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Avalanche Forecast

Archived

Jan 21st, 2022–Jan 22nd, 2022

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

Regions

Lizard-Flathead.

Strong wind and recent snow have formed reactive wind slabs.

Warm temperatures, weak cornices combined with a lingering deep persistent slab problem may equal large and destructive avalanches. Its a good time to approach the mountains cautiously and even expect surprises.

Confidence

Moderate - Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack.

Weather Forecast

A ridge of high pressure brings warming and sunny skies through the weekend. Alpine inversions may see temperatures as high as +1 degrees with valley bottoms seeing some cloud and cooler air pooling at the lower elevations.

Saturday: Mix of sun and cloud. Freezing levels rise through the day to 2000-2500 m with generally clear skies. Alpine temperatures near +1 C. Possible inversion may hold valley cloud and cooler temperatures down low. Ridgetop winds moderate from the northwest. Expect cooling overnight.

Sunday: Mix of sun and cloud with freezing levels hovering around 2000 m. Strong northwest wind. 

Monday: Cloudy with some flurries and strong ridgetop wind. Alpine temperatures -3 and freezing levels forecast to drop to 1000 m.

Avalanche Summary

On Friday, several wind slabs and storm slabs up to size 2 were reported. A natural cornice fall also occurred pulling a size 1.5-2 storm slab in the fans on the slope.  

Wind slabs may continue to be reactive on Saturday, especially where they have a poor bond to the underlying crust.

Most concerning is the warm weather this weekend. Warming, solar radiation, smaller surfaces avalanches, and cornice fall could trigger deep persistent slabs. This layer has been dormant but to no surprise could wake up. 

Snowpack Summary

Up to 15 cm of new storm snow fell Thursday with fairly light winds. Moderate to strong winds from the southwest on Friday formed fresh and reactive wind slabs. Wind slabs may be more reactive where they sit above older wind-affected and crusty surfaces. Around 2200 m and below, a crust caps the dense 15 to 30 cm of snow which has settled significantly with the past mild temperatures. The crust varies with aspect and elevation from thin (2 cm thick) in the alpine to (4 cm) thick below treeline. Below this, the midpack is well consolidated above the early December crust/ facet interface.  

The most notable layer of concern in the snowpack is a widespread crust that was formed in early December and is now down 100-270 cm. With warming last week this deep persistent slab problem woke up and produced several very large avalanches. The forecast warm weather and sunshine this weekend is concerning and we could see this avalanche problem wake up yet again. Warming, solar radiation, and cornice fall could all play a role in triggering deep persistent slabs. 

Terrain and Travel

  • Wind slabs may be poorly bonded to the underlying crust.
  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.
  • A moist or wet snow surface, pinwheeling and natural avalanches are all indicators of a weakening snowpack.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.