Mr. Trump’s errant forecast appeared to do little to diminish the zeal of those who did gather for an event that occasionally sounded more like a concert — there was a playlist that included Elton John’s “Rocket Man,” as well as a five-man band — than a political rally. But the night was punctuated with plenty of forceful reminders, including roaring chants of “U.S.A.! U.S.A.!” that Mobile had become an unexpected stop on the presidential campaign trail.

“Please focus your attention to the eastern sky,” a man’s voice announced on the loudspeakers at one point, “for the arrival in Mobile, Ala., of the next president of the United States, Donald J. Trump.” As advertised, Mr. Trump’s jet soon passed over the stadium.

By the time of the flyover, some Trump fans had been at the stadium for about 12 hours, attracted to an event whose potential for scale and boisterousness surged as the week went on. Mr. Trump’s campaign at first intended to hold the rally at a far smaller site. But as word spread that he would bring his red, white and blue road show to Mobile — an easy drive from New Orleans, Birmingham, Ala., and Jackson, Miss. — his aides said that interest was outpacing their plans.

And so Mr. Trump turned to one of the state’s largest venues for a rally of such a profile that Mobile’s mayor issued a news release filled with details about parking, shuttles and the weather. Before the gate opened at 5 p.m., two hours before Mr. Trump’s scheduled appearance, hundreds of people stood in a line snaking well into a parking lot.