* Blizzards, freezing rain in lower Midwest
* Chicago bracing for biggest storm in decades
* Thousands of flights canceled
By Mary Wisniewski
CHICAGO, Feb 1 (Reuters) - A colossal winter storm stretching from New Mexico to Maine hit the middle of the United States on Tuesday with blizzards and freezing rain, and experts said the worst is still to come as it moves northeast.
The storm, expected to affect as much as a third of the U.S. population, blanketed a wide swath with ice and sleet in the lower Midwest, creating dangerous travel conditions and forcing the cancellation of thousands of flights.
The National Weather Service said Chicago could get one of its biggest blizzards in 40 years, with two feet (70 cm) of snow.
In the U.S. Northeast, already facing a wintry mix of snow and sleet, the storm is expected to dump 12-18 inches (30-45 cm) of snow on Boston. New York was in line for a wintry mix of snow and freezing Tuesday and Wednesday with dangerous icing possible.
Wall Street financial markets were operating normally on Tuesday morning but officials were making plans for Wednesday.
In Washington, the federal government said that workers could take unscheduled leave or telecommute on Tuesday because of the treacherous travel conditions.
Strong winds and heavy snow created blizzard conditions in southern and eastern areas of the Plains wheat growing region.
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.
Chicago faces a blizzard warning through Wednesday afternoon, and many traders at the Chicago Mercantile Exchange were planning to stay downtown on Tuesday night. A CME Group <CME.O> official said the exchange was monitoring the weather forecasts but had not put any contingency plans in place.
A number of suitcases and overnight bags could be seen among the racks in the coat-room of the largest U.S. futures exchange. Road travel is expected to be treacherous and public transportation subject to delays as snow accumulates quickly.
FLIGHTS CANCELED
The Dallas-Fort Worth international airport closed for 2 1/2 hours early on Tuesday due to ice just as thousands of American football fans began arriving in the city for the Super Bowl game on Sunday.
The top U.S. airlines United Continental <UAL.N>, Delta Air Lines <DAL.N> and American Airlines <AMR.N> canceled more than 4,000 flights ahead of the storm. Aviation data service Flightaware said more than 6,000 flights have been scrubbed.
Snow, sleet and freezing rain made for a messy, slippery commute in New York City. Subways and buses were running but with delays and crowding due to weather-related problems.
"It's far from a picturesque scene. It's an icy, wet mess," said meteorologist John Davitt of NY1 television.
Traffic was slowed on highways in and around New York City, and speed restrictions were imposed on the George Washington Bridge and the Tappan Zee Bridge, both of which span the Hudson River, due to the weather conditions, authorities said.
By the time the storm front reaches the upper Midwest later on Tuesday, the National Weather Service said, the ice and sleet could turn into one of the biggest blizzards in more than 40 years in Chicago, with as much as two feet of snow expected.
The snow is expected to be accompanied by high winds of more than 40 miles per hour (64 kph) and plunging temperatures.
Missouri and Oklahoma were among several states that declared a state of emergency even before the storm hit.
"Everyone should stay inside today and not drive," Missouri Governor Jay Nixon said on local television.
In Tulsa, Oklahoma, conditions are so bad that the Tulsa World newspaper will not publish for the first time in its history, said publisher Robert E. Lorton III. Many subscribers couldn't find their Tuesday editions under the snow.
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.
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. (Additional reporting by Ros Krasny, Corrie MacLaggan, Ann Saphir, Carey Gillam, Wendell Marsh, Lauren Keiper, Ellen Wulfhorst and Ed Stoddard; writing by Greg McCune and Anthony Boadle; editing by Mohammad Zargham)
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