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Avalanche Forecast

Archived

Mar 18th, 2020–Mar 19th, 2020

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

Regions

Purcells.

Wind slabs may still be found and deeper layers may still persist. Make observations and assess conditions continually as you travel.

Confidence

Moderate - Uncertainty is due to limitations in the field data.

Weather Forecast

Wednesday night:1 Partly cloudy, light east wind, alpine temperature -2 C, freezing level valley bottom.

Thursday: Mostly cloudy, isolated flurries with trace accumulations, light east wind, alpine temperature -7 C, freezing level 1400 m.

Friday: Mix of sun and cloud, light northwest wind, alpine temperature -6 C, freezing level 1700 m.

Saturday: Mostly clear, light southwest wind, alpine temperature -6 C, freezing level 1700 m.

Avalanche Summary

Over the past several days, avalanche activity was reported as natural loose wet avalanches (size 1-2.5) running in steep, sun-exposed terrain in the alpine. One large (size 2.5) cornice failed as a result of explosive mitigation.

On Monday, there was a report of a skier remotely triggering (from a distance) a large (size 2) deep persistent slab avalanche at 2600 m on a northwest aspect. The avalanche was triggered from a thin spot, was 40-80 cm deep and ran on depth hoar near the base of the snowpack.

The number of data sources for the region is diminishing as professional operators close. If you're out in the mountains, consider sharing your observations via the Mountain Information Network. Heightened diligence and conservative risk management is recommended at this time.

Snowpack Summary

Previous moderate to strong northeast and east wind redistributed snow and formed wind slabs that may remain possible to trigger in isolated areas. With recent sun and relatively warm temperatures, expect to find either moist surface snow or crust on slopes facing the sun. New snow from last week may sit on a weak surface hoar layer and a sun crust on steep solar aspects. 

A weak layer of surface hoar buried February 22 may be found 50-100 cm deep. Sheltered north, northeast, and east facing slopes near treeline are the most likely locations to find this layer. The last reported avalanche activity on this layer occurred March 11th. There is a low likelihood of triggering an avalanche on this layer, but the consequences of doing so would be high. Read more about surface hoar on our forecaster blog.

Deeply buried facets lurk near the bottom of the snowpack. This layer has been responsible for sporadic deep persistent slab avalanches, usually triggered from shallow, rocky start zones.

Terrain and Travel

  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Small avalanches can have serious consequences in extreme terrain. Carefully evaluate your line for wind slab hazard before you commit to it.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Make observations and assess conditions continually as you travel.

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.

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.