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Avalanche Forecast

Archived

Mar 4th, 2020–Mar 5th, 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 possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

North Columbia.

A spicy persistent slab problem is not showing any signs of letting up. Don't get complacent about considerable danger. Avalanches have been large and human triggering remains likely.

Confidence

High -

Weather Forecast

Wednesday night: A trace of new snow. Moderate southwest wind. Freezing level valley bottom.

Thursday: 5-10 cm new snow. Strong southwest wind. Freezing level 1500 m.

Friday: 10-20 cm new snow. Light southwest wind. Freezing level 1100 m.

Saturday: 5-10 cm new snow. Light variable wind. Freezing level valley bottom.

Avalanche Summary

We've been flooded with reports of persistent slab avalanche activity on the February 22 surface hoar since Saturday, with no signs of slowing down. A natural cycle was observed Monday and Tuesday, with several cases involving cornice fallstriggering wind slabs which stepped down to persistent slabs, size 2-4. 

A deluge of skier triggered and remote triggered avalanches, size 1-2 have been reported by nearly every ski operation region wide even as professionals tiptoe around, avoiding 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 accidentally trigger the slab. 

Observations are from all aspects and elevations, but especially concentrated on north to east aspects around treeline. They are also occurring at unusually high elevations for surface hoar.

Snowpack Summary

Incremental snowfall and strong winds are building reactive wind slabs in the alpine and open areas at treeline.

A weak layer of widespread surface hoar sits 40-70 cm deep. It may sit over a crust on solar aspects. The overlying snow has been cohered into slabs by incremental loading through successive storms, wind and mild temperatures. As slab character and depth increase, so do reactivity and size of avalanches failing on the weak layer. Read more about surface hoar on our forecaster blog!

Terrain and Travel

  • Avoid freshly wind loaded terrain features.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.
  • Even brief periods of direct sun could produce natural avalanches.
  • In times of uncertainty conservative terrain choices are our best defense.

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.