Backcountry Advisory 

Crusty surfaces increase avalanche hazard

In the spring, avalanche conditions are typically most stable in the morning, after a clear cool night which allows heat from the snowpack to escape into the atmosphere, resulting in the formation of a melt-freeze crust on the snow surface. This surface crust is what gives the snowpack its strength. In its absence an isothermal snowpack is extremely unstable and unpredictable. Exposure to the sun and warm temperatures will quickly break down any surface crusts resulting in isothermal conditions and an increasing avalanche hazard. This transition will often happen very quickly.

Cloud cover and/or warm temperatures will inhibit the release of heat from the snowpack into the atmosphere and prevent the formation of surface crusts. In this scenario, the avalanche hazard may remain significant even early in the day. Spring snowstorms are common. Although it may be raining at and below treeline, significant snowfall accumulation can occur in the alpine. Winter-like slab avalanche conditions can quickly return with the formation of windslabs overlying a smooth melt-freeze crust. These potentially unstable windslabs are more of a factor during and immediately after storms when you can expect to encounter mid-winter avalanche conditions. This type of instability can accelerate rapidly in the spring as the storm-snow layers moisten and begin to sluff with exposure to the sun, after which returning to the classic spring melt-freeze cycle.

Route planning is often much more critical in the spring than any other time of year. Your timing in crossing certain terrain features can be critical. Study the map closely and plan a route that takes exposure to the sun and daytime warming into consideration.

A typical spring touring plan looks like this: Start your day early (often before sunrise) to take advantage of the surface crust and easy traveling conditions. Plan to cross any sun-exposed slopes or lower elevation avalanche terrain early in the day. Stay well clear of any large overhanging cornices as they could fail at any time. Higher elevation, north-facing terrain can often remain quite winter-like throughout the day, so travel on these aspects later, when the crust is gone on other aspects. Plan to be finished by mid afternoon, when daily maximum temperatures have been reached and you can be enjoying your afternoon siesta!

Have fun and travel safely. This column will reappear next winter. Until then, the backcountry advisory for the areas adjoining the Whistler-Blackcomb ski area boundaries will be updated daily until June 4 at 604-905-7386.

— Whistler/Blackcomb Snow Safety

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