Avalanche Forecast

Archived

Feb 27th, 2020–Feb 28th, 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 and human triggered avalanches likely.

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.

Anticipate increasing avalanche hazard as new snow accumulates and as winds increase. The incoming storm has potential to bring cornices and deeper weak layers to their tipping point. Conservative terrain use is warranted for Friday.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system. Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

Thursday night: Cloudy, 5 to 10 cm of snow, moderate southwest wind, alpine temperature -3 C.

Friday: Cloudy, 5-15 cm of snow, southwest winds increasing to strong by end of day, freezing level 1300 m.

Saturday: Cloudy, 25-30 cm of snow, light southwest winds, freezing level 1000 m.

Sunday: Mostly clear, light southwest winds, freezing level 1000 m.

Avalanche Summary

On Wednesday, a large (size 2) human-triggered avalanche releasing on the February 22 surface hoar was reported. Small point releases in steep rocky south facing terrain around treeline have been recently observed.

Reports from earlier in the week include a natural storm and wind slab avalanche cycle up to size 2, primarily on northeast aspects in the alpine and upper treeline, releasing on the February 22 interface. This MIN post includes a photo of a skier triggered wind slab.

Snowpack Summary

By Friday afternoon, 15-25 cm of forecast snow is expected to form a new layer of concern. Strong winds from the southwest will likely drift the new snow into touchy wind slabs on lee features and bring cornices to their breaking point. Buried 40-70 cm deep, a weak layer of near surface facets facets or surface hoar from February 22 still lingers and has recently shown reactivity.

Faceted snow and crusts exist near the base of the snowpack in much of the region. This layer is has been dormant recently.

Terrain and Travel

Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
Minimize exposure during periods of heavy loading from new snow and wind.
Wind slabs are most reactive during their formation.
Be aware of the potential for large avalanches due to the presence of buried surface hoar.

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.