How the Fort McMurray blaze spread, and why firefighters are at the mercy of the weather

CTV National News: Alberta weather outlook CTV Calgary Meteorologist Dave Spence breaks down the weather conditions expected in the next 24 hours for the Fort McMurray area.

Hundreds of firefighters are now on the ground in Fort McMurray, ready to battle a massive blaze that has already destroyed hundreds of homes. But at least one forest fire expert says crews are really limited in what they can do at this point, at least until the weather changes.

The fire is being fuelled by all the ingredients of vicious fire conditions: gusting winds, high temperatures, and low relative humidity that has essentially sucked away any moisture left in the brush.

Under those conditions, firefighters are simply at the mercy of the weather, says forestry expert Jen Beverly, with the University of Alberta’s Department of Renewable Resources.

“When you have extreme fire burning conditions like we have right now, there’s very little you can do in terms of effective methods for slowing or stopping the fire,” she told CTV’s Canada AM Thursday from Edmonton.

“What’s needed is a change in the weather.”

After a relatively warm and dry winter, the forest floor is covered in dead, dry leaves and pine needles, with not much spring growth to bring any humidity to the trees.

“We’re dealing with spring burning conditions, so we have extremely dry fuels that are very easily ignited,” Beverly said. “You combine that with wind and you get extremely fast rates of spread.”

Kerry Anderson, a fire research officer at Canadian Forest Services, told CTV News Channel that the extreme heat led to what meteorologists call an “inversion.”

That is, the air above the city had grown warmer than the cool air at the ground, acting like a lid on the air, much the way smog gets trapped in city air on hot summer days.

In this case, when the ground eventually warmed up, the inversion broke and caused what Anderson called a “convective column," creating winds that fuelled dramatic and unpredictable fire behaviour.

In order to put out the fire, fire crews need to eliminate one or more of the elements the so-called “fire triangle” of heat, oxygen and fuel.

Normally, crews might try removing fuel by clearing a swath of trees and brush ahead of the advancing fire line. They might also try driving the fire toward existing breaks in the fuels, such as a river or a highway.

But this fire is burning so big and so hot, it has shown repeatedly that it can “jump” highways and roads, and even find its way across the wide Athabasca River.

Beverly says fires “jump” through something called “spotting” and the science of convection.

“When you have a really intense fire, they have a lot of convective activity and those (winds) draw up embers from the fire and send them aloft in advance of the flames,” she said.

“They can jump over the barriers that would have otherwise restricted the spread of the fire.”

She said when those embers are thrown up with extreme winds, they can be carried through the air for distances of up to a kilometre or more and still be hot enough when they land to or spark another blaze – a phenomenon known as “spotting.”

It’s this “spotting” that allows forest fires to ignite rooftops in residential areas, even when the flames are still far off.

Anderson said fire crews can sometimes use heli-torches to fight the fire, which are helicopters that drop an ignited fuel mixture onto trees ahead of the fire line.

The deliberate burn helps to destroy any potential fuel for the wildfire ahead of its advancement, and the resulting convective winds can steer the fire in directions that firefighters want it to go.

Fire crews are also able to use water, fire retardant and dirt dropped from air tankers and helicopter buckets to try to cool the fire’s fuel and cut off its oxygen supply. But that will be difficult with this fire because it is so large and creating so much thick smoke, Beverly said.

“The smoke will definitely constrain the ability of aircraft to drop retardant and water,” she said.

“When there is a lot of smoke, that makes it difficult for them to identify and drop on the flaming fire front.”

Fire officials are also fully aware of the dangers of air drops, which may play a factor in their firefighting decision-making. In 2011, a pilot died while fighting the big fire near Slave Lake, Alta., when his firefighting helicopter crashed. Another pilot died last year in a water-bomber crash while fighting a wildfire near Cold Lake, Alta.

With files from The Canadian Press