This new round of scorching temperatures began on Saturday. It will continue through early next week, will bring above-average temperatures to the Pacific Northwest region, the central Rockies, and South-central Canada. The epicenter of the heat wave will be in Montana, where parts of the state will be 20 to 25 degrees or more above historical averages. 

The Western US has been plagued with four heat waves this year. An extremely hot spring/beginning to summer has resulted in megadroughts, water shortages, and wildfires. 

"A major concern on Sunday and Monday is the prospect of dry thunderstorms, from the Sierra Nevada mountain range northward through much of northern Nevada, eastern Idaho, and central Montana. These storms could unleash cloud-to-ground lightning that ignites new blazes," Washington Post said. 

Wildfire statistics in California, through July 11, show a dramatic increase over the prior year, which may suggest a dangerous fire season is ahead. 

Tens of millions of people could experience temperatures topping 100 degrees across the Pacific Northwest, the central Rockies, and south-central Canada. The heat wave will intensify Sunday and continue through Tuesday. 

Steve Bowen, head of catastrophe insight at Aon, told Axios that "much of the West is a tinderbox right now and if dry lightning or human actions lead to even more fires, the potential for explosive growth is seriously concerning."