* Authorities warn of power outages, travel delays
* Region could see 12-18 inches of snow (Adds quote from New York City mayor, details on flight cancellations)
By Daniel Trotta and Scott Malone
NEW YORK/BOSTON, March 13 (Reuters) - A fast-moving winter storm was expected to hit the U.S. Northeast United States, forecasters warned on Monday, threatening to snarl travel and knock out power while prompting some city officials to order schools to close on Tuesday.
The National Weather Service issued blizzard warnings for parts of Pennsylvania, New Jersey, New York and Connecticut, with forecasts calling for up to 2 feet (60 cm) of snow in places by early Wednesday, with temperatures 15 to 30 degrees below normal for this time of year.
Some 50 million people along the Eastern Seaboard were under storm or blizzard warnings and watches.
"This should be a very serious blizzard, one that everyone should take seriously," New York City Mayor Bill de Blasio told reporters. "The next day or two are going to be very challenging."
The storm comes near the end of an unusually mild winter along much of the East Coast, with below-normal snowfalls in cities such as New York City and Washington.
Boston was braced for up to a foot (30 cm) of snow, which forecasters warned would fall quickly during the storm's expected peak on Tuesday, making travel dangerous.
"During its height we could see snowfall rates of 1 to 3 inches (2.5-7.6 cm), even up to 4 inches (10 cm) per hour," said Alan Dunham, a meteorologist with the National Weather Service in Taunton, Massachusetts.
Winds were forecast to gust up to 60 miles per hour (100 km per hour) in spots, potentially causing power outages and coastal flooding.
The looming storm even had an impact on international diplomacy. German Chancellor Angela Merkel, who was due to meet President Donald Trump in Washington, postponed her trip to Friday, the White House said.
New York City and Providence, Rhode Island, canceled public school sessions in anticipation of the storm.
Closing arguments in the trial of a former principal of a Massachusetts pharmacy at the heart of a deadly 2012 meningitis outbreak, which had been expected on Tuesday, were postponed by at least a day due to the forecast storm.
The Port Authority of New York and New Jersey prepared hundreds of pieces of snow equipment at the three major New York area airports. Thousands of tons of salt and sand were prepared for airport roads, parking lots, bridges and tunnels.
Airlines had pre-emptively canceled some 3,850 flights ahead of the storm, according to tracking service FlightAware.com. The worst-hit airports were Newark International Airport in New Jersey and Boston Logan International Airport.
American Airlines canceled all flights into and out of New York's three metropolitan area airports and JetBlue Airways also reported extensive cancellations.
But the New York Stock Exchange vowed to remain open for the tiny fraction of trades that still take place on the trading room floor on Wall Street.
"We haven't closed due to inclement weather since 1996. We have a range of contingencies, and as of this moment it will be business as usual," spokeswoman Kristen Kaus said.
Washington, which often bogs down with even low levels of snow, was expecting 5 inches (13 cm) and twice that in outlying areas. (Additional reporting by Laila Kearney and Alana Wise in New York, Nate Raymond in Boston and Timothy Mclaughlin in Chicago; Editing by Bill Trott and Jonathan Oatis)