Avalanche Forecast

Tuesday night: Cloudy, another 30-40 cm of snow, strong southwest winds, alpine temperatures near -1 C with freezing level dropping from 1600 m overnight.

Wednesday: Cloudy, 5-10 cm of snow, moderate southwest winds, alpine high temperatures near -3 C with freezing level near 1000 m.

Thursday: Mostly cloudy, 5-10 cm of snow, light southwest winds, alpine high temperatures near -5 C with freezing level dropping below 500 m.

Friday: Cloudy, 60-80 cm of snow possible overnight and through the day, strong south winds, alpine high temperatures near 0 C with freezing levels rising to 1300 m.

Numerous human-triggered and explosive-triggered avalanches were reported Tuesday as snow accumulated over a previously buried weak interface of surface hoar from late last week. These avalanches were small (size 1-1.5), but expect avalanche size to increase as snowfall continues.

Reports from last week captured a widespread avalanche cycle that included large to very large (size 2-3) natural, human, and explosive-triggered persistent slab avalanches. Many of these avalanches either failed on the mid-November weak layer or stepped down to it, even scouring the lower snowpack away to reveal ground. A few of these avalanches were remotely triggered. See here for some photos of one of them.

The possibility for large human-triggered persistent slab avalanches remains a serious concern, especially as more sensitive storm slabs create the potential for avalanches to step-down to these layers.

The overnight storm is expected to rapidly add a critical new load to our snowpack. This will create a widespread storm slab problem at all elevations that will need to be managed conservatively. At higher elevations, strong southwest winds are expected to exacerbate the reactivity of the new snow in drifted areas. The new snow is falling on another recent layer of surface hoar that has been showing increasing reactivity as snow accumulates.

Buried deeper in the snowpack, there are multiple weak layers, which include a variable layer of surface hoar and crust from mid-December (down 70-90 cm) as well as a deeper layer of sugary facets and crust buried in late-November (down 120-200 cm). Both of these persistent weak layers produced many large and destructive avalanches during and in the days after last week's storm. Snowpack tests continue to produce sudden and propagating results on these layers (like this MIN report from Disease Ridge on Sunday). This fundamentally unstable snowpack structure remains a serious concern as new snow and wind add a critical load and increase the likelihood of triggering these large and destructive avalanches.

• Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
• If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.