Avalanche Forecast

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Sunday night: Mostly clear, light west winds, alpine temperature -16 C.

Monday: Mostly clear, light southwest winds, alpine high temperature -6 C, freezing level 1000 m.

Tuesday: Increasing cloud, 5-15 cm of snow, moderate southwest winds with strong gusts, alpine high temperature -4 C, freezing level 1200 m.

Wednesday: Mostly cloudy, 5-10 cm of snow overnight, light west winds, alpine high temperature -5 C, freezing level 1100 m.

Over the past several days, a natural wind slab avalanche cycle was observed. These avalanches were small to large (size 1-2.5) breaking 25-50 cm deep on leeward features at upper elevations. In several cases, cornice falls triggered these wind slabs. Wind slabs and cornice falls have the potential to step down to the buried surface hoar layer, forming very large avalanches. Avoid sun-exposed slopes during intense solar radiation, as rapidly warming snow and weakening cornices could trigger loose or slab avalanches.

Human-triggered and remotely-triggered avalanches releasing on the February 22 surface hoar have been reported by nearly every operation region-wide over the past week. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab. Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. Avalanches have also occurred at unusually high elevations for surface hoar, with several cases of wind slabs stepping down in the alpine. This combination has produced very large (size 3-4) avalanches. While the number of reported avalanches has decreased, the size has increased, with more of the activity releasing size 2+.

Incremental snowfall and wind have formed wind slabs in the alpine and open areas at treeline that remain possible to human trigger. Cornices are large and looming after a week of successive storms. Intense solar radiation is rapidly destabilizing recent snow on steep, sun-exposed slopes and weakening cornices.

A widespread weak layer of surface hoar is buried 50-90 cm deep. On solar aspects, this layer may sit over a crust. This persistent slab problem is transitioning into a low probability/high consequence scenario. Snowfall, wind, temperatures, and solar radiation have increased the depth and slab properties of the snow above the weak layer, making it more difficult to trigger and masking obvious clues that the problem is present (i.e. cracking, whumpfing). However, if triggered, avalanches will likely be larger in size and getting caught could have more serious consequences. Managing this problem requires a patient and diligent mindset, implemented by avoiding suspect slopes and maintaining conservative terrain margins. This persistent weak layer will likely pose the threat of a low probability/high consequence avalanche until there is a substantial change in the snowpack. Read more about surface hoar on our forecaster blog.

• Be aware of the potential for large avalanches due to the presence of buried surface hoar.
• Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
• Minimize exposure to sun-exposed slopes when the solar radiation is strong.
• Stay off recently wind loaded slopes until they have had a chance to stabilize.