Diana Mayhew, the festival’s president, said officials made the decision on Tuesday to begin five days earlier than planned, on March 15, after representatives from the Park Service briefed them on their forecast.

Ms. Mayhew said she expected as many as a million and a half people to take part in the five-week festival, with many of them crowding the blossom-shaded pathways that line the Tidal Basin from dawn to well past dusk.

The cherry trees date to 1912, when Japan gave some 3,000 of them to the United States as a gesture of friendship.

Only a handful of the original trees remain, but thousands more have been planted since.

Michael Litterst, a spokesman for the National Park Service, said the agency made its annual bloom prediction based on history, natural indicators on the trees and the weather forecast.

The trees are considered to be in peak bloom when 70 percent of the basin’s most common variety, Yoshino, are in flower. In a typical year, that comes on April 4.

But not this year. Given the mild winter and rare February warmth, the Park Service says the peak should come between March 14 and 17, depending on the weather in the coming weeks.