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 by 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 the affected areas to limit time outdoors, stay hydrated and ensure access to air conditioning or cooling centres.
New York City Mayor Zohran Mamdani activated a heat emergency plan with hundreds of cooling centres set to open across the city on Wednesday.
Detroit, Michigan, where temperatures could reach 100F (38C), opened a dozen recreation centres 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 hit 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, Georgia.
Philadelphia, meanwhile, is moving parts of its Fifa World Cup Fan Festival into cooled tents on Thursday and Friday.
