Avalanche Forecast
Issued: Mar 20th, 2022 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
zryan,
Avalanche Canada
Make conservative terrain choices as recently buried weak layers have been reactive in the Selkirks and may exist in other areas.
Crusts on solar aspect are becoming more the norm. Dry snow will exist on the more shaded aspects but watch for wind slabs at higher elevations.
Summary
Confidence
Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.
Weather Forecast
SUNDAY NIGHT: Mainly cloudy, light flurries. Light to moderate westerly winds. Freezing level dropping to 700 m.
MONDAY: Cloudy, isolated flurries, up to 5 cm of accumulation. Light to moderate westerly winds. Freezing level rising to 1500 m in the afternoon.
TUESDAY: Cloudy with light flurries. Light to moderate westerly winds. Freezing level rising to 3000 m in the afternoon.
WEDNESDAY: Mainly cloudy, isolated flurries. Moderate to strong southwest winds. Freezing level rising to 2500 m in the afternoon.Â
Avalanche Summary
There have been some notable human-triggered avalanches in the Nelson area and in the South Columbia region in the past few days (read more in this blog). These have been larger slab avalanches above weak layers, failing on either a buried sun crust on solar-aspects, or surface hoar on shaded-aspects. This MIN report from an incident on Saturday near Whitewater is an example of this problem.
On Friday, an operator south of Nelson reported a large slab avalanche, triggered by a helicopter landing initiating a cornice failure. This avalanche stepped down to a deeper weak layer in the upper snowpack, suggesting that these deeper layers are still possible to trigger with large loads.
Snowpack Summary
10 to 15 cm of recent snowfall overlies last week's 30 to 50 cm of denser, more settled snow. New snow depths taper significantly with elevation, with moist and crusty surfaces below 1800 m and on south aspects into the alpine.Â
Last week's snow is bonding poorly to underlying layers in many areas. In the Selkirks, especially north of Nelson, it appears this snow is bonding poorly to the underlying sun crusts on solar aspects and surface hoar on shaded-aspects. In the Monashees, it appears this snow has formed a stronger bond to underlying crusts. There are several other crust layers found 50 to 100 cm deep, and the snow is mostly well bonded to these crusts.
Terrain and Travel
- Be aware of the potential for large avalanches due to the presence of a persistent slab.
- Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
- Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
- Use ridges or ribs to avoid areas of wind loaded snow.
- Minimize your exposure time below cornices.
Problems
Persistent Slabs
Last week's 30 to 60 cm of storm snow has formed a persistent slab above weak layers in some areas. There have been reports of a poor bond to underlying sun crust layers on south slopes and surface hoar crystals on north slopes, especially north and south of Nelson in the Selkirk Range. At this point, we are uncertain about how long this problem will take to heal and suggest a more conservative approach to terrain to handle this uncertainty.
Several other crust/facet layers exist in the upper 150 cm of the snowpack. Human-triggered avalanches on these layers are unlikely, but large triggers such as a cornice failure, or a smaller avalanche stepping-down to these layers is possible. Be extra cautious during period of rapid loading, significant warming, or strong solar radiation.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Wind Slabs
10-25 cm of recent snowfall and southwest winds are expected to form wind slabs in lee features in the alpine and treeline.
Cornices have grown significantly in the past two weeks. Give them a wide berth when travelling on ridge-lines and minimize overhead exposure to large cornices, especially when the temperature is warm or solar radiation is strong.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 21st, 2022 4:00PM
