Winter storm updates: Dozens dead across US in wake of massive snowfall, deep freeze

Three brothers -- ages 6, 8 and 9 -- died after falling through an icy pond in Fannin County, Texas, north of Dallas, on Monday, according to the Texas Game Wardens.

The 8-year-old and 9-year-old were pulled from the pond by a neighbor and first responders and they were rushed to a hospital where they died, officials said.

"The 6-year-old did not resurface but was later recovered by Texas Game Wardens after an extensive search of the pond," the officials said, adding, "Our thoughts and prayers are with the family and all affected by this tragic incident."

As Washington, D.C., digs out from the snow, federal offices will remain closed on Tuesday, as well as the National Zoo and the Smithsonian museums.

Some 220 million Americans are on alert for extreme cold temperatures forecast to follow the weekend's record-breaking winter storm.

The extreme cold warnings and cold weather advisories stretch from Texas and Florida in the South to New York in the Northeast and Detroit in the Midwest.

Cold air is expected to produce another round of lake-effect snow on Tuesday for western Michigan and western New York -- an additional 8 to 16 inches of snow is possible in some areas.

Record low temperatures are possible in the South, where temperatures will feel as low as the 20s and 30s in Florida and the 20s along the Gulf Coast.

Wind chill is expected to drop into single digits on Tuesday in the Northeast, ahead of even lower temperatures later this week. Multiple days of below-zero wind chills are forecast for the Northeast from Wednesday into the weekend.

-ABC News' Melissa Griffin

At least 540,363 customers were without power as of 6:00 a.m. ET on Tuesday, according to PowerOutage.us, with Tennessee, Mississippi, Louisiana and Kentucky hit the hardest.

At least 173,997 customers were without power in Tennessee, 140,375 customers in Mississippi, 99,614 customers in Louisiana and 28,072 in Kentucky, PowerOutage.us said.

Also without power were 27,920 customers in Texas, 14,132 in South Carolina, 12,984 in Georgia, 10,412 in Virginia, 6,506 in West Virginia and 4,794 in Florida.

The Nashville Electric Service -- which as of Monday was the hardest-hit utility in the country according to PowerOutage.us -- said it planned to "double its workforce" to respond to the outages.

-ABC News' Aidan Gellert and Charlotte Slovin