An air tanker approaches a wildfire in the southeastern Interior. (B.C. Wildfire Management Branch)

B.C. firefighters are battling a surge of new forest fires this week after a huge thunderstorm triggered at least 20 new blazes on northern Vancouver Island.

Lightning strikes left dozens of "hangover" fires from Denman Island to Alert Bay, and dozens more could flare up when the weather dries out later this week, said Donna MacPherson, fire information officer for the Coastal Fire Centre.

"When lightning comes through like this, the strikes that go through the trees can start the trees on fire from the inside out, or they can catch the roots on fire underground," said MacPherson.

"What we're anticipating is as soon as this rain dries out, which is probably going to be about three or four days from now, we'll be getting lots more — and lots more," she said.

Lightning also sparked a wildfire in the Noorish Creek area near Mission. Fifty-three firefighters, seven helicopters and five air tankers were fighting the fire high in the mountains east of Vancouver.

Helicopters struggled with poor visibility caused by low cloud cover, and crews were unable to bring in heavy equipment because the slope was too steep. Still, officials expect fire crews to make good progress.

Elsewhere in the province, a forest fire in the southeastern Interior discovered on Sunday has been contained by fire crews.