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Avalanche Forecast

Archived

Feb 11th, 2020–Feb 12th, 2020

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

Regions

South Rockies.

See the Avalanche Summary for some great information about a near miss with a deep avalanche on Sunday, it's a humbling account of the current conditions. Wind slabs may remain sensitive to human triggering, especially in steep features immediately lee of ridgecrest.

Confidence

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

Weather Forecast

Aside from a few bouts of potentially strong wind, we continue to be in a rather benign weather pattern at least until Sunday when there is a hint of another strom...

TUESDAY NIGHT: Freezing level returning to valley bottom, moderate north/northwest wind, 1 to 5 cm of snow possible.

WEDNESDAY: Scattered cloud cover initially with some clearing expected in the late afternoon, freezing level near valley bottom, light variable wind at most elevations with moderate northwest wind at the highest elevations, trace of precipitation possible.

THURSDAY: Scattered cloud cover, freezing level rising to around 1500 m in the afternoon, moderate to strong west/southwest wind, trace of precipitation possible.

FRIDAY: Scattered cloud cover, freezing level rising to around 1300 m in the afternoon, moderate to strong wind shifting around from the west, southwest and northwest, trace of precipitation possible.

Avalanche Summary

On Monday reported avalanche activity was limited to small dry loose avalanches and a single small wind slab that succumbed to explosive control work.

Over the weekend numerous natural, human triggered and control work induced avalanches to size 2 were observed, there is a great MIN that serves as an example of this kind of activity here.

There was also a bigger avalanche reported in this MIN on February 9th (check out the photos, they're humbling). The very large avalanche ran on a north facing feature at 2400 m and was triggered by a snowmobile. The group was using excellent travel protocol and thankfully no one was hurt. 10 or so climbs had been made in the same area in the days leading up to this avalanche without incident and that's what makes dealing with persistent slabs so tough.  

Before the above avalanche, the last avalanche cycle to include persistent slabs was during the first weekend in February when some large (size 2.5) deep persistent slab avalanches in the Elk Valley (see photos in this MIN report) ran. These were similar to avalanches in that area from mid-January, which suggests the deep persistent slab problem may resurface during stormy periods.

Snowpack Summary

5 to 15 cm of storm snow from last weekend's storm adds to the variable amounts of wind affected snow that sits above a widespread rain crust that exists up to roughly 2100 m. The storm snow has been formed into slabs by wind from a variety of directions. The Elk Valley has 5-20 cm of snow above the crust and the eastern slopes have 15-35 cm above the crust. There have been observations of surface hoar forming in the Elk Valley previous to Friday Night's storm, check that out here. A well consolidated mid-pack overlies generally weak basal facets, that may be possible to trigger in isolated shallow rocky start zones.

Terrain and Travel

  • Avalanche hazard may have improved, but be mindful that deep instabilities are still present.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Caution around slopes that are exposed to cornices overhead.

Problems

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.

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.