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Avalanche Forecast

Archived

Dec 19th, 2020–Dec 20th, 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 possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Sea To Sky.

Snow and strong wind tonight will form new slabs and continue to load buried weak layers.

Confidence

Moderate - Forecast precipitation (either snow or rain) amounts are uncertain.

Weather Forecast

SATURDAY NIGHT: Snow, accumulation 10 to 20 cm with local amounts up to 30 cm possible, 60 to 80 km/h southwest wind, alpine temperature -2 C, freezing level 1700 m.

SUNDAY: Cloudy, 40 to 60 km/h west wind, alpine temperature -7 C, freezing level 1000 m.

MONDAY: Snow, accumulation 20 to 30 cm, 30 to 50 km/h south wind, alpine temperature -5 C.

TUESDAY: Clear skies, 10 to 20 km/h north wind, alpine temperature -9 C.

Avalanche Summary

A few large avalanches were triggered on the buried weak layers described in the snowpack summary on Saturday. They were triggered using explosives as well as by riders. They occurred on northerly aspect in alpine terrain (2100 m) and were generally 30 to 50 cm deep.

Otherwise, small to large (size 1 to 2) storm and wind slab avalanches were triggered by riders and explosives on Friday and Saturday. They generally occurred at treeline and alpine elevations on northerly terrain features and within the top 50 cm of the snowpack.

Snowpack Summary

Saturday night's snowfall will accumulate with strong southwest wind, forming wind slabs in exposed terrain. These will overly previous storm layers within the top 50 cm of the snowpack. The freezing level is expected to reach around 1700 m, so rain followed by a cooling trend will limit avalanche concerns below that elevation.

This snow will continue to load a couple weak layers in the snowpack. The shallower layer, being around 50 to 80 cm deep, includes feathery surface hoar crystals. There is uncertainty in the distribution of this layer, but the most likely place to find it would be in terrain features sheltered from the wind around treeline and lower alpine elevations. The more widespread weak layer includes sugary faceted grains that overlies a hard melt-freeze crust. This layer is generally found around 50 to 100 cm deep and is widespread up to around 2200 m.

The remainder of the snowpack is well-settled.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Problems

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.

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.