Warning Signs of a Weakening Snowpack

Identifying the warning signs of snowpack instability is a critical component of safety for those traveling in mountainous terrain. Avalanches, defined as the rapid flow of snow down a hill or mountainside, can occur on any steep slope given the right conditions, and understanding the red flags that indicate a weakening snowpack is essential for risk assessment.

According to the National Weather Service, three primary factors are required for an avalanche to occur: a slope generally steeper than 30 degrees, an unstable snowpack, and a trigger. Triggers can include new snowfall, wind, or the presence of people.

Physical Indicators of Instability

Several audible and visual cues can signal that a snowpack is unstable. One of the most prominent signs is a whumpfing sound heard while walking, which indicates the snow is settling and a slab may be released. Other signs of unstable snow include cracking or collapsing, and drum-like sounds.

Visual evidence of previous activity is also a major warning. Seeing a natural avalanche occur or finding evidence of previous slides indicates that the current snowpack is unstable. A moist or wet snow surface and pinwheeling are indicators of a weakening snowpack.

Loading and Snowfall Rates

The process of adding weight to the snowpack, known as loading, occurs through rainfall, snowfall, or wind-transported snow. Avalanche conditions are sensitive to both the total amount of loading and the rate at which it is added. Rapid loading increases the likelihood of an avalanche, as a weak snowpack may fail if too much weight is added too quickly.

Avalanche.org identifies several red flags regarding excessive loading over a 24-hour period, including:

• One foot of new snow
• One inch of snow water equivalent (SWE)
• Heavy wind transport

Rain also contributes to loading and adds stress to the snowpack, which can make the snow unstable for several days.

Temperature and Environmental Factors

Temperature fluctuations play a significant role in snowpack stability. Significant warming or rapidly increasing temperatures can elevate the risk of slides. Extended periods of temperatures above freezing, strong solar radiation, or rain can cause avalanche danger to rise through rapid melting.

Wind is another significant factor in avalanche activity. Wind-blown snow creates drifts and cornices on steep slopes. Safety guidelines advise avoiding travel on slopes located below cornices.

Long-term and Human Factors

Some instabilities persist longer than others. Persistent slabs may remain a threat and can be triggered weeks after a storm has passed. Because of this, checking local avalanche forecasts is necessary to determine if such slabs exist in a specific area.

Human activity is frequently a catalyst for these events. The National Weather Service reports that in 90 percent of avalanche incidents, the slides are triggered by the victim or someone within the victim’s party.