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Avalanche Forecast

Archived

Mar 10th, 2020–Mar 11th, 2020

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, 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.

The recent snow has added load to buried weak layers and may still be reactive to human triggers. Conservative terrain selection is recommended.

Confidence

Moderate - Uncertainty is due to the complexity of the snowpack’s structure.

Weather Forecast

TUESDAY NIGHT- Mainly cloudy with clear periods / west wind, 30-60 km/h / alpine low temperature near -12

WEDNESDAY- Cloudy with sunny periods and isolated flurries / west wind, 40-60 km/h / alpine high temperature near -11

THURSDAY- Cloudy with sunny periods / northwest wind, 20-30 km/h / alpine high temperature near -12

FRIDAY- Sunny / northeast wind, 30-60 km/h / alpine high temperature near -13

Avalanche Summary

Fresh snow and strong winds have promoted storm slab development, especially at upper elevations. Humans may be able to trigger these slabs, especially in wind loaded areas and in the trees, where a buried surface hoar layer may make these slabs more reactive.

Last week there were reports of wind slab, persistent slab (failing on the February 19 surface hoar layer), and deep persistent slab (failing on basal facets) avalanches. This pattern highlights loading and surface avalanches from recent stormy weather straining multiple weak layers in the snowpack.

Snowpack Summary

The storm on Monday night and Tuesday delivered 10-15 cm of new snow to the region with strong winds. This new snow is likely sitting on a weak layer of surface hoar, and/or a crust on solar aspects. There are likely widespread storm slabs at upper elevations within the new snow.

Loading from new snow and wind has made several persistent weak layers problematic over the past week. A combination of surface hoar and sun crust layers were buried on February 19 and currently sit 30-60 cm below the surface that 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.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.