EAGLE COUNTY, Colo. — Multiple avalanches closed highways in the High Country for most of the day Thursday, including one that buried four cars late in the afternoon. 

Everyone who was in those cars has been accounted for and is safe, according to the Colorado State Patrol. No one was injured in the earlier slides. 

RELATED: 4 cars buried by avalanche on Highway 91

Other highways were closed due to mitigation work and adverse weather conditions.

Here are the most recent updates on each closure:

I-70 | OPEN Frisco to Vail

Interstate 70 was closed for hours in the High Country due to adverse conditions, avalanche concern and mitigation work. The highway reopened in both directions shortly before 7 p.m. on Thursday, but the Traction and Passenger Vehicle Chain Laws remain in effect. 

The Traction Law requires that vehicles have snow tires, tires with mud/snow designation or four-wheel drive. All tires must have a minimum of one-eighth inch tread.

The Passenger Vehicle Chain Law requires that all vehicles on the road have chains or an alternative traction device (like AutoSock).

HWY 91 | OPEN Copper Mountain to Leadville

Another avalanche on Thursday afternoon closed Highway 91 from Copper Mountain to Leadville. According to Colorado State Patrol, four cars were buried in the natural slide but no one was injured. 

The highway reopened at about 8 p.m. 

avalanche
CSP

LOVELAND PASS | Closed

U.S. 6 over Loveland Pass is closed due to adverse conditions, according to CDOT. 

MONARCH PASS | Closed

Follow the latest road conditions and closures here:

SKI RESORTS

RELATED: Disney slide: Avalanche mitigation causes snow to pile onto highway for first time since 1957

Arapahoe Basin closed for the day due to extreme avalanche concerns and the U.S. 6 closure. 

Copper Mountain delayed their opening today as well. According to a tweet at around 11:30 a.m. Copper Mountain, said some lifts were operating.

RELATED: A-Basin closed Thursday due to extreme avalanche danger

In a Facebook post Thursday morning, the Colorado Avalanche Information Center (CAIC) said four backcountry zones have gone to extreme (5 of 5) Avalanche Danger for the first time since the 10-zone forecast format began. 

WATCH BELOW: Colorado Avalanche Information Center elevates avalanche danger to 'extreme'

Historic avalanches are already occurring, the CAIC said. It urged people not to travel in or near avalanche terrain. 

Avalanche warnings are in place for the Front Range, Steamboat and Flat Tops, Vail and Summit County, Sawatch, Aspen, Gunnison, Grand Mesa and San Juan Mountains.

The last time an Extreme Avalanche Danger warning was issued was on Jan. 11, 2017, according to the CAIC.

The decision to elevate to extreme is subjective by the forecasters, but generally, when avalanche danger is extreme, it means the probability of avalanche is almost "certain" and the size would be "historic", according to the CAIC.

On Sunday there were two avalanches along I-70 in the Officer's Gulch area, which is about 10 miles east of where the slide happened Thursday morning. The second slide on Sunday covered the lanes of the highway and swept vehicles out of the eastbound lanes and into the median.

RELATED: 'Just hold onto something': Family describes surviving avalanche that swept them off I-70

On Tuesday, CDOT stopped traffic on I-70 to allow for avalanche mitigation and a slide that the agency triggered came down and covered that highway with several feet of snow. A large section of the highway was closed in both directions for about 9 hours. US 40 was also closed on Tuesday for avalanche mitigation work.

RELATED: I-70 reopens between Georgetown and Vail Pass after 9 hours of avalanche work

SUGGESTED VIDEOS | Local stories from 9NEWS