After receiving reports of dozens of avalanches from popular backcountry areas across the state, Colorado avalanche officials are warning that conditions are trending toward a difficult-to-predict persistent slab problem.

State avalanche officials lifted a special avalanche advisory Sunday night, Dec. 1, that had been put in place after a snowstorm dumped more than 2 feet of snow on parts of the Colorado Rocky Mountains ahead of Thanksgiving.

But the Colorado Avalanche Information Center wrote in its forecast Monday, Dec. 2, that avalanche danger remains moderate to considerable throughout most of the state, including the northern mountain region. 

“You can trigger deadly avalanches nearly anywhere in the region,” Colorado avalanche officials wrote in the northern mountain forecast.

In recent days, backcountry observers have reported avalanches at Loveland Pass, Berthoud Pass, Bald Mountain near Breckenridge and other popular backcountry spots. Since Thanksgiving on Thursday, Nov. 28, the Colorado Avalanche Information Center has received nearly 20 field reports detailing avalanches in the Vail and Summit County region and more than 15 reports of avalanches on the Front Range.

On Nov. 30, backcountry skiers triggered a small avalanche on Loveland Pass on an east-facing slope near the tree line, according to the Colorado Avalanche Information Center. The skiers reportedly did not observe any cracking or collapsing in the area before the avalanche. Avalanche officials said that this demonstrates that just because a slope doesn’t give obvious signs of instability, doesn’t mean it won’t avalanche.

“As we move further away from the storm, avalanches will become more difficult to trigger, but also more difficult to predict,” Colorado avalanche officials wrote in a social media post Sunday, Dec. 1. “The avalanche conditions are headed towards the always scary lower-likelihood, higher-consequence setup with a Persistent Slab avalanche problem across most of the state.”

Persistent slab avalanches are responsible for most avalanche accidents, including about ⅔ of all avalanche fatalities in Colorado. A persistent slab problem forms when a persistent weak layer is buried by additional layers of snow, according to the Colorado Avalanche Information Center. These types of avalanches can be difficult to predict because they can occur days, weeks or months after the initial avalanche cycle. 

In the Flat Tops, Front Range and eastern Summit County, basal weak layers are the primary weak layer of concern because if a slide breaks in these deeper weak layers, it is likely to propagate and become a larger avalanche, the avalanche forecast states. This avalanche problem started way back when late October and early November storms dropped snow that then faceted on shady aspects and melted off sunny aspects, according to the Colorado Avalanche Information Center. Northerly and easterly aspects that held the most of this preserved snow are reportedly the most hazardous.

The 2-3 feet of snow received last week is now connecting terrain that previously wasn’t connected, and there have been reports of large avalanches connecting whole bowls and wrapping around terrain features, the avalanche forecast states. There have also been reports of avalanches triggered from a distance and from below.

While persistent slab avalanches pose the larger hazard, there have also been reports of wind slab avalanches in the northern mountains, according to the Colorado Avalanche Information Center.

In general, Colorado avalanche officials advise backcountry travelers to stick to slopes less than about 30 degrees that are sheltered from the wind. Avalanche danger could decrease to moderate in place throughout the week, but the hard-to-predict persistent slab problem will remain a danger, according to the Colorado Avalanche Information Center.

The most up to date avalanche forecast can be found at Avalanche.state.co.us.

“The bottom line is that you need to keep making conservative decisions,” Colorado avalanche officials wrote in the forecast. “Make sure you aren’t traveling below overhead hazards and be extra cautious about the slopes you commit to.”