What is a heat dome and why is it so deadly?
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 for several days. The Oregon state medical examiner said that the states death toll reached at least 79, with most occurring in Multnomah County, which encompasses Portland. In Canada, British Columbias chief coroner, Lisa Lapointe, said her office received reports of at least 486 sudden and unexpected deaths over a five-day period. Normally, she said about 165 people would die in the province in that time frame. 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 heatthen it was consumed by a wildfire, 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. 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 in 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. Whats 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 theres 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 regions 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 June 28, Portland hit 116 degrees, one degree less than Vegas all-time max. As forecasters warned of a record-breaking heatwave 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 during the heatwave. Many of the dead were found alone, in homes without air conditioning or fans. Some were elderlyone as old as 97. Among the dead was a farm laborer who collapsed 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 states 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 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 heatwaves is 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 dont 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 Seattles Harborview Medical Center, which treated an unprecedented number of severe heat-related cases. We need to be really augmenting our disaster response.