Severe storms move toward East Coast after spawning deadly tornadoes in Midwest

More than half a million customers were without power in the United States early Monday after a weekend of deadly tornadoes and severe thunderstorms.

The severe weather outbreak over the weekend spawned at least 20 reported tornadoes across three Midwestern states -- Iowa, Michigan and Indiana. On Sunday alone, there were four reported tornadoes in Indiana, killing at least one person and damaging or destroyed dozens of homes, according to local authorities.

Another person was killed in Georgia's Fulton County by a falling tree, local authorities said.

Meanwhile, thunderstorm winds brought down trees in multiple areas southwest of Knoxville, Tennessee, and baseball-sized hail was spotted 35 miles northwest of Indianapolis, Indiana.

Overall, a potent line of storms impacted a swath of the nation from the U.S.-Canadian border to the Gulf Coast, with tens of millions of Americans under tornado and severe thunderstorm watches on Sunday.

By Monday morning, nearly 430,000 customers were without power across the South, namely Tennessee, Kentucky, Arkansas and Georgia. An additional 71,000 were without power in Michigan, according to data collected by PowerOutage.us.

The massive weather system is forecast to move toward the East Coast on Monday, with severe storms expected to hit from New York and Connecticut all the way down to South Carolina, potentially impacting more than 86 million Americans. The highest threat will be damaging winds, large hail and even a few tornadoes.

The cities in the bullseye of Monday's storms will be Raleigh, North Carolina; Richmond, Virginia; Washington, D.C.; and Philadelphia, Pennsylvania. New York state's Hudson Valley and New York City as well as parts of Connecticut are also under threat for severe storms on Monday, but less acute than in the South.

The weather system could bring flash flooding to parts of New Jersey, New York and Pennsylvania on Monday.

ABC News' Matt Foster and Mariama Jalloh contributed to this report.