The fourth Thursday in November marks Thanksgiving Day in the United States, a day of giving thanks for the blessing of the harvest.

Thanksgiving Day Parade.

Thanksgiving Day Parade, 2014. Image by martha_chapa95 / CC BY 2.0

This American Christian tradition can be traced back to New England in 1621, when the first Pilgrim settlers from England declared a three-day feast with the local Native Americans to celebrate a bountiful harvest.

Thanksgiving was declared a national holiday by Abraham Lincoln in 1863, and today many Americans spend a four-day weekend with family and friends and enjoy a traditional feast.

But for those travelling over the Thanksgiving weekend this year, severe weather conditions threaten to cause some disruption.

Through much of the weekend, a slow-moving cold front extending from the Great Lakes to Texas will bring some very active weather to central areas of the United States.

On the warm side of the front, some torrential rain will develop across parts of the Central and Southern Plains and the Mississippi Valley. Rainfall totals across north-east Texas, southern Oklahoma and Arkansas could exceed eight inches by Monday, bringing the risk of some flash flooding.

Meanwhile, just a short distance to the north and west, the precipitation will be more of the wintry variety as cold air plunges down from the north.

Ice storm warnings are in effect for western Oklahoma, Kansas and the Texas Panhandle as the region braces itself for two days of freezing rain.

This occurs as falling snow encounters a layer of warm air aloft, causing it to melt. If the rain then falls into a layer of sub-freezing air just above the surface, it can freeze on contact and form a layer of ice.

Ice accumulations of more than half an inch are possible by Saturday, bringing dangerous driving conditions, damage to trees and the threat of power outages.

Snow will be an additional hazard across parts of the Central Plains, the Midwest and Rockies, with accumulations of several inches for some, while temperatures will be more than ten degrees below normal in places.

The storm system will be fuelled not only by abundant moisture from the Gulf of Mexico, but also by moisture from an unusually late hurricane over the East Pacific, carried in on the subtropical jet stream.

On Thursday, Sandra became the latest major hurricane ever to have formed in the Eastern or Central Pacific with winds of 145mph.

Whilst now weakening, the storm is still expected to bring heavy rain and rough seas to the south of Baja California and Sinaloa over the weekend.

Abnormally high sea surface temperatures and low wind shear, characteristic of El Nino conditions, have contributed to a very active season over the Pacific and allowed systems to develop later into the year compared with normal.

(Press Association)