Avalanche Forecast

Wednesday night: Clear with cloudy periods and isolated flurries, moderate northwest wind, alpine high temperature -12 C, freezing level at valley bottom.

Thursday: Mix of sun and cloud with isolated flurries, light northwest wind, alpine high temperature -12 C, freezing level 500 m.  

Friday: Mostly cloudy, scattered flurries with up to 5 cm of snow accumulation, moderate easterly wind, alpine high temperature -18 C, freezing level at valley bottom.

Saturday: Mostly cloudy with isolated flurries, moderate northeast wind, alpine high temperature -22 C, freezing level at valley bottom.

Only a couple naturally and human triggered wind slab avalanches up to size 2 were observed during the storm on Tuesday.

Over the weekend, a natural storm slab avalanche cycle was observed. These avalanches were small to large (size 1-2.5) and breaking 20-40 cm deep. Shallow slabs in the recent snow have the potential to step down to the buried surface hoar layer, creating very large avalanches. 

Over the past week, human-triggered avalanches releasing on the February 22 surface hoar have been consistently reported. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab. These avalanches have been remotely-triggered and have propagated widely across terrain features, warranting conservative terrain margins (see this MIN or this MIN for a helpful example). Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. 

Last Thursday, professionals reported large (size 2.5) explosive-triggered avalanches releasing on weak layers near the ground. These avalanches occurred on north and east facing slopes above 2300 m, and they confirm that the deep persistent slab problem remains a concern. 

The storm on Tuesday delivered up to 30 cm in the north and west of the region and up to 10 cm in the south and east of the region. Human triggering remains likely especially where the wind has transported the snow into deeper drifts. 

A weak layer of surface hoar from February 22 may be found 50-100 cm deep. It may sit over a crust on solar aspects. With successive storms, more areas have reached critical loading and developed a slab over this layer. 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 remain a problem until there is a substantial change in the snowpack. Read more about surface hoar on our forecaster blog.

Deep basal facets lurk near the bottom of the snowpack. This layer has been responsible for sporadic deep persistent slab avalanches, usually triggered from shallow, rocky start zones.

• Be aware of the potential for large avalanches due to the presence of buried surface hoar.
• Use conservative route selection and resist venturing out into complex terrain.
• Stay off recently wind loaded slopes until they have had a chance to stabilize.
• Avoid shallow, rocky areas where the snowpack transitions from thick to thin.