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Avalanche Forecast

Archived

Mar 9th, 2020–Mar 10th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

Between fresh wind slabs and a reactive persistent weak layer, human triggered avalanches are likely. Keep in mind it's not always the first rider who triggers the slab and keep terrain choices conservative on Tuesday.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Monday night: Mostly clear, light west wind, alpine high temperature - 12 C. 

Tuesday: Cloudy, 3-8 cm of snow, moderate southwest winds gusting strong at ridge-top, alpine high temperature -5 C, freezing level 1300 m.

Wednesday: Mostly cloudy, 5-10 cm of snow, light northwest winds, alpine high temperature -4 C, freezing level 1300 m. 

Thursday: Mostly cloudy, isolated flurries with trace accumulations, light west winds, alpine high temperature -6 C, freezing level 800 m.  

Avalanche Summary

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 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. 

On 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. 

Snowpack Summary

Snowfall amounts from the weekend storm ranged from 20-40 cm. Human triggering remains possible where the wind has transported the snow into deeper drifts. Incremental snowfall and wind forecast for Tuesday is expected to add to this wind slab problem.

A weak layer of surface hoar from February 22 may be found 35-70 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.

Terrain and Travel

  • 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.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.