SALEM, Ore. (NEXSTAR) — Hundreds of people are believed to be dead after temperatures soared in the Pacific Northwest and Western Canada, fueled by a “heat dome” above the region.

The temperature in Oregon and Washington has since fallen, but an excessive heat warning remained in effect for parts of the interior Northwest and western Canada Thursday.

The death toll in Oregon alone reached 79, the Oregon state medical examiner said Thursday, with most occurring in Multnomah County, which encompasses Portland.

In Canada, British Columbia’s chief coroner, Lisa Lapointe, said her office received reports of at least 486 “sudden and unexpected deaths” between Friday and Wednesday afternoon. Normally, she said about 165 people would die in the province over a five-day period.

She said it was too soon to say with certainty how many deaths were heat related, but that it was likely the heat was behind most of them. A town in British Columbia recorded 121-degree heat – then it was consumed by a wildfire Thursday, sending residents fleeing for their lives.

Washington state authorities have linked more than 20 deaths to the heat, but authorities said that number was likely to rise.

How does a heat dome work?

A heat dome is formed when scorching summer air becomes trapped under an atmospheric lid, according to the National Oceanic and Atmospheric Administration (NOAA). A main cause of the dangerous phenomenon is a “strong change in ocean temperatures from west to east in the tropical Pacific Ocean during the preceding winter.”

NOAA researchers say that temperatures in the western Pacific have risen more than those of the eastern Pacific, and the temperature difference causes warm air to move east. Northern jet stream forces trap the air and move it over land where it sinks, creating heat waves.

Typically, heat domes set up shop right over the Four Corners region in the very arid section of the southwest U.S. What’s troublesome with the heat domes is the process of descending air. That downward motion on the atmosphere compresses air molecules which in turn get excited and warm rapidly at the surface.

If there’s nothing to push the heat dome away, it will just hang over the area, creating a cycle of very hot days with warm air trapped overnight.

Heat far above the region’s norm

The easterly wind fueling the heat dome effect brought Las Vegas-like heat to a state whose residents are largely unaccustomed to needing air conditioning. On Monday, Portland hit 116 degrees, one degree less than Vegas’ all-time max.

As forecasters warned of a record-breaking heat wave in the Pacific Northwest and western Canada last weekend, officials set up cooling centers, distributed water to the homeless and took other steps. Still, hundreds of people are believed to have died from Friday to Tuesday.

Many of the dead were found alone, in homes without air conditioning or fans. Some were elderly — one as old as 97.

Among the dead was a farm laborer who collapsed Saturday and was found by fellow workers at a nursery in rural St. Paul, Oregon. The workers had been moving irrigation lines, said Aaron Corvin, spokesman for the state’s worker safety agency, Oregon Occupational Safety and Health, or Oregon OSHA.

In Bend, Oregon, a scenic town next to the snowy Cascade Range, the bodies of two men were found Sunday on a road where dozens of homeless people stay in trailers and tents.

Volunteer Luke Richter said he stepped into the trailer where one of the men, Alonzo “Lonnie” Boardman, was found.

“It was very obviously too late. It was basically a microwave in there,” Richter told Oregon Public Broadcasting.

Weather experts say the number of heat waves are only likely to rise in the Pacific Northwest, a region normally known for cool, rainy weather, with a few hot, sunny days mixed in, and where many people don’t have air conditioning.

“I think the community has to be realistic that we are going to be having this as a more usual occurrence and not a one-off, and that we need to be preparing as a community,” said Dr. Steven Mitchell of Seattle’s Harborview Medical Center, which treated an unprecedented number of severe heat-related cases. “We need to be really augmenting our disaster response.”

This week’s heat wave was caused by what meteorologists described as a dome of high pressure over the Northwest and worsened by human-caused climate change, which is making such extreme weather events more likely and more intense.

Seattle, Portland and many other cities broke all-time heat records, with temperatures in some places reaching above 115 degrees Fahrenheit (46 Celsius).

The Associated Press contribute to this report.