Avalanches are the most complex aspect of mountain environments. Knowing when the snowpack is unstable – ready to avalanche – is extremely difficult. The most effective way to stay alive in avalanche terrain is to look for red flags. Red flags are clues to instability that indicate when the avalanche danger is increasing. Red flags are also known as bull’s eye clues in the book Snow Sense, by Jill Fredston and Doug Fesler. When decisions become difficult, avalanche professionals resort to red flags.

If you observe red flags, a general rule is to stay off avalanche terrain (30-45 degree slopes). To help put boundaries on red flags, avalanche professionals use red flag values, which are guidelines to help make decision about the red flags.

Recent Avalanches
This is nature’s way of screaming “DANGER!” If you see evidence of a resent avalanche, or see an avalanche happen, then you know the snowpack is unstable. Red flag values for recent avalanches are within the past 36 hours in maritime climates such as Washington, or the past 48 hours in continental climates such as Colorado. The question to ask is: do the conditions that created the avalanche still exist? In other words, are the weak layer and the trigger still present?

Wind
Wind forms avalanche slabs (a slab is a stronger layer of snow over a weaker layer of snow). If the wind is blowing, or has blown recently, then it has probably built avalanche slabs. Blowing snow can load weight onto slopes 10 times faster than precipitating snow. The red flag values are wind strong enough to move snow (about 15 miles per hour), for several hours or more, onto aspects where you will travel. If you have fine-tuned avalanche eyeballs, then you can see these wind-deposited slabs; where the snow has been scoured and where it has deposited.

Heavy Precipitation
The seasonal snowpack doesn’t like rapid change. The more it snows, and the faster it snows, the more unstable the snow becomes. Red flag values for precipitation are snowfall rates greater than one inch per hour or 12 inches in 12 hours. Another recipe for immediate avalanching is rain or wet snow falling on cold, dry snow.

Whoomphing
You are happily breaking trail when your heart rate jumps to 200 and you soil your Gore-tex. That’s from whoomphing – when the weak layer within the snowpack collapses, putting out a creepy “whoomph!” sound. Put that whoomph on a 38-degree slope and you have an avalanche. Sometimes these whoomphs will produce visible cracks on the surface of the snow. Red flag values for whoomphing are any human or natural-triggered whoomphs that propagate more than 10 feet. If you hear a whoomph then stay on low-angle slopes and away from avalanche terrain. When I hear a whoomph, I stop right there and bust out my shovel and dig to the layer that collapsed. Digging only takes three minutes and then I know my weak layer.

Rapid Temperature Rise
Once again, snowpack doesn’t like rapid change, especially over 32-degrees F. The red flag trend to watch out for here is rising temperatures during a storm, which causes an upside-down layer cake, with heavier layers on top. Another red flag value is prolonged warm periods above 32-degrees F, like in spring when the snowpack becomes saturated and avalanches rip to the ground. Also, an avalanche slab can form if the powder you’re skiing in the morning becomes wet and heavy during the day.

Probably the best online resource for avalanche education, including red flags, is the National Avalanche Center. To learn about red flags in the field, take a course from the American Association for Avalanche Research and Education.

Liked it? Share it!