Utah officials have issued a rare extreme avalanche danger warning for the Salt Lake region following significant snowfall.

On Wednesday morning, the Utah Avalanche Center advised people to avoid all avalanche terrain in the Salt Lake mountains, some of the most popular skiing destinations in the country, as avalanche forecasters issued their highest level of danger.

“Natural and human-triggered avalanches are certain,” the center said in a social media post. “Avalanches may run historic distances and create new avalanche paths.”

Utah has had only two days in the past four years with an extreme avalanche danger rating. This is the first time Salt Lake City has had one since 2013, which is as far back as the Utah Avalanche Center’s online records go.

The unprecedented warning comes after 60 inches of snow fell in parts of Utah in a 48-hour period this week, making the snowpack very unstable.

Forecasters have been sounding the alarm on avalanche danger in backcountry regions, outside of ski resort boundaries, for weeks. A dry start to the winter, with not much snowfall, created a very weak base layer of snowpack across the western U.S. As snow builds on top of that weak layer, avalanches are easier to trigger, and may also be wider.

So far this winter season, 25 people have died in avalanches in the U.S., according to the Colorado Avalanche Information Center. The deadliest was Feb. 6 in Utah, when four people died while skiing in the Salt Lake backcountry.

The most recent fatalities occurred on Sunday, when two people in separate accidents in Colorado and a third person in Montana died.

A search mission was underway in Northern Colorado Wednesday morning after a snowmobiler went missing in an avalanche the day before, according to the center.

Ski resorts employ avalanche mitigation measures on their trails. But one resort in Utah’s high-risk area even suggested customers wear their avalanche beacons — used to help locate buried victims — while skiing on the resort’s trails amid the “historic avalanche danger.”

“There isn’t any reason to go out of bounds today, you can find ample powder inbounds and we even suggest carrying a beacon inbounds today,” Brighton Resort, in Brighton, Utah, said in a social media post Wednesday. “Our ski patrol do their best to keep you out of harm’s way, but now it’s time to do your part to keep yourself and others safe.”