During the last few weeks, large fires have initiated and grown over northern Alberta, resulting in massive smoke plumes (see the image below from one week ago). Here in Washington State, we experienced a few days of smoke aloft from these fires last week.
The setup for the fires earlier this month was nearly perfect. In early May, an intense ridge of high pressure developed over southern Canada (see a plot of the difference from normal heights at 500 hPa pressure below for 1-15 May). Red and orange indicate MUCH higher than normal pressure in the lower atmosphere (around 18,000 ft).