Mayor Daniel Lurie said the declaration will let city departments buy equipment more quickly by waiving contracting requirements that can slow procurement. The city may need pumps, flood barriers and generators to protect infrastructure from rising water.

At a news conference on the Embarcadero, City Administrator Carmen Chu said crews plan to install temporary storm and flood barriers along the waterfront and maintain trees to reduce the risk of falling branches during high winds. Some water had already spilled onto the Embarcadero near the event.

The city is preparing for both intense rain and wind, which can flood streets, topple trees and disrupt power, and elevated coastal water levels that can flood low-lying waterfront areas even without rain, said Mary Ellen Carroll, director of the Department of Emergency Management. The first forecast series of storms was expected around Halloween weekend.

San Francisco residents can receive up to 10 free sandbags from the Department of Public Works. Department Director Carla Short said people whose homes have not flooded before may not need to collect them.

El Niño is a naturally occurring climate pattern that develops every two to seven years and influences weather around the world. Officials expect this year's event to be historically strong, with the potential for exceptionally high tides and heavy rain that could compound coastal flooding. California Gov. Gavin Newsom declared a statewide emergency in late September, and other local governments have also issued emergency declarations.