(CNN) - Pelting rains, whipping winds, mass evacuations: There is no doubt that Hurricane Sandy, by Sunday, had already made a mammoth impact on the U.S. East Coast.
And it should only get worse.
That's the consensus view, among forecasters and officials, as the Category 1 storm continued to chug northeastward parallel to the shore. Even with its eye still hundreds of miles away, those on the North Carolina and Virginia coasts felt its wrath Sunday.
But if, as expected, it turns toward the United States later in the day, Sandy will have an even more direct -- and potentially calamitous -- effect on millions of Americans. Forecasters warn it will likely collide with a cold front from the West to spawn a "superstorm" that could slog along the Eastern Seaboard for days -- meaning even more flooding, even more power outages, even more potential danger.
Millions of people could experience flash flooding or river flooding by the time the storm has passed, which might not happen until Wednesday, said Rick Knabb, director of the National Hurricane Center. That Miami-based agency, in its 5 p.m. advisory, warned of no less than "life-threatening storm surge flooding the Mid-Atlantic coast, including Long Island and New York Harbor," all in addition to hurricane-force winds.
"Sandy has a tremendous amount of energy... It could be bad, or it could be devastation," U.S. Coast Guard Rear Adm. Steven Ratti told CNN. "It's so far out, so we don't know exactly where it will hit."
Already on Sunday, power was knocked out in places such as Hampton Roads, Virginia, as rough waves crashed along the coast, said Penelope Penn. Another CNN iReporter, Elizabeth Switzer, reported waves topping 12 feet further south along Carolina Beach outside Wilmington, North Carolina. And by 5 p.m., floodwaters were washing over roads between Dewey and Bethany beaches in Delaware, said Gov. Jack Markell, adding even more urgency for people to get out before they get trapped.
"We're seeing more flooding than you normally do, and particularly since the storm's not here," Markell said.
Hurricane-force winds extended 175 miles out from Sandy's eye, according to the National Hurricane Center's 5 p.m. update, meaning it is much larger than most storms of its type.
Sandy prompted evacuation orders on North Carolina's Outer Banks, New Jersey's barrier islands, in downtown Ocean City, Maryland, and in flood-prone coastal communities in southern Delaware. Low-lying areas of New York City, including Coney Island and parts of Manhattan, are being cleared out as well.
Jim Brady was among those who heeded the call, leaving his Cape May home about three blocks from the Atlantic and heading 85 miles north to his sister-in-law's house in Toms River. Having packed what they can and stashed bigger valuables as high as possible, what happens next is now out of the hands of those heading elsewhere -- knowing that it might not be until after Monday night when the storm really rolls in, with a possible 12-foot storm surge, that they know if their home has skirted disaster.
"We'll just hunker down and wait for it to pass, basically," Brady said.
Many other communities, big and small, are bracing for the worst. Subway service will halt beginning at 7 p.m. Sunday in New York City, the city that never sleeps. Other mass transit systems soon followed suit in suspending service on Monday, including Washington's Metro service and Southeastern Pennsylvania Transportation Authority trains and buses around Philadelphia.
Across the bay from Brooklyn in Sea Bright, New Jersey, Yvette Cafaro pleaded on the plywood that covered up her seaside burger restaurant, "Be kind to us Sandy." The seaside area largely dodged last year's Hurricane Irene, and they are hoping for -- but not expecting -- any more reprieves.
"Everything that we've been watching on the news looks like this one will really get us," Cafaro said. "We're definitely worried about it ... But we're doing everything we can to prepare. And hopefully, she'll spare us."
Officials from North Carolina to Maine have been raising alarms, and taking preventive steps like the subway shut-down, for days.
By Sunday evening, officials already had canceled classes Monday for well over 2 million public school students in districts such as New York, Philadelphia, Boston and Baltimore, while federal government offices in Washington and government offices in states like New Jersey were preemptively closed.
Sandy also turned presidential campaign plans upside down, not to mention throwing into question how millions of kids will celebrate Halloween since the storm, and its effects, could linger for days.
Then there are the travel nightmares that the storm has already, and will continue, to cause with thousands of flights called off, Amtrak train runs scuttled, and hundreds of roads and highways expected to flood.
A full moon, which always brings out higher than normal tides, should exacerbate storm surge problems
on Monday in coastal areas. In addition to 8 a.m., high tide is scheduled for around 8 p.m. -- meaning parts of Delaware and New Jersey, for instance, could see significant flooding then even if Sandy has come ashore by then. Irrespective of the tides, the National Weather Service is forecasting potentially harmful storm surges of between 6 to 11 feet in New York Harbor and Long Island Sound.
"This is not a typical storm. It could very well be historic in nature and in scope and in magnitude because of the widespread anticipated power outages, flooding and potential major wind damage," Pennsylvania Gov. Tom Corbett said, speaking of his state but just as well about many others in the Mid-Atlantic and Northeast.
"Essentially, this is a hurricane wrapped in a nor'easter."
CNN's Jareen Iman, Chelsea J. Carter, Alison Kosik, Catherine Shoichet, George Howell, Athena Jones, Shawn Nottingham, Alden Mahler Levine, Joe Sutton and Devon Sayers contributed to this report.