A prolonged and dangerous heatwave will intensify across a large swath of the US this week, bringing scorching daytime heat, high humidity and stifling overnight temperatures, forecasters predict.

Some 120 million people, external nationwide are under an extreme heat warning as of Tuesday, as Americans head into the Fourth of July holiday weekend, the National Weather Service (NWS) said.

To the north, much of Canada is experiencing its own heatwave starting on Tuesday, with temperatures up to 37C (99F) expected in Ontario.

It comes after Europe’s unprecedented early summer heatwave scorched several countries over recent days, with temperature records broken across the continent.

The heatwave is forecast to hit a large area of the country, from the US east coast to central areas of the country.

The extreme temperatures caused by a heat dome, will peak in the Midwest and Mississippi Valley through Thursday, the NWS predicted. It will shift east into the Ohio Valley and East Coast on Thursday and into the holiday weekend, according to the weather service.

Temperatures ranging from 95F-105F (35C -40.6C) combined with high humidity will bring the heat index to 100F-115F (38C-46C).

Daily temperature records are expected to be broken on Thursday and Friday, with some monthly and all-time records possible.

Officials are warning people living in impacted areas to limit time outdoors, stay hydrated and ensure access to air conditioning or cooling centers.

New York City Mayor Zohran Mamdani activated a heat emergency plan with hundreds of cooling centers set to open across the city on Wednesday.

Detroit, where temperatures could reach 100F (38C), opened a dozen recreation centers with air conditioning for residents to cool down.

With the intense weather, some World Cup matches could have heat indexes exceeding the threshold where global players’ union Fifpro consider it too unsafe for play.

The hottest weather will be in Texas, which will mostly impact fans travelling to and from the air-conditioned stadiums in Houston and Arlington. This may also be the case for the England v DR Congo match on Wednesday in Atlanta where it will also feel hot and humid outside of the air-conditioned stadium.

Philadelphia, meanwhile, is moving parts of its Fifa World Cup Fan Festival into cooled tents on Thursday and Friday.

In Canada, temperatures are expected to remain high through Thursday, particularly in Ontario, where it could hit 37C (99F) this week.

Amid the heat, Toronto is scheduled to host a World Cup game on Thursday – a day when temperatures are predicted to reach 35C (95F).

The country has issued orange heat warnings – the second level of a three-tier alert system – in Ontario.

Meanwhile, central and eastern parts of Ontario, including Montreal, are also predicted to see high temperatures. Yellow heat warnings, the lowest level, are in place.