Another season of forecasting has come and gone for Avalanche Canada, and though forecasts are done, the organization says backcountry users should still be wary of a major warm-up as spring progresses.
Even though spring has arrived, there is still the potential for winter-like storms, which can result in storm slab and wind slab avalanche issues. Avalanche Canada adds that cornices are also always an issue in the spring.
Additionally, according to an April 28 Avalanche Canada update there are currently two snowpacks in the region. Below 2000 metres elevation, spring has arrived, with regular melt-freeze cycles forming thick surface crusts and causing lower elevation snowpacks to melt away.
However, above 2000 metres, especially on north aspects where the sun doesn’t reach as much, there is still cold, dry powder stashes at high elevations that haven’t been affected by the influence of moderately high freezing levels and intense sunshine.
“Perhaps most importantly, the snowpack at alpine elevations has yet to experience the kind of sustained warming event we normally would have felt by this time of year,” AC said in the release.
Once that warming does occur, the crust and facet layer formed back in December that exists below the bottom of the snowpack will be tested. Only after that happens will spring conditions truly arrive.
Spring is challenging because of the wide variety of conditions that each have their own challenges they present to back country users.
The organization broke down the types of conditions people are likely to encounter into four scenarios.
The first spring condition is Cold and Snowy —when a winter storm rolls through in the spring, which is of course not unheard of. These storms can dump significant quantities of cool, dry snow in the alpine, with the snow usually transitioning to moist or wet snow at lower elevations and rain closer to valley bottom.
Cornice development can happen during these storms, as the warmer the snow, the more cohesive it becomes, especially when pressed or moved by the wind. Dense and reactive wind and storm slabs may also be found during and following these storms, particularly higher up the mountain.
Fortunately, winter storm and wind slabs settle more quickly and bond better than cold slabs, but the presence of melt-freeze crusts common in the spring, can inhibit and slow down the bonding process at the interface of new and old snow. Avalanche Canada says it’s important to remember that snow that is well-bonded when cool will destabilize rapidly when it first sees the sun, which is inevitable in the spring even if it is cold and snowy.
The next scenario is daily-melt freeze cycles, which is what happens during clear spring weather that develops a cycle of warming from sunshine during the day and cooling overnight. This can produce increasingly strong crusts on the surface of the snow, with dense snow below the crusts.
These conditions are very dynamic and can change dramatically throughout a 24-hour period. When frozen, supportive crusts lock the snowpack in place making avalanches unlikely, but travel is sketchy on the hard, slippery surfaces.
When temperatures rise, the frozen surface softens and a thin layer of wet, slushy snow forms on top, creating a condition known as corn snow, which is desirable for riding.
When frozen, these conditions don’t create high risk for avalanches, but as temperatures rise throughout the day and the slushy surface forms, loose wet avalanches are the primary concern.
An all melt, no freeze scenario, created by warmer than average temperatures with no overnight freeze, creates a progressively weak, wet and less cohesive snowpack. Without a frozen, supportive crust developing over night, the upper snowpack weakens and stability deteriorates rapidly with daytime warming.
Avalanche activity in this scenario is likely to be more intense, with a higher likelihood of large, destructive slab avalanches.
“As surfaces lose cohesion due to melting, loose wet avalanches become common in steeper terrain,” AC said. “Basically, the more the snow feels like a Slurpee, the more likely loose wet avalanches will become.”
The more the snowpack warms up and weakens, the more you need to be conservative with terrain selection, as this is not the best condition for hill climbing and you should reduce exposure to overhead avalanche terrain.
Finally, an all freeze, no melt scenario — a cooling pattern following several days of warming and potentially rain, freezes previously wet snow into a strong, supportive crust that locks the snowpack in place.
Deep, persistent weaknesses from early season or at the base of the snowpack are highly unlikely to trigger without a very large input like a cornice or explosives in this scenario.
For more information on all these scenarios you might encounter this spring, with terrain and travel advice for each, visit www.avalanche.ca/spring-conditions