In a post on X, city officials said umbrellas will not be permitted along the parade route because they block the view for others.
The city suggests wearing rain jackets or ponchos so everyone can enjoy the show.
If the forecast holds, it will be the first time rain has fallen on the floral procession since 2006.
Rose Parade could see rain for first time in 20 years
The parade has traditionally managed to avoid precipitation, with rain dampening the event less than a dozen times in its previous 136 years.
National Weather Service forecasters said people heading for the parade should expect to get wet.
"... The chances of a wet New Year's parade, and including the night before when people came out before the parade, are extremely high, 80-90% at this point," forecasters said in a statement Monday afternoon.
You can watch the full Rose Parade on New Year's Day starting at 8 a.m. on ABC7.
City News Service contributed to this report.