(Adds Georgia emergency declaration, local authorities frustrated with pace of federal assistance; updates flight cancellations)
By Ian Simpson
WASHINGTON, Jan 23 (Reuters) - A powerful storm system plowed up the U.S. Eastern seaboard with torrential showers and high winds on Monday, hindering airline and rail travel, after killing at least 21 people in the South, many in mobile homes demolished by tornadoes.
The storm, which unleashed deadly twisters in Mississippi and Georgia over the weekend, turned cooler as it advanced on the Northeast, where residents faced potential flooding, downed power lines and high surf late on Monday and early Tuesday, said Tim Morrin, the National Weather Service's New York observation program leader.
"The best place to be is inside," Morrin said.
Two commuter rail lines operated by New Jersey Transit, which serves about 100,000 passengers daily, suspended service during the evening rush hour between New York City and parts of New Jersey due to fallen power lines, the agency said on its website.
About 750 U.S. airline flights were canceled, with New York metropolitan airports bearing the brunt of the disruptions, according to the air traffic website FlightAware.
New York City emergency management officials warned that wind speeds 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 receive more than 6 inches (15 cm) of wet snow, producing dangerously slick roads, and AccuWeather said traffic along several interstate highways through the region may be impeded.
Forecasters said winds topping 50 mph (80 kph) could whip the East Coast from Delaware to Maine, where the state legislature canceled all business for Tuesday in anticipation of harsh weather.
As the storm front reached the Middle Atlantic Coast on Monday evening, a flood warning was posted for parts of southwestern Virginia, and a high wind advisory was in effect for western North Carolina.
The storm was expected to take a northeastward 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 has suffered from moderate to extreme drought, according to the U.S. Drought Monitor.
Thunderstorms and tornadoes spawned by the same weather system killed 16 people on Sunday in Georgia, according to a tally by the Storm Prediction Center in Oklahoma. Four of those fatalities were confirmed, and a toddler was reported missing, in Dougherty County after a twister left a path of destruction up to a half mile wide.
A trailer park was especially hard hit, and local authorities said they expected the death toll to rise.
"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.
Cohilas expressed frustration at the pace of the federal government in furnishing emergency help, calling it "disgraceful."
"I would ask President Trump to take some significant steps to cut through the red tape and get us some people on the damned ground," he said at a news conference on Monday.
Governor Nathan Deal, announcing that he extended a state emergency declaration to 16 Georgia counties, said Trump had called him on Sunday and promised he would be "ready and willing to respond" to an expected request for federal disaster relief.
A tornado killed four people, three of them in mobile homes, in the southern Mississippi town of Hattiesburg on Saturday, and one person was reported killed in northern Florida when a tree toppled onto a house during a severe thunderstorm, authorities said. (Additional reporting by Letitia Stein in Tampa, Florida, and Laila Kearney, Frank McGurty and Chris Michaud in New York; Editing by Scott Malone, Lisa Shumaker and Leslie Adler)
Our Standards: The Thomson Reuters Trust Principles.