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Avalanche Forecast

Archived

Dec 4th, 2020–Dec 5th, 2020

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

Northwest Inland.

Temperatures will be on the rise through Saturday afternoon. Avoid consequential terrain and overhead hazard at peak warming.

Confidence

Low - Uncertainties in both the snowpack structure and the weather forecast limit our confidence. Uncertainty is due to the limited number of field observations.

Weather Forecast

Friday night: Mostly cloudy, 5-10 cm of snow above 1500 m, moderate southwest wind with strong gusts, freezing level rising to 1600 m

Saturday: Mostly cloudy, up to 5 cm of snow above 1600 m, moderate southwest wind, freezing level rising to 1600 m and then falling to 1400 m.

Sunday: Mix of sun and cloud, 5-10 cm of snow above 1400 m, moderate southwest winds, freezing level dropping to 1200 m.

Monday: Partly cloudy, 5-10 cm of snow above 1200 m, light south winds, freezing level dropping to 800 m.

Avalanche Summary

Avalanche observations are limited. If you're out in the field, please consider sharing a photo or description via the Mountain Information Network (MIN).

On Wednesday, a series of notable avalanches were reported in the northern half of the region. These were explosive-triggered and natural releases that resulted in very large deep persistent slab avalanches (size 3-3.5), failing on weak snow near the ground. 

 

Snowpack Summary

Isolated flurries, moderate southwest winds, and fluctuating temperatures are forecast for Saturday. Look for signs of warming and back off consequential terrain if the surface becomes moist or wet with rising temperatures.

Below a well-settled mid-pack, the bottom half of the snowpack reportedly consists of weak snow interrupted by a crust from early November. Further north in the region, we're learning that weak snow developing near the ground has produced very large deep persistent slab avalanches. Although we lack data on the distribution of this problem, the bottom line is that smaller avalanches have the potential to step-down to deeper layers and to produce large, destructive avalanches.

Snowpack depths are variable across the region, ranging from 60 cm in eastern parts of the region up to over 100 cm in western parts of the region. Lower elevation terrain may still be below the threshold for avalanches. 

Terrain and Travel

  • Back off slopes as the surface becomes moist or wet with rising temperatures.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.