A wildfire burning in Larimer County Monday prompted officials to order an immediate mandatory evacuation of thousands of residents west of Loveland.

The U.S. Forest Service said a plane flew over the blaze, dubbed the Alexander Mountain Fire, and determined it had grown to about 950 acres as of 7:45 p.m. on Monday and was still growing.

The fire was zero percent contained at 7:45 p.m. and the cause was undetermined, according to the USFS.

Anyone along Highway 34 from Drake to Dam Store, including Storm Mountain and Palisade Mountain, was urged by the Larimer County Sheriff’s Office to evacuate immediately, according to a NoCo Alert.

Residents near Waltonia Road, Eden Valley to Sunrise Ranch, and Sylvan Dale to Ellis Ranch were also ordered to evacuate, the sheriff’s office said in a social media post on X.

A mandatory evacuation was also issued for residents off County Road 18E from Pole Hill to Pinewood Reservoir, the sheriff’s office said.

Voluntary evacuations were ordered for residents off of County Road 86C, north of Rustic, and for residents off of Glade Road, from Highway 34 north to Indian Creek, the sheriff’s office said.

The fire is burning near Storm Mountain in the Roosevelt National Forest, north of Highway 34, according to a tweet by the USFS.

It was measured by aircraft at 247 acres as of 12:45 p.m., according to the USFS.

As of 4:30 p.m., crews were mitigating the wildfire that has not stopped growing westward but it was not threatening any buildings, sheriff’s officials said at a news conference.

So far, authorities have contacted 2,635 people inside the evacuation zone, according to the sheriff’s office. 

Officials from Larimer County Natural Resources also announced that Horsetooth Reservoir is being utilized in support of wildfire operations and response and they are urging bystanders to avoid the area. 

Temperatures in Loveland were in the low 90s as the fire broke out, with 18% humidity, according to the National Weather Service. The high in the area was forecast to reach 98 degrees on Monday.

The weather service said that wind speeds in the area were variable at about 7 mph. 

Local engines and crews battled the blaze and air support was ordered, officials said.

Highway 34 has been closed between County Road 29 and County Road 43.

Evacuation centers were set up at Namaqua Elementary School, at 209 N. Namaqua Ave., and at the Estes Park Events Complex in Estes Park, at 1125 Rooftop Way.

Large animals that need to be evacuated can be brought to The Ranch, at 5290 Arena Circle in Loveland. Small animals, like cats and dogs, can be evacuated to the NoCo Humane Society at 3501 E. 71st St. in Loveland. 

Other operations that can help with large animals are the NoCo Humane Society, Horsealert.org, and 911 Horse Evacuation. 

Valerie Johnson and several other large animal evacuation specialists moved the horses at 2939 N. Co Road 31D to a location about 5 miles away.

“We have a convoy of horse trailers,” said Johnson. “The fire is running down the mountain, so it’s spreading pretty fast.”

She advised anyone with livestock in the area not to wait. 

Some websites suggested that livestock owners spray paint their phone numbers on the sides of their animals and let them loose, but Johnson said not to do that because of the flammable nature of the paint.

Instead, she said to “write your phone number on a waterproof tag and braid it” into the horse’s mane. 

Johnson, who worked the Cameron Peak fire in Larimer and Jackson counties in 2020, also said that putting a halter on a horse is a mistake because halters can get caught up on fences and trees. 

Area resident Steve Pierson saw smoke from the fire when it started just before noon.

“It started small but it blew up in a hurry,” he said, noting that the beetle kill is terrible in the area of the fire. 

Air support arrived on scene just after 2 p.m., the USFS posted on social media. It includes four helicopters, one large air tanker, aerial supervision, one multi-mission aircraft, and one additional Type 1 helicopter coming in from Wyoming.

Poudre Fire Authority posted on social media that it is responding, and the sheriff’s office said other partner agencies are also responding to help.

An information hotline was set up for the Alexander Mountain Fire at (970) 980-2500.

Denver Gazette reporters Noah Festenstein and Carol McKinley, and news partners 9NEWS, contributed to this story.