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Avalanche Forecast

Archived

Mar 17th, 2021–Mar 19th, 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.
Below Treeline
Natural avalanches unlikely.

Regions

North Rockies.

 Southwesterly winds continue to form wind slabs on lee and cross-loaded terrain features in the alpine. Use caution when transitioning into wind affected terrain and avoid wind loaded slopes below ridgetops. 

Confidence

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

Weather Forecast

  

WEDNESDAY NIGHT: Clear / Strong, southwest ridgetop wind / alpine low temperature -9 / Freezing level valley bottom.

THURSDAY: Increasing cloud in PM, then 5-10 cm. overnight / Strong, southwest ridgetop wind / alpine high temperature 0 / Freezing level 1900 m.

FRIDAY: Mix of sun and cloud / Strong, southwest ridgetop wind / alpine high temperature -2 / Freezing level 1500 m.

SATURDAY: Mix of sun and cloud / Moderate, northwest ridgetop wind / alpine high temperature -9 / Freezing level 800 m.

Avalanche Summary

A few naturally triggered size 2 wind slabs on northerly aspects in the alpine were reported on Tuesday.

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

The northern portion of the region around Pine Pass got around 35 cm. of snow last weekend, but much less fell in the rest of the region. Southwesterly winds have formed wind slabs on lee and cross-loaded features below alpine ridgetops. 

Sun crust exists on solar aspects and a temperature crust may exist on all aspects up to 1800 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.

About a metre of snow covers a weak layer of facets buried mid-February. Slightly deeper there is a widespread persistent weak layer from late January/early February that consists of surface hoar. It is most prevalent around treeline elevations, but likely reaches into the alpine and in openings below treeline too. 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 and steep, open slopes at treeline 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.
  • Avoid slopes that sound hollow or drum-like.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

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.