At a Home Depot in Yonkers, where propane cylinders were prominently displayed near the cash registers, generators were sold out by 6:30 on Saturday morning, within 30 minutes of opening, said Kareem Hiland, a store employee. “The line for them was out the door,” he said. “For batteries, too.”Experts warned that even if Hurricane Sandy decreased in strength, it would remain a danger because of the unusual convergence of several weather systems.

A system known as a midlatitude trough — which often causes severe winter storms — is moving across the country from the west. It is expected to draw in Hurricane Sandy, giving it added energy. A burst of arctic air is expected to sweep down through the Canadian Plains just as they are converging. That could lead to several feet of snow in West Virginia and lighter amounts in Pennsylvania and Ohio.

The full moon on Monday could cause even greater flooding, with tides at their peak.

The hurricane was forecast to come ashore between the Delmarva Peninsula and Long Island. But as it continued to churn north, it began to spread out, with tropical storm-force winds extending about 520 miles from its center. On Saturday, it was still moving slowly north and had yet to make its predicted westward swing, at which point it will likely become clearer where it will make landfall.

Forecasters cautioned that the course of the storm could change, but officials from the National Hurricane Center said that it was no longer a question of if the storms would converge — but where and with how much force.

Dr. Knabb of the National Hurricane Center said the storm’s intensity was unlikely to change. “The center of circulation is only going to be a very small part of the story,” he said. “This is not just going to be a coastal event.” People from Virginia northward should be prepared for a “long-duration event,” he said.

Utility companies were rushing to put crews in place to deal with power failures, which state officials warned could be extensive and long lasting. Gov. Chris Christie of New Jersey told residents that they should be prepared to go 7 to 10 days without electricity. On Saturday evening, Amtrak began to cancel train service to parts of the East Coast, including between Washington, D.C., and New York.