Freezing Weather Spreads as Storm Heads toward East Coast

(Adds details about Dallas flight cancellations, deaths, winter weather advisories and possibility of flooding)

Nov 24 (Reuters) - Thanksgiving travel may be snarled by a wintry storm system heading for the East Coast, bringing snow, sleet and rain to a region already plunged in to sub-freezing temperatures, the National Weather Service said Sunday.

Forecasters warned the winter storm now in the Southwest could cause flight delays and icy roads for some of the millions of people traveling for the holiday, depending on the path it takes as it heads eastward.

"There's still a whole lot of uncertainty," Lora Wilson, a National Weather Service meteorologist, said in an interview. For now, the forecast was for heavy rain on Tuesday and Wednesday for much of the East Coast and Central Plains.

"If that's the case it will be your typical soggy-rainy-delay-type situation," she said, adding that skies will largely clear for Thanksgiving itself and the rest of the weekend.

The Thanksgiving holiday weekend is one of the most heavily traveled in the United States. Some 39 million people are expected to hit the roads from Wednesday to Sunday, centering around Thanksgiving Day, travel group AAA said this week.

In Texas, said at least 650 flights were canceled in and out of Dallas-Fort Worth International Airport on Sunday, according to FlightAware.com. The website that tracks delays and cancellations said at least more than 300 flights have been canceled for Monday at the southern U.S. travel hub.

The National Weather Service has issued winter weather advisories, some until Tuesday morning, for parts of Texas and Oklahoma, New Mexico and northern Arkansas as snow and freezing rain was expected to accumulate throughout the region.

At least 13 people were killed in storm-related accidents over the weekend in Oklahoma, Texas, California, New Mexico and Arizona, according to NBCNews.com.

The storm was expected to bring heavy rain to the Southeast on Tuesday before heading north up the East Coast. The heavy rain could possibly lead to flooding problems in the deep South and Mid-Atlantic, according to the National Weather Service.

Meanwhile, an Arctic air mass was expected to chill much of the Central Plains and the East Coast through Monday, the advisory said, with wind-chill temperatures dropping into the single digits overnight in some places.

"These conditions would even be considered cold by January standards," the advisory said. "The only warm weather across the country will be in California, the lower elevations of Arizona, and Florida."