North American heat wave spreads east as alerts affect more than 100 million

A record-breaking heat wave was spreading Tuesday from the Mountain West toward the eastern United States and Canada, with more than 100 million people under heat alerts.

Hot and dry conditions also contributed to intense wildfires in southwestern Ontario and northern Minnesota. Air quality was expected to worsen across parts of the Midwestern and Northeastern United States in the coming days.

The heat dome has already broken all-time temperature records in western states. Billings, Montana, reached 111F, or 44C, above its previous record of 108F. Salt Lake City reached 109F, above its previous record of 107F.

Extreme heat and humidity continued in the Mountain West, while the heat was spreading toward the more densely populated East Coast, which had already faced severe heat earlier in the month. Canada's Ontario province was also affected, while Montreal was affected to a lesser extent.

The US National Weather Service said above-average temperatures and dangerous heat were forecast to enter the Northeast on Tuesday, with the strongest heat expected Wednesday and expanding into the Mid-Atlantic. Cities from Richmond, Virginia, to Boston, Massachusetts, were forecast to see temperatures from the upper 90s to near 100F, with daily high temperature records at risk.

In Ottawa and Toronto, highs were expected to reach 100.4F on Tuesday, with humidity making conditions feel hotter. Environment and Climate Change Canada said hot and humid air can worsen air quality and push the Air Quality Health Index close to the high-risk category.

In Montreal, the sky turned yellow on Tuesday morning because of smoke from wildfires burning several hundred miles away in northern Quebec and northwestern Ontario.

Cooler temperatures were expected by the end of the week, but the heat dome was expected to remain over the US Mountain West for the rest of July. Heat domes are large high-pressure areas in which sinking air limits clouds and rain, allowing heat to build over days and weeks.

Marc Alessi, a climate scientist at the Union of Concerned Scientists, said heat domes are a natural part of the climate system, but said the current climate system is fundamentally different because of fossil fuel-driven climate change. He said warmer air and oceans are releasing more heat into the atmosphere.

Scientists from the World Weather Attribution group released an analysis saying the hot and humid conditions during the earlier East Coast heat spell around July 4 would have been virtually impossible without human-caused climate change.

Scientists also said a developing super El Nino in the equatorial Pacific could be affecting the heat dome. Record-warm sea surface temperatures in the central Pacific are shifting where tropical storms form and release energy, which can distort the jet stream over the western United States and help trap hot air.

US forecasters expect El Nino to peak between October and December at potentially record-breaking levels, with the strongest temperature increase to follow in 2027. Along with human-induced climate change, the last El Nino contributed to making 2023 the second-hottest year on record and 2024 the hottest on record.