A blast of arctic air is expected to sweep down through the Canadian Plains just as the two storms converge. That could lead to several feet of heavy, wet snow in West Virginia and lighter amounts in Pennsylvania and Ohio that could bring down trees and power lines if already chilly temperatures drop below freezing.

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

The possibility of a higher surge was one reason that Mayor Michael R. Bloomberg of New York ordered mandatory evacuations in low-lying areas, just as he did before Tropical Storm Irene. One city official said there was particular concern about Con Edison’s Lower Manhattan infrastructure, noting that if the storm surge washed over the bulkheads, it could damage the utility’s electrical and steam networks. If the surge runs as high as forecast, Con Ed will shut off two electrical networks in Lower Manhattan,

As for the subway shutdown, Mr. Bloomberg said that if the Metropolitan Transportation Authority had not suspended service, but instead had left itself vulnerable to the storm, the city would have risked being without its mass transit network for even longer.

“They do have to make sure that their equipment doesn’t get damaged,” Mr. Bloomberg said. “Otherwise we would not have subway trains when this is over or buses when it’s over.”

Joseph J. Lhota, the chairman of the authority, said he expected the transit systems to restore at least some service about 12 hours after the storm ended. But he warned that the city could be without mass transit for as many as two full work days. “I do think Monday and Tuesday are going to be difficult days,” Mr. Lhota said.

But while the mayor said schoolchildren could take Monday off, city workers could not: He said that city offices would be open for business.