Avalanche Forecast

Issued: Mar 15th, 2020 4:00PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs, Persistent Slabs and Deep Persistent Slabs.

Avalanche Canada astclair, Avalanche Canada

A rise in freezing level with intense sun is expected to increase avalanche hazard. Be alert to signs that snow is warming and weakening, and be prepared to dial back terrain choices. Avalanches could potentially run full path.

Summary

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather. Uncertainty is due to difficult to forecast freezing levels.

Weather Forecast

Sunday night: Clear, light southeast winds, alpine temperature -14 C.

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

Tuesday: Clear, light east winds, alpine high temperature -2 C, freezing level 1000 m. 

Wednesday: Increasing cloud, alpine high temperature -1 C, freezing level 800 m. 

Avalanche Summary

Avalanches are expected to become more reactive with rising freezing levels and intense solar radiation.

On Saturday, there were multiple reports of large (size 2-2.5) cornices failing naturally. Several of these cornice falls triggered slabs on slopes below, and one triggered a deep slab avalanche releasing on weak snow near the ground.  

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. 

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 rising with intense solar radiation. In the southern half of the region near Smithers, pockets of above freezing air are forecast up to 2200 m on Monday. This warm-up is expected to rapidly destabilize surface snow and cornices as well as add significant strain to buried weak layers. Cornices have grown large with the recent weather, and a cornice failure has the potential to release very large avalanches on slopes below.

Over the past few days, strong easterly winds drifted the 15-30 cm of recent snow into wind slabs in a reverse-loading pattern (check out this MIN from HBMR). 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.  

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. The warming weather 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.
  • Avoid steep slopes when air temperatures are warm, or solar radiation is strong.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

Problems

Wind Slabs

An icon showing Wind Slabs

Strong winds have drifted recent snow into wind slabs that are possible to trigger, especially in areas where surface hoar sits at the interface. A significant warming event on Sunday is expected to rapidly destabilize slabs, initiate wet loose avalanches, and bring cornices to their breaking point. This activity is likely to be most prominent on steep slopes that receive direct day-time solar radiation (slopes that face south through west). 

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

40 to 90 cm of settled snow rests above several layers of buried surface hoar that have produced avalanches over the past week. These slabs have been most sensitive to triggering in sheltered areas at treeline. Warming is expected to increase the likelihood of triggering these layers.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1.5 - 2.5

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Forecast warming may aggravate the region's deep persistent slab problem. Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine. Cornice falls are a likely trigger for these slabs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2.5 - 3.5

Valid until: Mar 16th, 2020 5:00PM