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Avalanche Forecast

Archived

Mar 6th, 2020–Mar 7th, 2020

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

Regions

North Columbia.

The snowpack is not to be trusted. Reports of large, human-triggered avalanches on a buried weak layer continue across aspects and elevations. Signs of the problem are becoming less obvious and the consequences more critical. Keep it conservative on Saturday.

Confidence

High -

Weather Forecast

Friday night: Partly cloudy, isolated flurries with trace accumulations, light variable winds, alpine temperature -10 C. 

Saturday: Cloudy, 3-8 cm of snow, light variable winds, alpine high temperature -5 C.

Sunday: Partly cloudy, another 3-8 cm of snow overnight, light southwest winds, alpine high temperature -6 C.

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

Avalanche Summary

On Wednesday and Thursday, 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. 

There has been a flood of reports of avalanches releasing on the February 22 surface hoar since Saturday. Both human-triggered and remotely-triggered avalanches have been reported by nearly every operation region-wide, even as professionals tiptoe around and avoid suspect terrain features. The problem is touchy but tricky. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab. 

Avalanche size has increased since the weekend, with more of the activity releasing size 2+. Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. Avalanches are also occurring 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.

Snowpack Summary

Incremental snowfall and recent strong winds on Wednesday/Thursday formed reactive wind slabs in the alpine and open areas at treeline that remain possible to human trigger. Snow falling Friday night and into Saturday may obscure the evidence of these wind slabs.

A weak layer of widespread surface hoar is buried 40-80 cm deep. It may sit over a crust on solar aspects. Incremental loading from successive storms, strong winds, and mild temperatures have strengthened the slab character and increased the depth of the snow above, creating the potential for larger avalanches with serious consequences. Read more about surface hoar on our forecaster blog!

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Use conservative route selection and resist venturing out into complex terrain.

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.