* Winter storm system brings snow to U.S. South
* Tornado touches down in Mobile, Ala. (Updates with Mobile tornado, changes dateline)
By Kaija Wilkinson
MOBILE, Ala., Dec 25 (Reuters) - A major winter storm system swept through the southern United States on Tuesday, spawning tornadoes, including one that struck downtown Mobile, Alabama, and bringing a rare white Christmas to a number of states.
The National Weather Service said a tornado struck Mobile, a U.S. city with a population of about 200,000, at about 5 p.m. local time (2300 GMT). There were reports of damage to trees and widespread power outages, along with some structural damage.
"Right now our initial reports are downed trees, power lines down. That's the main damage that's being reported so far ... not aware at this any time of any injuries or fatalities ...," said John Kilcullen, director of operations at the Mobile County Emergency Management Agency.
Tuesday's storms also contributed to a 21-vehicle pile-up that shut down a major highway in Oklahoma, thousands of power outages and the death of a Texas man.
The storm system surging east from Kansas and the Texas Panhandle included tornados and severe thunderstorms along its southern fringe, from southeastern Texas to Alabama, the NWS said.
CenterPoint Energy reported more than 20,000 customers without power in the Houston area Tuesday afternoon.
The storm was expected to expected to evolve into a blizzard from Arkansas to southern Illinois on Tuesday night, with snowfall of up to a foot (30.5 cm) in some areas, according to Accuweather.com.
Accuweather.com senior meteorologist Kristina Pydynowski warned on the website that travel will be "extremely treacherous, if not impossible, as the snow clogs roads, such as interstates 24, 55 and 57, and the blowing snow severely lowers visibility."
The snowstorm will shift on Wednesday to the eastern Great Lakes and northeastern region, she said.
Southern Indiana is under a blizzard warning starting early Wednesday morning, according to NWS meteorologist Crystal Pettet. Indianapolis could see its biggest snowfall in four years, with a possibility of 10 to 12 inches (25 to 30.5 cm) of snow.
"Conditions should be pretty bad in time for rush hour," Pettet said.
A 25-year-old Texas man was killed Tuesday when a tree fell across a road in Harris County, in the Houston metropolitan area, according to Thomas Gilliland of the county's sheriff's office.
A tornado destroyed a building 13 miles (21 km) southeast of Crockett, Texas, and a bank lost a section of its roof, according to Accuweather.com.
Freezing drizzle overnight led to 10 separate collisions on Interstate 40 at Oklahoma City just before 3 a.m., said Betsy Randolph, a spokeswoman for the Oklahoma Highway Patrol.
The 21-vehicle pile-up included three tractor-trailers and shut down the westbound lanes for about five hours, she said. Twelve people were taken to hospitals, and troopers were checking on the severity of their injuries.
In a rare taste of Christmas snow, Oklahoma City was forecast to get 3 to 6 inches (7.5 to 15 cm) of the white stuff on Tuesday. The city's biggest Christmas snowfall was 6.5 inches (16.2 cm) in 1914, and measurable amounts have been recorded only a handful of times on the date.
The FlightAware website, which tracks flight delays, reported departure delays of 40 minutes from Dallas/Fort Worth International Airport late Tuesday afternoon and 32 minutes from Chicago/O'Hare International Airport.
San Francisco International Airport had delays for inbound flights of over an hour due to low clouds.
Ahead of the storm's path, parts of eastern West Virginia are under a winter storm warning. Ice accumulations of up to half an inch (1.25 cm) are expected in higher elevations, the NWS said. (Reporting by Kaija Wilkinson, Ian Simpson and Mary Wisniewski; Editing by Sandra Maler, Todd Eastham and Paul Simao)