New snow has formed widespread, reactive storm slabs. In the north of the region, avalanches may step-down to more deeply buried weak layers. Stick to a conservative mindset and to simple terrain as the snowpack adjusts.
A problematic weak layer is buried 50-90 cm deep in the snowpack in the north of the region. Recent snow and wind has added a critical load to this fundamentally unstable snowpack structure. This problem is largely absent in the south of the region.
Recent snow has created a widespread storm slab problem that now buries another surface hoar layer observed in the north of the region late last week. Triggering a large storm slab is still possible on Thursday, with wind-loaded areas seeing more slab formation and sheltered areas harboring a weaker interface.
Wednesday night: Mostly cloudy, 3-8 cm of snow, strong northwest winds, alpine temperatures near -7 C with freezing levels dropping to valley bottom.
Thursday: Cloudy, 3-5 cm of snow, moderate southwest winds, alpine high temperatures around -8 C.
Friday: Mostly cloudy, 30-40 cm of snow falling overnight and through the day, strong southwest winds, alpine high temperatures near 2 C with freezing levels rapidly rising to 2000 m.
Saturday: Mostly cloudy, 5-10 cm of snow, moderate southwest winds, alpine high temperatures around -3 C, freezing level near 1200 m.
Several large (size 2) natural avalanches were reported in the region on Wednesday (check out this MIN from Zoa Peak). This avalanche activity occurred in the storm snow.
Few observations are available from the northern part of the region where the deeper, persistent slab problem exists. In the widespread avalanche cycle a week ago, avalanches were observed running to valley-bottom and failing on deeply buried weak layers. See this MIN report of an avalanche involvement from last week for an example of this avalanche problem.
The possibility for large human-triggered persistent slab avalanches remains a concern in the northern part of the region (e.g., Duffey, Hurley, etc.), especially as more sensitive storm slabs create the potential for avalanches to step-down to these layers.
Storm snow totals came to 20-30 cm for most of the region, creating a widespread storm slab problem. At higher elevations, strong southwest winds are expected to exacerbate the reactivity of the new snow in drifted areas. The storm covered another recent layer of surface hoar and a sun crust on steep sun-exposed aspects.
The snowpack below varies significantly between the northern and southern parts of the region. In the north (e.g. Duffey, Hurley), the upper snowpack consists of around 50-90 cm of snow that overlies a weak layer from late November composed of sugary faceted grains and hard melt-freeze crust. This presents a persistent slab problem that may remain problematic for weeks to months with the potential to produce large and destructive avalanches. This persistent weak layer is largely absent in the south of the region.
