* Sandy forecast to come ashore on Monday night
* New York, Philadelphia, Washington, Boston in its path
* Obama asks residents to heed orders of local authorities
* New York, New Jersey, Amtrak transit systems to shut
* New York Stock Exchange trading floor to close on Monday
NEW YORK, Oct 28 (Reuters) - Authorities shut transit systems and ordered some evacuations as tens of millions of people on the U.S. East Coast braced on Sunday for Hurricane Sandy, a gigantic storm forecast to deliver battering winds, dangerous flooding and even heavy snowfall.
Sandy, expected to come ashore late on Monday, could deliver a harsh blow to major cities including New York, Philadelphia, Washington, Baltimore and Boston. Its center was forecast to strike the New York-New Jersey area and then move inland toward Philadelphia and the rest of Pennsylvania.
The sheer size of the storm meant its effects would be felt from the mid-Atlantic states to New England. Officials warned of widespread power outages that could last for days.
Officials ordered school closures in many locations, New Jersey casinos and Broadway theaters prepared to close, airlines got ready to cease flight activity in the New York area, and residents cleared store shelves of vital supplies and food.
President Barack Obama asked residents to heed the orders of state and local authorities to protect themselves from Sandy.
``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.''
Mayor Michael Bloomberg ordered the evacuation of low-lying areas of New York City, including parts of lower Manhattan, that are home to some 375,000 people.
New York City, New Jersey and Philadelphia transit authorities said they would begin shutting down service on Sunday afternoon. Maryland's transit system, serving some suburbs of Washington, said it would not open on Monday.
Amtrak, the U.S. passenger rail service, said it canceled nearly all service on the Eastern seaboard on Monday and would halt its service north of New York along the Northeast corridor beginning at 7 p.m. on Sunday.
Transit systems in Washington and Boston said they planned to operate as usual on Monday as long as it was safe to do so.
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 2 feet (60 cm) of snowfall in the Appalachian Mountains from West Virginia to Kentucky.
The New York Stock Exchange trading floor will close on Monday for the first time since 1985's Hurricane Gloria, exchange officials said. All stocks listed on the exchange will trade electronically, NYSE Euronext said.
Nasdaq planned to open on Monday despite the transit shutdown and evacuation orders, with big banks putting up key personnel in hotels overnight Sunday so that they would be able to make it in Monday morning.
The CME said it would suspend floor trading on the NYMEX oil market on Monday, as its building is located in the New York City evacuation zone near the Hudson River. It said that electronic trading would go on as usual.
Airlines flying into and out of New York's three major airports were all expected to cease flight activity at those facilities on Sunday night, according to the Port Authority of New York and New Jersey.
Europe's air traffic controllers told airlines operating over the Atlantic that Sandy could bring winds of more than 50 knots (58 mph/93 kph) to Washington, Philadelphia, Boston and New York airports starting mid-day on Monday.
The Port Authority said that New York and New Jersey tunnel and bridge closures were being evaluated and were highly likely beginning on Monday.
Worried residents in the hurricane's path packed stores, searching for generators, flashlights, batteries, food and other supplies in anticipation of power outages.
New York City, Boston and Washington were among the jurisdictions announcing that schools would be shut on Monday.
Broadway's theaters planned to close on Sunday night and remain closed on Monday.
New Jersey Governor Chris Christie said his state would face the brunt of the storm and urged all residents to evacuate barrier islands.
``We need to prepare for the worst here,'' Christie told a news conference. The state ordered buses to Atlantic City to evacuate stranded vacationers before the storm's arrival.
Gale-force winds were already starting to hit Virginia and could reach other parts of the mid-Atlantic coast on Sunday night. Tens of millions of people will feel its bluster for as long as two days, officials said.
It could be the largest storm to hit the United States, according to the National Oceanic and Atmospheric Administration.
In Washington, Obama said officials had assured him that they had all the resources they needed in place, and he stressed that ``it is important for us to respond big and to respond fast'' to the hurricane's onslaught.
``We're going to cut through red tape and we're not going to get bogged down in a lot of rules,'' said Obama, who was having to juggle both his re-election bid and his efforts to stay on top of the storm's impact just nine days before Election Day.
Sandy blew the presidential race off course, forcing Obama and Republican challenger Mitt Romney to cancel scheduled campaign stops. It fueled fears that the storm could disrupt early voting ahead of the Nov. 6 election.
INSURERS PREPARE FOR STORM
Insurers also prepared for the storm's arrival, activating claims teams, staging adjusters near the locations most likely to be affected and generally getting ready to pay for a potentially huge volume of losses.
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, wearing down trees, roofs and buildings and piling up rainfall and storm surge.
Hurricane-force winds extended 175 miles (280 km) from the center of the asymmetrical storm, while its lesser tropical storm-force winds spanned 850 miles (1,368 km) in diameter.
``That's gigantic,'' said Chris Landsea, the hurricane center's science and operations officer.
At high tide, it could bring a surge of seawater up to 11 feet (3.4 meters) above ground level to Long Island Sound and New York Harbor, forecasters said.
Sandy was centered about 270 miles (440 km) southeast of Cape Hatteras, North Carolina, or 575 miles (930 km) south of New York City, at mid-afternoon on Sunday, the National Hurricane Center said. It pushed seawater up over the barrier islands off North Carolina known as the Outer Banks.
Sandy was moving northeast over the Atlantic, parallel to the U.S. coast, at 14 mph (22 kph). It was forecast to make a tight westward turn toward the U.S. coast on Sunday night.
Sandy killed at least 66 people as it made its way through the Caribbean islands, including 51 in Haiti, mostly from flash flooding and mudslides, according to authorities.