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Avalanche Forecast

Archived

Feb 7th, 2022–Feb 8th, 2022

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

Regions

Lizard-Flathead.

Wind slabs are expected to remain reactive in exposed high elevation terrain on Tuesday. It may still be possible to trigger a buried weak layer in isolated areas but the likelihood is decreasing with the cooler weather. 

Confidence

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

Weather Forecast

Dry and mainly sunny conditions are expected for the week except for a weak disturbance Tuesday night which may bring light flurries. A major warming event is current forecast to arrive late Wednesday which may persist for several days. 

Monday Night: Light flurries in the evening then clearing, light to moderate SW wind, freezing level dropping to valley bottom.

Tuesday: A mix of sun and cloud with a chance of flurries overnight, moderate SW wind, freezing levels reaching around 1500 m.

Wednesday: A mix of sun and cloud, moderate NW wind, freezing levels climbing to around 2000 m.

Thursday: Mainly sunny, moderate SW wind, freezing levels reaching near 2500 m with an inversion.

Avalanche Summary

An early report from Monday includes explosives triggering numerous of cornices, with some subsequently triggering slabs on the slope below. 

On Sunday, a ski cut triggered a size 1.5 wind slab at 1950 m on a north aspect which was 15-25 cm thick and a skier accidently triggered a size 2 wind slab which failed down 30-60 cm on the mid-January crust. An explosive-triggered size 2.5 wind slab was also reported which failed on the mid-January crust down 30 cm and then stepped down to an older layer 100 cm deep in the snowpack. 

This MIN report summarizes the recent conditions well and describes natural and human-triggered slab avalanches size 1-2 as well as whumphing and shooting cracks. 

On Saturday, a natural cornice fall triggered a size 2 storm slab which failed down 30 cm and then stepped down to deeper layers in the snowpack. Several natural wind slabs size 1.5-2.5 were also reported in alpine terrain on NW-NE aspects. A small cornice failure was also reported which did not trigger a slab. 

Snowpack Summary

A new melt-freeze crust is expected on solar aspects into the alpine and on all aspects at lower elevations. Ongoing periods of strong wind from the southwest through northwest have formed reactive wind slabs and large cornices in exposed terrain at higher elevations. 

The January 30 interface is now typically down 20-40 cm and consists of a melt-freeze crust at lower elevations and on solar aspects extending into the alpine, spotty surface hoar in sheltered areas at and below treeline, and small facets in some areas. 

The widespread January 18 rain crust is now around 50-70 cm deep with weak faceted snow above. This layer appears to have woken up with the weekend warming and at least three avalanches likely involved this layer.

The early December crust/facet persistent weak layer is now 100-200 cm deep. This layer produced numerous large avalanches in January but is now considered dormant. See this forecaster blog on how to manage this layer as it may come into play again later this season. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • Cornices become weak with daytime heating or solar exposure.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.