President Donald Trump is scheduled to deliver a keynote address on the National Mall on Saturday evening as the centrepiece of the 250th anniversary of American independence, an event the White House has promoted as “the largest pyrotechnics display in the history of the world.” The celebration, which follows a Friday night speech and fireworks at Mount Rushmore, unfolds as a heat dome blankets the eastern United States, pushing temperatures in Washington, New York, and Philadelphia past 38°C and triggering heat advisories for more than 180 million people. The National Weather Service has warned that the combination of heat and humidity will produce a heat index as high as 46°C in the capital on 4 July.

Internal National Park Service documents reviewed by The Washington Post project that the 40-minute, 850,000-shell fireworks show will generate concentrations of fine particulate matter (PM2.5) between 600 and 1,200 micrograms per cubic metre around the Mall, with a worst-case scenario exceeding 2,000 micrograms. The draft analysis advises people nearby to “avoid prolonged exposure,” and a separate Park Service document recommends N95 masks outdoors. George Thurston, a professor of population health at New York University, told the Post that attendees should minimise exposure, while Anthony Wexler, director of the Air Quality Research Center at UC Davis, described the combination of extreme heat, stagnant air, and the record-scale pyrotechnics as “a disaster” for vulnerable populations. The US Secret Service has designated the Mall event a National Special Security Event, placing the agency in charge of a security apparatus that includes thousands of National Guard troops, miles of fencing, airport-style screening, and Coast Guard patrols along the Potomac.

The anniversary has exposed political fissures. Freedom 250, an organisation aligned with the White House, has taken the lead in programming, sidelining the bipartisan America250 group created by Congress a decade ago. Several Democratic-led states declined to send delegations to the Great American State Fair on the Mall, where some pavilions stood empty. Trump has described the gathering as “the most spectacular TRUMP RALLY of them all,” and the New York Times reported that donors contributing more than $2.5 million would be granted speaking slots during the evening. An April survey by the Associated Press-NORC Center for Public Affairs Research found that roughly four in ten US adults feel “proud” about the anniversary, while three in ten described themselves as “excited.”

Power grids across the eastern interconnection are under strain. PJM Interconnection, which serves nearly a fifth of the US population, declared an emergency for the third consecutive day and ordered all plants to maximum output. Consolidated Edison asked 133,000 customers in northern Brooklyn to conserve energy while crews repaired equipment. Amtrak cancelled some Northeast Corridor trains because of heat-related track risks. The World Cup match between France and Paraguay in Philadelphia on Saturday is expected to begin with a wet-bulb globe temperature above 28°C, prompting FIFPRO to call for delays and FIFA to mandate hydration breaks. The heat dome is forecast to weaken early next week as a cold front moves in, bringing a risk of severe thunderstorms and flash flooding to the Midwest and Northeast.