Cameron Peak Fire crews had a "successful day" fighting the largest wildfire in Colorado history Thursday but were bracing for conditions similar to those that led Wednesday's big fire run Friday.

In spite of this, a mandatory evacuation order has been called for the a stretch of U.S. Hwy. 34 through the Big Thompson Canyon west of Loveland due to the Cameron Peak Fire's continued southeast encroach.

Fire officials said winds have picked up, grounding fire suppression aircraft. The fire has increased south of Signal Mountain and is well-established in Miller Fork between Signal Mountain and Glen Haven, as winds continue to push the blaze south.

The blaze, which grew by more than 34,000 acres this week, is now 169,153 acres with 56 percent containment, according to the incident command site InciWeb.

After concern about whether high winds would prevent air resources from assisting firefighting efforts, three large air tankers and four Type 1 helicopters were able to get in the air and be "very busy" Thursday, the incident command team said.

The main focus for crews Friday is to limit any fire growth to the south. 

But firefighters are facing another day of challenging weather Friday despite a crisp morning in the Fort Collins area. A red flag warning has been issued for the fire area starting at 11 a.m. Friday and continuing through 8 p.m. Saturday. 

A red flag warning means warm temperatures, very low humidity and strong winds can combine to produce an increased risk of fire danger.

"Realistically, with a red flag condition and miles of open fire line, that fire may advance may occur today," operations section chief Paul Delmerico said. "... We're doing everything we can to make sure that we're prepared for a fire advance to the communities of Glen Haven all the way to Masonville.”

Conditions Friday will be reminiscent of those seen Wednesday, when the fire grew to become the largest in state history, Delmerico said. “We’re going to do the best we can to keep the fire from progressing south, but keeping in mind that public and firefighter safety is our No. 1 priority.”