New snow may form touchy wind slabs in exposed areas and may test the strength of deeply buried weak layers. Step-down avalanches could be possible. Time to adopt a mindset of stepping back to simple terrain while the snowpack adjusts.
10-20 cm of snow and southwest winds are expected to form fresh wind slabs that are likely to be reactive to human triggering. Expect this problem to be most pronounced in the immediate lee of wind-exposed terrain features. Wind slab releases currently carry a serious risk of triggering a deeper weak layer, especially in shallow, rocky areas.
Several different deeply buried weak layers can be found in the snowpack that have produced large avalanches. These weak layers may be tested by additional loading from new snow and wind. Shallower but more touchy wind slab releases carry the risk of triggering one of these deeper weak layers to create larger, more destructive avalanches.
Monday night: Cloudy, isolated flurries with 1-3 cm of snow possible, light to moderate southwest winds, alpine temperatures around -7 C.
Tuesday: Cloudy, 10-20 cm of snow, moderate to strong southwest winds, alpine high temperatures near -5 C with freezing levels around 700 m.
Wednesday: Mostly cloudy, another 10-20 cm of snow overnight, moderate west winds, alpine high temperatures near-3 C with freezing levels around 900 m.
Thursday: Decreasing cloud, isolated flurries with a trace of accumulation, light to moderate northwest winds, alpine high temperatures around -5 C.
The likelihood of triggering avalanches increases with the incoming storm.
Avalanche activity has been diminishing; however, several persistent slab avalanches (size 1.5-3), both human and explosive-triggered, were reported over the weekend. These avalanches released on both the December surface hoar and November crust layers across a variety aspects. This recent persistent slab avalanche was observed Sunday. It scrubbed into the lower snowpack and to the ground and was noted for being triggered on a shallow, rocky, convex slope.
The possibility for large human-triggered persistent slab avalanches remains a serious concern, especially as newly formed wind slabs create the potential for smaller avalanches to step-down to these layers.
The storm brings new snow for moderate southwest winds to drift into touchy wind slabs in exposed areas near and above tree line. Forecast snowfall will cover another recent surface hoar layer buried late last week by snow that fell over the weekend.
Last week's big storm deposited 70 to 100 cm of snow and a significant load to multiple weak layers. These layers include a feathery surface hoar layer (down 70 to 110 cm), an older surface hoar layer with a melt-freeze crust on steep south aspects (down 90 to 130 cm), and a layer of sugary faceted snow, surface hoar, and melt-freeze crusts from late November found in the bottom half of the snowpack.
Snowpack tests continue to produce sudden and propagating results on these layers (like this MIN from Big White on Sunday and this MIN from Rossland on Monday) This fundamentally unstable snowpack structure remains a serious concern as new snow and wind add an additional load and increase the likelihood of triggering large and destructive avalanches.
