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Avalanche Forecast

Archived

Dec 20th, 2019–Dec 21st, 2019

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

Regions

North Rockies.

The stormy weather has ended, but human triggered avalanches are still likely in steep and open terrain. A cautious approach to backcountry travel is recommended.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

FRIDAY NIGHT: Partly cloudy, 30-50 km/h wind from the southwest, alpine temperatures drop to -8 C.

SATURDAY: Mix of sun and cloud with some isolated flurries in southern parts of the region, 20-40 km/h wind from the southwest, alpine high temperatures around -4 C.

SUNDAY: Mix of sun and cloud, 20-40 km/h wind from the west, alpine high temperatures around -6 C.

MONDAY: Mix of sun and cloud, light wind, alpine high temperatures around -6 C.

Avalanche Summary

Recent reports include natural wind slab avalanches in steep alpine terrain (see this MIN report) and touchy storm slabs around Hasler (see this MIN report). As we move out of the stormy period, natural avalanches are still possible in wind affected alpine terrain, while human triggered avalanches remain a concern at all elevations.

Please share you observations and photos from this weekend on the Mountain Information Network!

Snowpack Summary

Stormy weather the past few days delivered 20-30 cm of new snow to most parts of the regions, with some areas receiving up to 50 cm (e.g. around Pine Pass). Steady wind from the southwest is likely blowing this snow into unstable wind slabs in open terrain, while soft powder will be preserved in sheltered areas.

The storm snow sits above three weak layers buried between 30 and 60 cm below the surface. These surface hoar and facet/crust layers showed signs of being reactive prior to the storm and have the potential to produce large avalanches the next few days while they adjust to the weight of the new snow. Reports suggest these layers are widespread around Pine Pass and the McGregors, but harder to find around McBride. 

The lower snowpack contains several crust layers. In higher snow areas, these are likely well bonded. However, in shallow areas along the eastern slopes, these could be of concern.

Total snowpack depths are approximately 240 cm in the south of the region, 170 cm around Pine Pass and more like 120 cm in areas to the east.

Terrain and Travel

  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Give the new snow several days to settle and stabilize before pushing into bigger terrain.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.

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.