Officials in the Canadian province of Nova Scotia say a day of blistering heat, strong winds and low humidity could lead to “extreme fire behaviour” as they work to control wildfires that have forced more than 20,000 people from their homes. Fire crews said on Thursday they were concerned about a phenomenon known as “crossover”, which occurs when temperature rises above 30C, humidity drops below 30% and winds exceed 30km/h. Officials warned that during a crossover event, fuels such as twigs and leaves on the forest floor could combust, making it easier for existing wildfires to spread. “We are looking at possible extremely fast and aggressive spread rates for firefighters on the ground,” said David Steeves of Nova Scotia’s department of natural resources. Steeves said the blaze near the community of Tantallon, which has destroyed more than 150 homes and displaced 18,500 people, has been 50% contained and has not grown. But with the extreme heat forecast, he said the situation outside Halifax remained dangerous. “We do not want to give a false sense of security so that folks think, ‘Well, everything is great in this particular area.’ That is not the reality,” he said. “The reality is that we are still dealing with a very, very volatile situation.” In addition to forcing residents from their homes, the fires have also seriously degraded the region’s air quality. Environment Canada has issued warnings for communities near the Barrington Lake fire, the largest wildfire in the province’s history, spanning more than 170 sq km. On Wednesday, the province also increased the fine for illegal burning, raising it to C$25,000 (US$18,500). Despite pleas from elected officials, multiple illegal fires have been reported in recent days. “You’re endangering yourselves, your families, your neighbours if you do that and you’re adding unnecessary undue stress to firefighters who have a lot on their hands at the moment,” Halifax’s mayor, Mike Savage, told reporters. “Think about other people, don’t be selfish. Don’t be stupid. Think about how it would feel to be personally responsible for the situation that we’re seeing unfold right now.” On Wednesday, the Nova Scotia premier, Tim Houston, said he had asked the federal government for military firefighters, ignition specialists, firefighting equipment including hoses and foam, as well as trucks and helicopters. To help combat the blazes, 100 US firefighters are headed to Nova Scotia over the weekend, officials in Ottawa said at a Thursday morning briefing. Officials also said nearly 200 fire fighters from South Africa are ready to assist in the province, although they might be shifted to areas of higher need. On Wednesday, officials in Alberta evacuated the entire northern Alberta community of Fort Chipewyan, using aircraft and boats to move nearly 1,000 people. Federal officials outlined the devastating scope of the country’s spring wildfire season, with an “unprecedented” 2.7m hectares of forest that have already burned. More than 80 fires across the country are considered out of control. The natural resources minister, Jonathan Wilkinson, said it was a “simple fact” that Canada was experiencing the effects of climate change, which experts agree worsens fire risk. In Nova Scotia, a mild winter with little snowfall as well as a dry April set the stage for tinder-like conditions in the region’s boreal forests. But fire crews are hopeful that the cooler temperatures forecast and rains will help. “We need Mother Nature to get onside with us on this one,” said Steeves.