Snow, sleet and hail snarled transportation across swaths of the United States on Monday during one of the busiest travel weeks, after dozens died in U.S. storms that were part of a wild worldwide weather system seen over the Christmas holiday period.

Damage caused by a tornado is seen in a neighborhood in Birmingham, Alabama, December 26, 2015.

More than 40 people were killed by tornadoes and floods in the United States during the holiday season, where rare winter tornado warnings were issued in Alabama on Monday.

Alabama, Mississippi and the Florida panhandle were expected to bear the brunt of the of the day's strongest storms, AccuWeather senior meteorologist Michael Leseney said.

About 2,900 flights had been canceled at U.S. airports by 11 p.m. EST on Monday, according to FlightAware.com, while another 4,800 delays were reported.

Chicago-area airports were worst hit with hundreds of flights canceled as the city was swept by sleet and hail and United Airlines granted exemptions from fees for some travelers impacted by the storms.

More than a foot (30 cm) of snow was forecast for southwestern Wisconsin and southeastern Minnesota, and snow was also falling in Iowa, Nebraska and Missouri.

A flash flood warning was in effect in eastern Missouri and Southern Illinois, the National Weather Service said. Thirteen people died in flash floods in those two states during the weekend, including four international soldiers training at a military base in Missouri, the Army said.

The storms came as other countries struggled with extreme weather and stressed holiday infrastructure.

In Britain, hundreds of troops were deployed and a government agency said a "complete rethink" of flood defenses was needed after swathes of northern England were inundated by rivers that burst their banks.

Source: Reuters