Avalanche Forecast

Issued: Feb 18th, 2022 4:00PM

The alpine rating is high, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Storm Slabs and Persistent Slabs.

Avalanche Canada zryan, Avalanche Canada

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Greatest snowfall amounts are forecast for the Monashees, where touchy storm slabs will build with heavy snowfall throughout the day.

Concern for a buried weak layer necessitates conservative terrain travel. Be aware of above and adjacent slopes.

Summary

Confidence

Moderate - Forecast precipitation (either snow or rain) amounts are uncertain. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Overnight on Friday, a frontal system enters the region from the north. Moderate to heavy snowfall is forecast for Saturday, with a clearing trend in sight for early next week.

Friday Overnight: Cloudy with a chance of light precipitation. Freezing level around 1700m. Moderate to strong southwesterly winds.

Saturday: A stormy day, 10-30 cm of new snow accumulation. Freezing level around 1000 m. Moderate to strong southwesterly winds.

Sunday: Mainly cloudy with light snowfall. Freezing level around 500 m. Light northwesterly winds.

Monday: A mix of sun and cloud with lingering flurries. Freezing level at valley bottom with alpine temperatures around -10 C. Light northwesterly winds. 

Avalanche Summary

Natural and human-triggered storm slabs are likely on Saturday.

On Wednesday, we received numerous reports of reactive wind slab avalanches mostly up to size 1.5 and several larger ones up to 2.5 from alpine features. These wind slabs ran naturally and were easily triggered by the weight of a skier. Explosive control also initiated some larger persistent slabs up to size 2.5. 

Last week, near-daily reports came in of human-triggered persistent slab avalanches on the January 30th weak layer, with the associated surface hoar layer taking both recreationists and professionals by surprise. Reactivity has tapered, but we would continue to be cautious as the snowpack is tested with new load. The most reactivity on this layer has been seen between 1600-2200 m in open areas in the trees, in cut blocks and on steep convexities.

Snowpack Summary

Continued snowfall throughout Saturday will add to 10-25 cm of recent storm snow above the old hard surface. This old surface is comprised of near-surface faceting, and in some areas surface hoar growth overlying a melt-freeze crust at lower elevations, a sun-crust on steep solar aspects, and hard wind-affected snow in the alpine and exposed treeline.

Below this, from 60-120 cm down exists a weak layer (surface hoar. facets, crust) that was buried at the end of January. This surface hoar has been the dominating feature in the past few weeks and remains reactive creating surprisingly large avalanches. This is most prominent in sheltered openings at treeline and below treeline.

The facet/crust layer that formed in early December is now buried around 120 to 250 cm. The last reported avalanche was on January 31 from a large explosive, and before that on January 23. Although unlikely, the layer could be triggered from a large load like a cornice fall or a shallower slab avalanche could step down to this layer. 

Terrain and Travel

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.

Problems

Storm Slabs

An icon showing Storm Slabs

The new storm snow will likely have a poor bond to the underlying old snow surfaces. Storm slabs will be reactive, especially on leeward slopes where deeper and stiffer slabs exist due to wind loading. 

Dry-loose avalanches may be seen from steeper slopes, especially where a firm crust exists below the new snow.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2.5

Persistent Slabs

An icon showing Persistent Slabs

A weak layer formed in late January may be found 50 to 120 cm deep in the snowpack. This layer consists of faceted snow, a sun crust on steep solar aspect, and most notably large surface hoar crystals in sheltered terrain.

Reactivity has begun to taper, but a significant amount of new load may cause this layer to 'wake up'.

Terrain features to be particularly cautious around include the lee side of ridges, openings in trees, cut blocks, and burns, especially on Northerly aspects where the layer may not be capped by a firm crust. 

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1.5 - 3

Valid until: Feb 19th, 2022 4:00PM