Avalanche Forecast

Monday night: Cloudy, 10-20 cm of snow, moderate southwest winds, alpine temperature near -2 C with freezing levels near 1200 m.

Tuesday: Cloudy, 30-40 cm of snow, moderate southwest winds gusting strong at ridge top, alpine high temperatures near-2 C with freezing level rising to 1600 m overnight.

Wednesday: Cloudy, 10-15 cm of snow, light southwest wind, alpine high temperatures near -1 C with freezing level near 1300 m.

Thursday: Mostly cloudy, 2-4 cm of snow, light southwest winds, alpine high temperature near -6 C with freezing level dropping below 500 m.

With the incoming storm, a natural avalanche cycle is expected on Tuesday.

Avalanche activity has diminished over the past few days. A few small avalanches, both human and explosive-triggered, were reported over the weekend in the recent storm snow. These avalanches released on leeward aspects (north to northwest facing slopes) in the alpine, and one stepped down to the mid-November weak layer.

Reports from last week captured widespread 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 incoming storm is expected to rapidly add a new layer to our snowpack. The new snow will fall on 15-25 cm of snow from over the weekend overlying a weak interface. At higher elevations, winds from the southwest drifted this snow into slabs on leeward features. Elsewhere this recent snow remains unconsolidated. The interface below it may present as surface hoar in many sheltered locations.

The upper snowpack consists of around 70 to 120 cm of snow that overlies a variable weak layer of surface hoar and a crust from mid-December (down 70-90 cm), as well as a deeper weak layer of sugary faceted grains and a crust buried in late-November (down 100-200 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 with the incoming storm.

• Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
• Storm slab size and sensitivity to triggering will likely increase through the day.
• Be aware of the potential for large avalanches due to the presence of buried weak layers.