One climatologist said "a perfect storm" of climate extremes primed the western U.S. for one of its worst fire seasons in a decade. Meanwhile, Canadian wildfire smoke fills the air.

Over 800 wildfires are burning through forests in Canada, prompting heavy smoke warnings for millions of Americans.

Heavy smoke from several large wildfires blazing in Canada and Minnesota is expected to engulf large swaths of the Midwest and Northeast U.S. this week.