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.
Cornices may become weak with sun and warming.
Up to 40 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 most likely in the parts of the region that saw more recent snowfall. Extra caution is recommended around open slopes at and below treeline where the buried surface hoar is most prominent.
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 the potential for a temperature inversion in the valleys.
Saturday Night: Mainly clear, light to moderate NW wind, treeline temperature around -5 °C.
Sunday: Mainly sunny with a chance of valley cloud, moderate W wind, freezing level reaching over 2000 m in the afternoon with a possible temperature inversion.
Monday: A mix of sun and cloud, moderate to strong SW wind, freezing level around 1800 m with an inversion.
Tuesday: Mainly sunny, light to moderate W wind, freezing level reaching around 1800 m.
This MIN report from Saturday describes a natural size 1.5 slab avalanche on a south aspect as well as shooting cracks and whumpfing while travelling. It also suggests the surface snow is getting warm and consolidating.
On Friday, four natural size 1.5 wind slabs were reported by the field team in this MIN post failing down 30 cm on a crust in cross-loaded terrain. On Thursday, three natural size 1-1.5 wind slabs were reported on east through south aspects at 2200 m elevation and explosives triggered one size 1 wind slab.
Up to 40 cm of settling snow now overlies the January 30 interface which consists of a melt-freeze crust to around 2000 m and on solar aspects extending into the alpine, spotty 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 strong wind on Friday and Saturday, mainly from the southwest through northwest, has continued to develop wind slabs.
The early December crust/facet persistent weak layer is now 100-200 cm deep. This layer produced numerous very 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.
