Attendees of a July 4 event on the National Mall are being told to take shelter Saturday evening as severe thunderstorms are moving into the Washington, D.C., region. Thousands of people were on hand to watch an air show, which was supposed to be followed by musical acts, a speech by President Donald Trump and a 40-minute fireworks show. It is unclear when the event will resume.The big July 4 birthday party for the United States took place in Washington, D.C., as the country marks 250 years since the signing of the Declaration of Independence.WATCH SCRIPPS NEWS LIVE COVERAGE OF THE FESTIVITIES The Freedom 250 event includes military flyovers, live music, speeches, patriotic performances, an appearance by President Donald Trump and, of course, fireworks.The celebration comes as the National Weather Service has issued an extreme heat warning for Washington, D.C., through Saturday evening, with heat index values expected to reach 110 to 115 degrees.IN CASE YOU MISSED IT | Trump kicks off America 250 celebrations with speech warning of communismThe heat forced the cancellation of D.C.'s Fourth of July parade, which had been scheduled for 10:30 a.m. Saturday.People going to the National Mall for the afternoon and evening celebrations were encouraged to stay hydrated, take breaks in air-conditioned spaces when possible and avoid staying in the sun for extended periods.IN CASE YOU MISSED IT | America 250 v. Freedom 250: What's the difference?The Freedom 250 celebration was supposed to feature aerial demonstrations from the U.S. Navy Blue Angels and Air Force Thunderbirds, along with flyovers by military aircraft, including bombers and fighter jets. Organizers also planned a concert program with military bands, guest performers and tributes to U.S. service members, veterans and moments in American history. It is unclear if those events will be able to resume.Trump was scheduled to address the crowd during the evening program, shortly before a fireworks finale over the nations capital. Organizers say the fireworks show will last about 40 minutes and is intended to be the largest in history.Freedom 250, an organization aligned with the White House, organized the Washington celebration. It is separate from America250, a bipartisan group founded by Congress to help commemorate the nation's 250th anniversary. ...read more read less