The heat dome, an area of high pressure in the atmosphere, was visible on satellite imagery on Tuesday as an area of clear sky over the middle of the country, where the surrounding air moves clockwise. On the ground, it manifests as a hot, humid and smoky stew that can make just stepping outdoors an unpleasant task.<\/p>\n
Across much of the United States and southern Canada, weather officials have placed tens of millions of people under heat warnings, with high temperatures from the upper 90s to above 100 degrees expected in many places on Wednesday.<\/p>\n
The National Weather Service said that nearly 100 million people from the Dakotas into New England would be under its \u201cmajor\u201d or \u201cextreme\u201d warning levels for dangerous heat. Environment Canada, the country\u2019s weather service, said that, through Wednesday, southern Ontario would be under an orange alert, the second most severe in its three-tier system.<\/p>\n
At least 17 wildfires spanning thousands of acres have forced evacuations across Minnesota, according to state officials, as flames spread through the state\u2019s northeastern wilderness and into Canada.<\/p>\n
Erik Evans, a spokesman for the Minnesota Department of Natural Resources, said that, as of 1 p.m. on Tuesday, the five largest fires combined were estimated to be scorching at least 30,000 acres of land. So far, fire officials have not been able to contain any of those fires, he said.<\/p>\n
The fires prompted the U.S. Forest Service to close part of the Boundary Waters Canoe Area in Superior National Forest<\/a>, a vast network of lakes and forest along the U.S.-Canadian border. The agency\u2019s regional office said that three active fires were burning inside the Boundary Waters, and that a fourth had ignited on Monday just outside the protected area.<\/p>\n Two of the largest blazes \u2014 the Bear Trap and Thumb fires \u2014 have crossed the border into Canada, officials said.<\/p>\n \u201cFire management anticipate another very high fire danger day today with increased fire activity and fire growth,\u201d the agency said.<\/p>\n Already this week, state authorities around the Great Lakes have issued alerts for poor air quality. In northeastern Minnesota, the state\u2019s pollution control agency<\/a> said that the air quality would likely remain \u201cvery unhealthy for everyone\u201d through Thursday. Just across the border in southern Ontario, Environment Canada issued an orange-level alert on Tuesday for air quality and urged people to avoid spending time outside.<\/p>\n The heat dome has slowly been crossing North America over the past several days. It singed the Mountain West with record heat in places like Salt Lake City, where official readings surged to 109, two degrees above the previous high temperature recorded in 1960, 2002, 2021 and 2022.<\/p>\n Across the Northern Plains, the heat is setting consecutive days of daily records, and it could last through the end of the week.<\/p>\n New York and other cities across the Northeast could set daily records on Wednesday, forecasters warned.<\/p>\n That is, unless the smoke gets worse. If the smoke thickens, which some forecast models suggest that it will, the temperatures may not climb as high because the smoke could filter the sunlight throughout the day, according to forecasters in the New York City weather office.<\/p>\n In addition to the heat and the smoke, there is a third potential danger from the heat dome: Along its northern edge are what meteorologists call \u201cridge-riding storms.\u201d These intense thunderstorms form along the boundary of warm, moist air and cooler, dry air, and they can cause powerful winds, heavy rain, hail and even tornadoes.<\/p>\n At the start of this month, at the tail end of an earlier record-breaking heat wave, similar storms raced through rural Wisconsin, surprising boaters and leading to the deaths of three children<\/a>.<\/p>\n
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