Ominous weather is threatening the "Salute to America" celebration President Trump says will be among the greatest in the nation's history, although the Interior Department says the show will go on, rain or shine.

Shortly before noon, the National Weather Service (NWS) issued a flash-flood watch lasting through 8 p.m. for Washington, D.C.. Mr. Trump is scheduled to speak at the Lincoln Memorial at 6:30 p.m.

"Slow-moving showers and thunderstorms are expected to develop after 1 p.m. this afternoon before diminishing early this evening," NWS said in an update Thursday. "Torrential rainfall rates may lead to totals exceeding 2 inches in a short period of time. This may cause flash flooding of small streams and other poor drainage urban areas."

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Even if the president's "Salute to America" event continues without skipping a beat, weather could turn away waiting crowds. Just how many people will show is already in question, as the event was put together at the last minute.

"Salute to America," scheduled to run from 6:30 p.m. to 7:30 p.m., will also feature military plane flyovers and tanks. The White House and Pentagon aren't saying how much the event will cost taxpayers.

The president has defended the price tag, tweeting earlier this week, "the cost of our great Salute to America ...will be very little compared to what it is worth."

Earlier Thursday, Mr. Trump visited his golf course in Northern Virginia, while Vice President Mike Pence spoke at a naturalization ceremony for new citizens.