Extreme heat will greet fans and players this week at the World Cup, as an imposing "heat dome" settles over the central and eastern United States - plus parts of Canada - as the knockout rounds progress.

The weather phenomenon known as a "heat dome" - a large area of high pressure that traps heat and humidity - could lead to dangerously high temperatures, with heat indices set to hit 105 to 115 degrees Fahrenheit in parts of the Midwest and East Coast, according to the US National Weather Service.

Those conditions are set to extend through the Fourth of July holiday weekend in the US, which is celebrating its 250th anniversary, with matches from Toronto to Kansas City to East Rutherford, New Jersey and Philadelphia all feeling the heat.

"Even after the sun goes down, it's still going to be very hot," said AccuWeather Senior Meteorologist Alan Reppert. "We're at a pattern that's really going to be hot during the good portion of the afternoon and even into the evening hours."

"This week is going to be very, very hot," said Geoff Cornish, assistant chief video meteorologist for the weather forecasting company AccuWeather. "This is going be a significant heat wave, the likes of which we don't see in every single year."

Concerns about extreme heat during the games taking place in the US, Canada and Mexico have been mounting for months. Worsening climate change, caused by burning oil, gas and coal, is fueling dangerous heat across the globe. In the US, heat kills more people every year than all other weather-related events, including hurricanes, flooding and tornadoes, according to the National Oceanic and Atmospheric Administration.

Reppert projects that New York, which will welcome fans for a Round of 16 match in nearby New Jersey on July 5, will see temperatures reach their highest levels since 2013. He added that even matches played in the evening could be affected.

"The sun will be down," he said. "That will help make things better, but it's still going to be hot."

Here's a preview of how hot it will get, how cities, stadiums and Fifa prepared for this threat, and how to stay cool.

How hot will it get this week?

The National Weather Service issued an extreme heat watch for Philadelphia and surrounding regions starting Wednesday afternoon and into Saturday evening, with dangerously hot "feels-like" temperatures, another term for the heat index, up to 110 F (43.3 C) during the daytime. As a result, Fifa's Fan Festival in Philadelphia is shifting its hours this week, including Saturday's watch party, which will end at the conclusion of the 1 p.m. game.

"It's going to be sweltering and dangerous for anybody who's spending a long period of time outside unprepared for the heat," said Cornish.

An extreme heat warning is also in effect for Kansas City and other portions of west central Missouri until Friday night, with heat index values as high as 105 F to 110 F (40.6 C to 43.3 C) expected.

The New York City area could see near-record high temperatures of up to 109 F (42.8 C) with little respite overnight. An extreme heat watch will be in effect from Wednesday afternoon to Saturday evening.

"This level of heat can be deadly for those without adequate cooling and hydration," the National Weather Service said, adding that it could also impact transportation and electrical and water systems.

The heat index in Boston and much of southern New England could skyrocket to 111 F (43.9 C), with the weather warning starting Wednesday morning.

How could fans and players be impacted?Players have medical staff, three-minute cooling breaks midway through each half, regardless of temperatures, and other heat safeguards. But even the most highly trained, elite athletes are vulnerable to what's called exertional heat illness.

This happens when your body's temperature gets too hot and is accompanied by significant strain from intense exercise on a sizzling day. Symptoms include extreme fatigue, impaired performance, headache, irritability, nausea, dizziness, cramping and dehydration. Exertional heat stroke requires immediate medical attention and is the third leading cause of death in athletes.

Heat also lowers game intensity. Players adapt, for example, by reducing how much they sprint, the amount of distance they cover or the risks they take. In a 2023 survey by World Athletics, a global sports federation, 75 per cent of responding athletes said the impacts of climate change are negatively affecting their health and athletic performance.

For spectators, volunteers and workers, the greatest dangers could be outside the stadiums in fan zones, transportation routes, car parks and outdoor celebrations where they're likelier to be exposed to heat for hours. Many soccer fans are also drinking alcohol and watching the World Cup. In heat, doing that is risky.

Some cities and stadiums have increased access to shade, cooling areas and water for spectators and workers and have said they'll issue heat notifications to the public. Medical personnel will also be stationed and available in Fifa Fan Festivals and around stadiums during matches to manage heat-related illnesses.

Tips for staying safe in the heat

Drinking plenty of fluids, seeking shade or air-conditioning and staying out of the sun can help protect you from heat. If you have to be outside, wear light and loose-fitting clothing and limit strenuous activities during the hottest times of the day. Use cooling fans or towels and drink slushies or other icy beverages.

Limit the amount of alcohol you drink and hydrate before it gets really hot, said Maggie Aldousany, associate clinical professor in the Department of Kinesiology and Sport Sciences at the University of Miami.

"It's really important to realize that it's not enough to wait to get thirsty." she said, adding, "Even if you're going to go to one of the games in person or you're going to go celebrate in your hometown or have a backyard barbecue, you make sure that you're considering hydration even the couple days leading up to it and not just waiting until the morning of or until the event itself."

Spectators can check if they're adequately hydrating by looking at the color of their urine - a light yellow is a good sign.

Hydration breaks feature at tournament

The conditions have the potential to reignite the issue of player safety and welfare that came up a year ago during the Club World Cup, which was held in the U.S., when FIFPRO said the scorching weather should serve as a "wake-up call."

Fifa instituted mandatory three-minute hydration breaks in each half in every match for the World Cup this year, a measure that supporters say benefits player welfare but critics argue disrupts the flow of play at the tournament.

World Soccer's governing body did not immediately respond to a request for comment on any additional measures that could be taken for player or fan safety during the expected period of intense heat.

Toronto, which will host a Round of 32 match between Portugal and Croatia on Thursday, put its "Heat Relief Strategy" into effect after Environment and Climate Change Canada issued a heat warning that runs from Tuesday through Friday.

Three of the stadiums hosting matches under the umbrella of the heat dome - Atlanta, Dallas and Houston - have retractable roofs and air conditioning, offering significant relief.

But even the stroll to the modern home of the Dallas Cowboys can feel more like a firewalk, with the concrete-and-asphalt-laden Texas city built for cars and primed to absorb heat.

Alina Mitina, an emergency department physician for Hackensack University Medical Center in New Jersey, said fans should seek out shade as much as possible and watch out for telltale signs of heat-related illness, including dizziness.

"Shady areas will really save lives in these types of situations," said Mitina. "So as long as there's lots of shade and areas to buy water, I think they're going to be in really good shape."