Five people have died in avalanches in the Rocky Mountains since Valentine’s Day, adding to a mounting toll amid an especially perilous season. Three deaths occurred in Colorado, with fatalities also reported in Montana and Wyoming.

The western United States is only halfway through its snowy season but has already matched the annual average for national avalanche fatalities. According to Avalanche.org, deadly slides have occurred across eight states this year, killing 27 people. Twenty-one of those deaths have occurred just since Jan. 30.

According to a statement from the Northwest Avalanche Center, the United States experienced more recreational avalanche fatalities in the first week of this month than during any other period in the past 100 years. This stretch included a slide in Utah that claimed four lives, while a fifth person clung to a tree to survive.

Eleven of the fatalities this season have been in Colorado. Ethan Greene, director of the Colorado Avalanche Information Center in Boulder, said this season has been very active for human-triggered avalanches, with nearly 600 of the at least 2,800 avalanches reported this winter in Colorado caused by people.

“We have had more activity than what we typically see,” Greene said. “I’d put [it] at a 1-in-10 sort of occurrence, meaning the most active I’ve seen in about 10 years.”

He said human-triggered avalanches began to spike during the last two weeks of January and into early February as frequent heavy snow stressed an already unstable snowpack. The West saw a profound atmospheric river event that resulted in nearly nine feet of snow in parts of the Sierras. That snow compounded an already unstable snowpack, priming this season for dangerous avalanche conditions.

“Avalanche danger doesn’t subside; it gets easier to trigger with each weather event,” Greene said. “The [season’s] snow cover is a layered structure, so it builds over time. With each weather event, whether that’s a snowstorm or a wind storm or a period of sunny weather with radiation exchange, avalanches tend to break at those layers.”

Although they’re difficult to predict, avalanches are easy to understand. Like snow sliding off your roof on a warm day, avalanches are nothing more than snow sliding down a steep incline ― but scaled to the size of a mountain slope.

Avalanches can release massive amounts of snow — as much as 230,000 cubic meters, or the equivalent of 20 football fields filled three feet deep — according to the National Snow and Ice Data Center.

“It can be [a] sunny day, and if you step in the right place, hundreds of feet of a crack could form, releasing thousands of tons of snow,” Greene said.

Avalanches occur most often on mountains with steep angles between 30 and 45 degrees. Slopes of this steepness are much more common in the western United States. A number of avalanche-monitoring organizations regularly test and track snowpack conditions and issue alerts as danger builds.

“We’ll go out and do mechanical tests to assess the qualitative properties of the snow,” Greene said. “What we’re going for is for the snow to break and for the crack to extend on its own.”

Based on these tests, along with expected weather, avalanche experts can forecast avalanche danger days ahead. Greene said it is critically important for backcountry enthusiasts to understand the risk before venturing onto slopes.

“There is a lot you can do,” he cautioned. “People shouldn’t be afraid to go into the backcountry, but know what to expect by checking local conditions and ensure what you’re planning to do matches the conditions.”

Given the recent winter weather from coast to coast, avalanche danger continues to be high in many western mountains this week.

Those interested in checking their local avalanche danger can find information at avalanche.org and follow links to local monitoring organizations that serve their area. Also, monitor the latest weather to determine whether the backcountry is safe before you venture out.

Kerrin Jeromin is an American Meteorological Society certified broadcast meteorologist with more than 12 years of forecasting experience and has covered everything from winter storms to hurricanes to natural disasters and major climate events. She received her bachelor’s degree in meteorology from Lyndon State College in Vermont.