New Snow, Old Problems

Date

jeudi 20 février 2025

Auteur

Ryan Buhler, Forecast Program Manager

Dangerous avalanche conditions are expected to return to parts of Western Canada as a series of storms brings mild temperatures, heavy snowfall, and strong winds. The incoming snow will bury a weak upper snowpack which developed during the periods of extended cold and dry conditions through January and February, setting us up for a period of instability. Because the exact timing, intensity, and track of each storm system remain uncertain, it will be important to regularly check the avalanche bulletin heading through the weekend. One thing is certain, it’s time to dial back our exposure to avalanche terrain.

An avalanche that failed on the late January weak layer near Fernie recently. New snow and warming are expected to increase the likelihood and size of avalanches failing on this weak layer. Source: MIN user SAMIRSOMJI

The Current Snowpack

Extended periods of cold and dry conditions through January and February brought stable avalanche conditions and relatively low avalanche danger to much of southern BC and Alberta, but they also caused the formation of weak layers of surface hoar, facets, and/or crusts in the upper snowpack. There are currently at least two major weak layers that we’re expecting to become reactive as this series of storms arrives.

Depending on your location, you likely saw one or more weak layers form during January’s dry spells, which were then buried by a big storm at the start of February. Although the storm brought up to 80 cm of new snow to the south coast and 40 cm to the southern interior, heightened avalanche danger was short-lived. This is because the weather returned to cold and dry, preventing the new snow from settling into a widespread slab over that weak layer. We did see lots of natural avalanche activity in specific areas where the wind formed cohesive wind slabs, as well as numerous human-triggered avalanches failing on this layer, but avalanches generally remained on the smaller side. The storm snow simply lacked the cohesion and slab properties that would allow for wide propagations and widespread large avalanches. Crisis averted–but not indefinitely.

The cold and dry February conditions compounded this situation and added a second weak layer of note, formed in much the same way as the first.

(Psst—want to know more about what makes a cohesive slab? We have a blog about that! What Is a Slab? Forecaster Blog)

The weak layer from late January is a prominent feature in the snowpack throughout southern BC. Credit: MIN User ALPINELIFESNOWPACKREPORT

The weather forecast shows substantial precipitation for many regions from Thursday through Sunday. Source: Environment Canada

What to Do With the Coming Changes

The weather forecast for the weekend shows a major warming trend, with freezing levels reaching upwards of 2000 m over the weekend in southern BC. For northern BC, freezing levels might not get quite as high, but the warm conditions will still have an impact. The warming alone has the potential to wake up the buried weak layer from late January as the snow above it finally begins to settle into a cohesive slab. 

For coastal regions, the problem should be obvious. Periods of heavy precipitation, along with strong winds and rising freezing levels, will create high avalanche danger, and natural avalanches should be expected. It’s not an unusual scenario. If you’re recreating in these areas, managing the problem is simple: avoid avalanche terrain. Steering clear of the storm slab problem means you’ll also be out of the way of any surprising deeper releases or step-down avalanches.

For the interior, conditions will be trickier to manage. Incremental loading means that each consecutive snowfall will increase the load over our weak layers as a new storm slab develops. At some point, we will reach a tipping point where widespread avalanche activity is possible, but when will it be? Given the uncertainty, we need to take a conservative approach. Watch your local avalanche forecast for increasing danger, and stay alert to changing conditions in the field. 

Wherever you’re recreating this weekend, your choice of terrain needs to reflect the changing conditions. As avalanche danger ramps up, we’ll be limiting our exposure to avalanche slopes and enjoying lower-angle terrain without exposing ourselves to overhead avalanche hazards.

This avalanche failed on the weak layer from late January in terrain impacted by the wind near Whistler early in February. The weak layer remains a concern throughout southern BC. Source: MIN user STERLING.CHRISTENSON

The incoming storm snow will bury a weak layer which formed during the recent period of cold and dry conditions in February. Credit MIN User ANDREW.SACHS1