CHICAGO (Reuters) - Strong and gusty thunderstorms with tornadoes were threatening the U.S. Gulf Coast on Monday, following tornadoes and floods over the Christmas holiday season that killed at least 43 people in the South and Midwest and snarled transportation during a busy travel time.

The strongest storms will be in Alabama, Mississippi and the Florida panhandle, said AccuWeather senior meteorologist Michael Leseney. Tornado warnings were already in effect in Alabama, meaning a tornado had been sighted.

To the north, the Chicago area was being hit by sleet, which was expected to turn to rain, while snow was falling in Wisconsin, Iowa, Nebraska, Michigan and parts of western Missouri, Leseney said.

The heaviest snow, over a foot (30 cm), was expected in southwestern Wisconsin across and southeastern Minnesota, Leseney said.

Winter storms that brought ice and high winds to Oklahoma over the weekend led to snapped power lines, with 54,000 customers without power on Monday morning in Oklahoma City and surrounding areas, Oklahoma Gas & Electric said. Local news reports said there were 100,000 without power across the state.

At least 17,000 customers were without power in Texas on Monday morning, according to major utility companies.

Flight delays and cancellations were mounting, with 2,439 delays and 1,017 cancellations as of 8 a.m. CST (1400 GMT), according to the FlightAware website.

A flash flood warning was in effect in eastern Missouri and Southern Illinois, the National Weather Service said. Flash floods over the weekend killed at least 13 people in Missouri and Illinois.

In Missouri, Governor Jay Nixon declared a state of emergency on Sunday, saying continued rains would make already widespread flooding conditions worse.

Three adults and two children were killed near the village of Patoka, Illinois, 85 miles (137 km) east of St. Louis, Missouri, when their car was washed away by floodwaters on Saturday night, according to Marion County Coroner Troy Cannon.

In Texas, at least 11 people were killed in the Dallas area over the weekend by tornadoes, including one packing winds of up to 200 miles per hour (322 km per hour). The twister hit the city of Garland, killing eight people and blowing vehicles off highways.

Powerful tornadoes are a staple of spring and summer in central states but occur less frequently in winter, according to U.S. weather data.