear, light to moderate NW wind, treeline temperature around -6 °C.
Sunday: Mainly sunny with some valley cloud, light to moderate W wind, freezing level reaching over 2000 m in the afternoon with a temperature inversion.
Monday: A mix of sun and cloud, moderate to strong SW wind, warm air aloft breaking down in the morning, afternoon freezing level reaching around 1500 m.
Tuesday: Mainly sunny, light to moderate W wind, freezing level reaching around 1500 m.
An early report from Saturday includes natural and explosive triggered wind slabs in the north of the region which were 15 cm thick.
On Friday, ski cutting triggered several small loose dry avalanches failing down 10-30 cm and some natural loose dry avalanches were observed in extreme terrain features. In the deeper western part of the region, multiple small slab avalanches were reported failing on the January 30 interface as a slab started consolidating with warmer temperatures. On Thursday, a skier remotely triggered a size 1 slab from 30 m away in the southwest of the region which failed on the Jan 30 surface hoar down 40 cm.
Up to 50 cm of settling snow now overlies the January 30 interface which consists of a melt-freeze crust at lower elevations and on solar aspects extending into the alpine, widespread surface hoar in sheltered areas at and below treeline, and small facets in some areas. Prior to the weekend warming, the surface storm snow was cold and mainly unconsolidated, and had not formed a slab in most places except for where it was wind loaded. With warming and sun this weekend, rapid settlement is possible and this may result in the formation of a more widespread reactive slab over the weak interface.
Periods of strong wind over the past week has formed reactive wind slabs in exposed terrain at higher elevations and ongoing wind on Friday and Saturday, mainly from the southwest through northwest, has likely continued to develop wind slabs.
Several old surface hoar layers are now buried in the upper snowpack 40-70 cm deep. While these have been dormant recently, they will get tested this weekend with warming and sun.
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.
