Avalanche warning signs to recognize after California's deadliest in history

This week's deadly avalanche north of California's Lake Tahoe involved a group of 15 backcountry skiers and left only six survivors.

The slide quickly became the deadliest in state history and the deadliest in U.S. history in nearly five decades. It happened in an area of the northern Sierra Nevada mountain range that experts say is "one of the snowiest places in the western hemisphere."

Among the 15 people were four ski guides and 11 clients, who were returning from a three-day touring trip at Frog Lake near Donner Summit when they got caught in the avalanche.

Backcountry skiers usually carry items like beacons, probes, shovels or satellite phones, which can be used to precisely locate someone buried under snow and get them out.

Law enforcement officials said the six survivors in this week's avalanche were able to get out from under the snow on their own accord and wait for rescue in blizzard conditions. Officials confirmed one of them used the SOS satellite service feature on iPhones to make the initial avalanche report and call for help.

Before navigating treacherous terrain like the backcountry of California's Sierra Nevada during a peak winter storm, experts suggest looking for a bevy of potential warning signs to avoid getting caught in an avalanche.

Check the forecast for avalanche warnings or watches

Before heading out, checking the forecast is a critical step in trip planning. Most North American avalanche centers, including the Sierra Avalanche Center, issue forecasts by 7 a.m. daily during the winter months. In addition to these technical advisories, you should monitor local news station weather forecasts, which provide essential context on incoming storm systems, travel restrictions, and emergency alerts that may impact safety.

Recent avalanches and unstable snow

If an area has experienced recent avalanches, it could be a sign that the snowpack there is unstable. Other signs of unstable snow include collapsing snow, which the National Avalanche Center (NAC) defines as "a sudden drop in the upper snowpack caused by the fracture of a lower snow layer." Air escaping from the weak layer may create a drum-like "whumpf" sound.

Has there been significant snowfall or rain in the area?

Significant snowfall or rain is one of the most common red flags for avalanche activity because it adds immediate weight and stress to the snowpack. A general rule of thumb used by experts is that one foot or more of new snow in a 24-hour period makes avalanche conditions likely to be dangerous.

Persistent slabs

Persistent slabs are considered a more dangerous type because they can be triggered weeks or even months after a storm has passed. These form when a cohesive layer of snow sits on top of a weak layer and can be released when the bond between the two breaks, according to the NAC. Unlike other types of avalanches, persistent slabs can be triggered remotely, meaning you could be standing on flat ground and trigger a slide on a steep slope above or beside you.

Persistent slabs are one of nine types of avalanches, which include storm slab, deep persistent slab, wet slab, wet loose, dry loose, cornice fall and glide avalanche.

Wind-blown snow

Strong enough winds can quickly carry and move snow, creating wind slabs even on bluebird days without new precipitation. As wind transports snow over ridges and crests, it deposits it into thick, dense drifts on the downwind side of the mountain. These wind-loaded slopes are often characterized by smooth, pillow-like appearances and are extremely sensitive to the weight of a skier or rider.  

Rapidly melting snow

A rapid rise in temperature is a major warning sign that the snowpack is losing its strength. Strong sun, a sudden thaw, or even rain can cause the snow to creep downhill or become saturated with water, leading to wet avalanches. If you see "pinwheels," or small snowballs rolling down the slope, or if you are sinking into the snow past your ankles in slushy conditions, it is a clear sign that the snow is melting too quickly to remain stable.