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Avalanche Forecast

Archived

Mar 14th, 2020–Mar 15th, 2020

Alpine
Natural avalanches unlikely, human triggered possible.
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 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.

Regions

Northwest Inland.

Strong northeast winds have formed wind slabs on atypical aspects. Watch for signs of warming and weakening snow from strong sun and rising temperatures. Buried weak layers and cornices could reach their tipping points.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to how the snowpack will react to the forecast weather.

Weather Forecast

Saturday night: Clear, decreasing northeast wind, alpine temperature -16 C.

Sunday: Clear, light northeast winds, alpine high temperature -5 C, freezing level 600 m.

Monday: Clear, light variable winds, alpine high temperature -1 C, freezing level rising to 2200 m in the southern half of the region.

Tuesday: Clear, light northeast winds, alpine high temperature 0 C, freezing level 1000 m. 

Avalanche Summary

Avalanches are expected to become more reactive with a rapid warm-up and intense solar radiation.

Over the past several days, large (size 2) human-triggered avalanches have been reported releasing in a drifted snow/weak interface combination and breaking 20-80 cm deep. These avalanches primarily occurred on leeward features above 1400 m. Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below. 

Last week, there were reports of large (size 2-2.5) human-triggered avalanches failing on the March 1st surface hoar layer. Over the past two weeks, avalanches have been reported on an earlier surface hoar layer from February 19th as well as a deep persistent slab failing on basal facets. This pattern highlights how shallow avalanches in the surface snow have the potential to strain multiple weak layers in the snowpack and release larger avalanches.

Snowpack Summary

Temperatures are forecast to rise from -25 C to -5 C in the span of 24 hours and with intense solar radiation on Sunday. This significant warm-up is expected to rapidly destabilize surface snow and cornices as well as add significant strain to buried weak layers. 

Over the past few days, strong easterly winds drifted the 15-30 cm of recent snow into wind slabs in a reverse-loading pattern. Wind slabs will likely be more reactive where this snow rests on a weak layer of surface hoar and/or a crust on solar aspects. Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below.

Loading from new snow and wind has made several deeper weak layers problematic over the past week. A surface hoar layer from March 1st may be found 40-80 cm deep, and another combination of surface hoar and sun crust from February 19 may sit 60-90 cm deep. These layers seem to be most sensitive to human triggering at treeline elevations. 

A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface. Below that, an early season crust/facet layer lurks at the base of the snowpack. Sunday's warm-up may have the potential to re-awaken these deeper layers.

Terrain and Travel

  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

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.

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.