But American Airlines spokesman Tim Smith says the airline is hardly operating at Dallas because high winds make it unsafe for de-icing employees to work in the bucket trucks. Chicago O'Hare was seeing delays of more than two hours.
A United Airlines spokeswoman says about 300 departures will be canceled in Chicago starting in the afternoon, when the snow is expected to pick up.
The storm, expected to affect as much as a third of the U.S. population, created blizzard conditions from the southern Plains to the upper Midwest, paralyzing grain and livestock movement and threatening near-record snowfall.
Ice and sleet created dangerous travel conditions and forced the cancellation of thousands of flights. President Barack Obama was briefed on the storm and preparations for emergency relief.
The National Weather Service has issued storm watches, warnings and advisories in more than 30 states, and blizzard warnings for eight: Illinois, Iowa, Indiana, Kansas, Michigan, Missouri, Oklahoma and Wisconsin.
Moderate to heavy snowfalls of 8 to 15 inches (20 to 38 cm) will blanket the central and northern Midwest, with some places getting 20 inches (50 cm) more.
In Chicago, local forecasters expect to see accumulations of up to 2 feet (60 cm). In the U.S. Northeast, already facing a wintry mix of snow and sleet, the storm could dump 12-18 inches (30-45 cm) of snow on Boston through Wednesday.
After the snow lets up Wednesday, some affected areas will be in a deep freeze until the weekend, with daytime temperatures below freezing and "really dangerous wind chills," said National Weather Service spokesman Pat Slattery.
Wall Street financial markets were operating normally on Tuesday, but officials were making plans for Wednesday, when dangerous icing was possible.
In Washington, the federal government said workers could take unscheduled leave or telecommute on Tuesday because of the treacherous travel conditions.
"The largest area of the country we've seen so far this winter will be hit with moderate to heavy snow," said Mike Palmerino, a forecaster with Telvent DTN weather service.
"Transportation will be treacherous for the next 48 hours." The southern half of the United States will miss the snow but parts of it may get hit with freezing rain and ice.
Eleven states, from Oklahoma to Rhode Island, have taken the Federal Emergency Management Association up on an offer to deploy personnel as needed, and the agency has positioned items such as meals, blankets and generators for rapid delivery if needed.
Wheat, Cattle Threatened
The storm is expected to wreak havoc on agricultural operations in the Plains states, threatening the dormant winter wheat crop, cattle herds and grain deliveries.
Key farm states of Oklahoma, Kansas and Missouri are being hammered by what forecasters said could be a record combination of frigid conditions and snowfall.
Between 12 and 24 inches (30 and 60 cm) of snow are forecast in an arc from southeastern Kansas to southern Michigan and northern Ohio.
Grain elevators across the southern Plains were working with limited shifts and icing on Midwest rivers was expected to slow loading of grain barges headed to U.S. Gulf export markets.
Meat processor Cargill said it will reduce production at two U.S. Midwest pork plants ahead of the storm.
Chicago soybean futures rose more than 1 percent early on Tuesday, hitting their highest level since July 2008 as the frigid winter storm boosted feed demand.
Freezing temperatures were proving dangerous for Oklahoma's 5.1 million head of cattle, its Department of Agricultural Food and Forestry said.
"Hypothermia and dehydration are the two things we worry about," said spokesman Jack Carson.