By Ian Simpson

WASHINGTON (Reuters) - Torrential rain and strong winds threatened to snarl travel and cause more destruction on Monday as a major storm roared up the U.S. East Coast after killing at least 20 people in the South.

The weather system, which formed deadly tornadoes in the deep South, is turning colder as it moves northeast, where residents face potential flooding, downed power lines and high tides overnight, said Tim Morrin, the National Weather Service's New York Observation Program Leader.

"The best place to be is inside," Morrin said.

Two train lines operated by NJ Transit, which serves about 100,000 commuters daily, were suspended during evening rush hour between New York City and parts of New Jersey due to downed power lines, the agency said on its website.

About 665 flights were canceled, with Newark Liberty International Airport in New Jersey the hardest hit U.S. airport, according to the air traffic website FlightAware.

New York City emergency management officials warned winds could reach 60 miles per hour (97 km per hour) through Monday night, with more than 3 inches (7.5 cm) of rain possible. Flood advisories and watches were issued for much of the region.

Northern Pennsylvania and parts of New England were expected to get more than 6 inches (15 cm) of wet snow, producing dangerously slick roads. Winds topping 50 mph (80 kph) could whip the East Coast from Delaware to eastern Maine.

The National Weather Service said the front would reach the Middle Atlantic coast by Monday evening. A flood warning was in place for part of southwestern Virginia, and a high wind advisory was issued for western North Carolina.

The storm is expected to take a north-eastward path away from the U.S. coastline by Wednesday evening, the weather service said.

The rain and snow will help ease a dry spell in the northeast United States, where much of the region is suffering from moderate to extreme drought, according to the U.S. Drought Monitor.

Tornadoes and storms killed 15 people over the weekend in Georgia. In Dougherty County, four people were confirmed dead after a twister left a damage path at least a mile wide in spots on Sunday, according to the National Weather Service.

Families were separated and many people left without homes, food or hope, officials said. A mobile home park was particularly hard hit.

"It literally looks like God took half of the mobile home park and threw it across the street into the woods," Dougherty County Commission Chair Chris Cohilas said at a news conference.

Mississippi reported four dead from a tornado on Saturday, and one death was reported in northern Florida's Columbia County.

(Additional reporting by Letitia Stein in Tampa, Fla., Laila Kearney, Frank McGurty and Chris Michaud in New York; Editing by Scott Malone and James Dalgleish)