The Rural Municipality of Wood Buffalo said people should leave immediately and head for a lodge about 20 kilometres north of the city.
Residents were ordered to flee several neighbourhoods and a trailer park that had been evacuated on Monday was on fire, said Carina Van Heerde, a reporter with Fort McMurray radio station KAOS.
She said the fire was advancing towards a Super8 motel and a gas station.
“It’s chaos on the roads. People are panicking. It’s gridlock on the roads. Flames are right next to a gas station,” she said.
Radio reporter J.D. Deraadt said the fire flared up suddenly.
“I’m feeling nervous. It’s a bit of a surreal thing to see it go from nothing to big. It’s very disheartening.”
Pictures posted on Twitter showed long lines of traffic and skies darkened by thick smoke as flames licked the edges of roads.
Crews had seemed to be making progress controlling the blaze, burning since the weekend, but the situation worsened quickly on Tuesday. By mid-afternoon people from three neighbourhoods were told to leave immediately.
Sandra Hickey, who lives in a neighbourhood under an evacuation order, said the situation changed quite quickly.
“When I got in the shower earlier today the sky was blue. When I got out. The sky was black,” Hickey said. “It was fast. The wind picked up and changed direction.”
Fire officials had already warned earlier in the day that rising temperatures and low humidity could help the fire grow.
“Don’t get into a false sense of security,” fire chief Darby Allen said during a media briefing before things worsened. “We are in for a rough day.”
Unseasonably hot temperatures combined with dry conditions have transformed the boreal forest in much of Alberta into a tinder box. The wildfire threat ranging from very high to extreme.
The Alberta government banned all open fires except in designated camping sites in forested areas due to the threat. There were about 30 fires burning in the province.
The province was calling in more reinforcements to Fort McMurray, including 100 more firefighters and a giant helicopter that can dump more than 2,000 litres of water at a time.
Crews were busy cutting down a line of unburned trees in the path of the flames to deny the fire fuel. Air tankers were dropping carpets of fire retardant while other aircraft released water.
Bernie Schmitte, an Alberta wildfire manager, said spruce and pine trees growing in the area burn easily and hot. He said changing wind conditions were also a concern.
“It is going to challenge us throughout the day,” he said. “At this point we cannot say that there is any part or portion of the line that is 100 per cent secure or contained.”