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Avalanche Forecast

Archived

Mar 19th, 2021–Mar 21st, 2021

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

Regions

North Rockies.

Modest amounts of new snow and shifting winds may form wind slabs on a variety of aspects below alpine ridgetops. The snowpack is highly variable throughout this region and persistent weak layers may be reactive in isolated areas.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

  

FRIDAY NIGHT: Cloudy with flurries; 3-5 cm. / Strong, southwest ridgetop wind / alpine low temperature -9 / Freezing level valley bottom.

SATURDAY: Mix of sun and cloud with isolated flurries; 3-5 cm. / Strong, northwest ridgetop wind / alpine high temperature -5 / Freezing level 700 m.

SUNDAY: Flurries; 5-10 cm. / Strong, southwest ridgetop wind / alpine high temperature -4 / Freezing level 1200 m.

MONDAY: Mix of sun and cloud / Moderate, west ridgetop wind / alpine high temperature -2 / Freezing level 1300 m.

Avalanche Summary

On Thursday our field team reported several cornice failures, a couple of which triggered small slab avalanches on the slopes below. MIN report HERE.

Our field team found some large and disturbingly wide avalanches in the Hasler last week, with lots of compelling images in their MIN report here. It's suspected that these are running on facets that are about 80 cm below the surface.

Snowpack Summary

Modest amounts of snow over the weekend and strong southwesterly winds will likely form small wind slabs on lee features below alpine ridgetops. 

Sun crust exists on solar aspects to mountain tops and a temperature crust on all aspects up to 1500 m. Large cornices are present throughout the region and are capable of triggering large avalanches when they fail. Cornice failures are most common during windy or warm weather.

A weak layer of facets buried mid-February is down 100-120 cm. in many areas and was reactive a little over a week ago. (See avalanche summary for report) Slightly deeper there may be a persistent weak layer of surface hoar in isolated areas at treeline, though this layer has recently been unreactive. These layers are both significantly shallower in the east of the region. 

Steep, rocky slopes with a shallow or thin to thick snowpack in the alpine are the most likely places to trigger these persistent weak layers. Large loads such a cornice falls or multiple sleds on the same slope often trigger slopes that single rider couldn't trigger.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid lee and cross-loaded terrain.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid areas with a thin or variable snowpack.
  • Avoid exposure to slopes that have cornices overhead.

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.