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Avalanche Forecast

Archived

Feb 6th, 2022–Feb 7th, 2022

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 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

Lizard-Flathead.

Ongoing periods of strong wind continue to develop reactive wind slabs in exposed terrain and two buried weak layers remain a concern for persistent slab avalanches. Give the snowpack time to recover from the recent warming and continue to use a conservative mindset.

Confidence

Low - Uncertainty is due to rapidly fluctuating freezing levels. Uncertainty is due to how the snowpack will react to the forecast weather.

Weather Forecast

Freezing levels may stay elevated overnight and Monday morning but there is still a lot of uncertainty regarding the intensity and extent of any lingering warming. 

Sunday Night: Mainly clear, moderate SW wind, freezing levels 1500-2000 m with a temperature inversion.

Monday: Increasing cloud cover in the afternoon with a chance of flurries, moderate to strong SW wind, afternoon freezing levels around 1600 m. 

Tuesday: Mainly sunny, light to moderate SW wind, freezing levels reaching around 1600 m.

Wednesday: Mainly sunny, light to moderate W wind, freezing levels climbing to around 2000 m.

Avalanche Summary

An early report from Sunday includes an explosive-triggered size 2.5 wind slab which failed on the mid-January crust and stepped down to a deeper layer in the snowpack. 

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

20-40 cm of rapidly settling snow now typically overlies the January 30 interface which consists of a melt-freeze crust at lower elevations and on solar aspects extending into the alpine, surface hoar in wind-sheltered areas at and below treeline, and small facets in some areas. With the recent warming, the surface snow has settled into a cohesive slab over the weak layer and a new surface crust will likely form as temperatures drop.  

Periods of strong wind over the weekend, mainly from the southwest through northwest, have formed reactive wind slabs in exposed terrain at higher elevations. Ongoing wind on Monday will likely continue to develop reactive wind slabs where there is still loose snow available for transport. 

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 two 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.
  • Approach steep open slopes at and below treeline cautiously, buried surface hoar may exist.
  • Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.

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.