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Avalanche Forecast

Archived

Dec 29th, 2019–Dec 30th, 2019

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

Regions

Sea To Sky.

Although the likelihood of triggering avalanches is decreasing, the persistent slab problem warrants avoiding steep convexities and areas with a shallow, rocky, or variable snowpack.

Confidence

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

Weather Forecast

Sunday night: Cloudy with scattered flurries and 3-5 cm of new snow, light south winds, alpine temperatures around -1 C with freezing level around 1700 m.

Monday: Cloudy, isolated flurries with 1-3 cm of accumulation possible overnight, light to moderate southwest winds, alpine high temperature near -2 C with freezing level dropping from 1200 m.

Tuesday: Cloudy, 10-20 cm of new snow, moderate southwest winds, alpine high temperatures near-2 C with freezing level around 1000 m.

Wednesday: Cloudy, 15-25 cm of snow, light to moderate southwest wind, alpine high temperatures near -1 C with freezing level near 1300 m.

Avalanche Summary

Avalanche activity has diminished over the past few days. A few small avalanches, both human and explosive-triggered, were reported Saturday in the recent storm snow. These avalanches released on leeward aspects (north to northwest facing slopes) near tree line.

Reports from earlier in the week captured widespread large to very large (size 2-3) natural, human, and explosive-triggered persistent slab avalanches. Many of these avalanches either failed on the mid-November weak layer or stepped down to it, even scouring the lower snowpack away to reveal ground. A few of these avalanches were remotely triggered. See here for some photos of one of them.

Despite a decrease in avalanche activity, human-triggering large avalanches remains possible at higher elevations. Safe travel requires evaluating snow and terrain carefully.

Snowpack Summary

15-25 cm of new snow fell over the weekend, and winds from the southwest drifted this snow into slabs on leeward features at higher elevations.

Below the recent snow, the upper snowpack consists of around 70 to 120 cm from last weekend's storm. This overlies a variable weak layer of surface hoar and a crust from mid-December (down 70-90 cm), as well as a deeper weak layer of sugary faceted grains and a crust buried in late-November (down 100-200 cm). Both of these persistent weak layers produced many large and destructive avalanches during and in the days after the storm. Avalanche activity on these layers has been on a downward trend, but our fundamentally unstable snowpack structure remains a serious concern in the region. Snowpack tests continue to produce sudden and propagating results on these layers (check out this MIN report from the Musical Bumps on Saturday). It is atypical for the region, and it is expected to persist for some time.

Terrain and Travel

  • Avoid steep convexities or areas with a thin, rocky, or variable snowpack.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Uncertainty is best managed through conservative terrain choices at this time.

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.