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For the first time since forecasting began in 1973, the  (CAIC) has predicted extreme avalanche danger in four backcountry zones. The affected zones are Aspen, Vail and Summit County, the Sawatch Range, and Gunnison.

Avalanche conditions are exceptionally dangerous right now, says CAIC forecaster Spencer Logan. “[Avalanches] are running bigger and further than we’ve ever experienced.”

Logan says that the current storm cycle has caused some of the biggest slides in the past 60 years.

The late-season storm cycle that swept across Colorado this week piled thick, wet snow on top of an already unstable snowpack. Persistent weak layers left over from conditions earlier in the season can produce avalanches days, weeks, or even months after storms, and Colorado is dealing with them statewide. According to the CAIC, that combination of heavy snow (the kind that makes really good snowballs) and weak lower layers makes all sizes of slides more likely—and catastrophically large ones especially so.

The organization rates avalanche danger on a  from low to extreme. An average season won’t typically see extreme danger in a single zone. And in the past five years, only once has the CAIC forecasted extreme danger. According to Logan, this is could be a once-in-a-lifetime event.

Today’s extreme warning indicates that large avalanches are all but certain. “A lot of places that we think of as being relatively safe are not safe right now,” says Logan. “Avalanches are running all the way to valley floors.”

Traveling anywhere near avalanche terrain is really dangerous, even if that terrain has a low angle or is below tree line. Backcountry roads or trailheads near avalanche paths that are typically safe should be avoided. The Colorado Department of Transportation has even closed down large sections of Interstate 70 between Frisco and Vail.

Friday’s forecast in much of Colorado’s mountains will be reduced to high, meaning natural slides are likely and human-triggered slides are very likely, and experts recommend staying out of avalanche terrain entirely. Logan says that conditions can change rapidly.

“More storms are on the way, and weak layers can be unpredictable,” says Logan.

His advice on staying safe? “Avoid the backcountry.”