* Sandy forecast to come ashore on Tuesday morning
* New York, Philadelphia, Washington, Boston in its path
* Obama asks residents to heed orders of local authorities
* Public transit, Amtrak, airlines affected
* New York Stock Exchange to close Monday, possible Tuesday
NEW YORK, Oct 28 (Reuters) - Hurricane Sandy, which could become the largest storm ever to hit the United States, is set to bring much of the East Coast, including New York and Washington, to a virtual standstill in the next few days with battering winds, flooding and the risk of widespread power outages.
About 50 million people are in the path of the massive storm, which has already killed 66 people in the Caribbean and is expected to hit the U.S. Eastern Seaboard on Tuesday morning.
While the storm does not pack the punch of Hurricane Katrina, which devastated New Orleans in 2005, forecasters said it could be the largest in size when it strikes land. Sandy's winds stretched some 520 miles (835 km) from the storm's eye, meteorologists said.
New York and other cities and towns closed their transit systems and schools and ordered residents of low-lying areas to evacuate before a storm surge that could reach as high as 11 feet (3.4 meters).
All U.S. stock markets will be closed on Monday and possibly Tuesday, the operator of the New York Stock Exchange said late on Sunday, reversing its earlier plan to close the trading floor but offer electronic trading.
"The dangerous conditions developing as a result of Hurricane Sandy will make it extremely difficult to ensure the safety of our people and communities, and safety must be our first priority," NYSE Euronext said in a statement.
In addition, the United Nations, Broadway theaters, New Jersey casinos, schools up and down the Eastern Seaboard, and myriad corporate events are all being shut by the storm.
'DON'T BE STUPID'
Sandy also blew the presidential race off course, forcing President Barack Obama and Republican challenger Mitt Romney to cancel some campaign stops. It fueled fears that the storm could disrupt early voting before the Nov. 6 election.
Officials ordered people in coastal towns and low-lying areas to evacuate, often telling them they were putting emergency workers' lives at risk if they stay.
"Don't be stupid, get out, and go to higher, safer ground," New Jersey Governor Chris Christie told a news conference.
Forecasters said Sandy was a rare, hybrid "super storm" created by an Arctic jet stream wrapping itself around a tropical storm, possibly causing up to 12 inches (30 cm) of rain in some areas, as well as up to 3 feet (90 cm) of snowfall in the Appalachian Mountains from West Virginia to Kentucky.
Worried residents in the hurricane's path packed stores, searching for generators, flashlights, batteries, food and other supplies in anticipation of power outages. Nearly 284,000 residential properties valued at $88 billion are at risk for damage, risk analysts at CoreLogic said.
Transportation is set to grind to a halt on Monday, with airlines cancelling flights, bridges and tunnels likely to be closed, and the Amtrak passenger rail service scrapping nearly all service on the East Coast. The federal government told non-emergency workers in Washington D.C. to stay home.
"This is a serious and big storm," Obama said after a briefing at the federal government's storm response center in Washington. "We don't yet know where it's going to hit, where we're going to see the biggest impacts."
The second-largest refinery on the East Coast, Phillips 66's 238,000 barrel per day (bpd) Bayway plant in Linden, New Jersey, was shutting down and three other plants cut output as the storm affected operations at two-thirds of the region's plants. Benchmark gasoline prices rose 1 percent in early futures trading.
At 11 p.m. Sunday (0300 GMT Monday), Sandy was centered about 470 miles (760 km) south of New York City, the National Hurricane Center said. The storm was moving northeast over the Atlantic, parallel to the U.S. coast, at 14 mph (22 kph), but expected to turn north toward land.
"We're expecting the worst, hoping for the best. We're getting everything off the basement floor. We've got two sump pumps. But during Hurricane Floyd, we were down there for 17 hours straight sweeping water into the sump pumps," said Maria Ogorek, a Maplewood, New Jersey, lawyer and mother of three.
New York Mayor Michael Bloomberg ordered the evacuation of some 375,000 people from low-lying areas of the city, from upscale parts of lower Manhattan to waterfront housing projects in the outer boroughs.
"I'm nervous. It's the biggest storm to ever hit the city," said Lizzie Elston," a 28-year graduate student who returned to New York from a trip to Virginia on Sunday and caught one of the last subway trains of the night. "And I'm worried that the grocery stores will be closed. We have not prepared."
Not everyone heeded the warning. Mike Cain, a construction manager who lives in a high-rise building in Manhattan's Battery Park City, said he was staying put. "We have stocked up on water, food and taped up our windows. If the track or size of the storm changes we may leave after all, but for now we are staying here, we'll be OK," he said.
While Sandy's 75 mph (120 kph) winds were not overwhelming for a hurricane, its exceptional size means the winds will last as long as two days, bringing down trees and damaging buildings. The slow-moving storm brought lashing rains in coastal areas and snow farther inland.
"This is not a typical storm," said Pennsylvania Governor Tom Corbett. "It could very well be historic in nature and in scope, and in magnitude because of the widespread anticipated power outages, and the potential major wind damage."
Even with all the warnings, some people tried to carry on with their plans.
"I just don't buy into the hype," said Kate Sullivan, a 40-year-old computer specialist from Alexandria, Virginia, who was headed to Baltimore-Washington International airport for a planned flight to Los Angeles. "I'm pretty sure I'm going to end up in LA by the end of the night."