Heavy rain and wintry weather across the US promise post-holiday travel woes.

The holidays in the United States began with dense fog halting flights from the Midwest on Christmas Eve. Two days later, nearly 24 million people across the region remain under dense fog advisories.

A winter storm system is also moving through the upper Midwest and southern Canada, and is expected to bring fast-falling snow and ice that will reduce visibility and produce dangerous driving conditions.

The National Weather Service has posted weather warnings for southern California, as a new system is bringing heavy rain on Thursday, with the threat of flash flooding and mudslides in areas recently burned by wildfires.

The Los Angeles Basin and San Diego County are expected to receive as much as 50 millimetres of rain (two inches), with coastal areas seeing even higher amounts. It will also feel cold, with temperatures in LA around seven degrees Celsius below average, struggling to reach 13C on Thursday.

Heavy snow is expected across the mountains in southern California and the southern Rockies, with at least 30cm (one foot) accumulating throughout the day.

Strong, gusty winds will accompany the snow and could bring down trees and cause power outages.

The same weather system will then move into the Four Corners region, bringing rain and snow to higher elevations. The Southern Plains will see the rain arrive during the coming weekend.

By the end of the weekend, New England and southeastern Canada will be in the firing line, with snow and gusty winds moving in from the Midwest.

As people return home following the Christmas holiday, the wintry weather could lead to widespread travel delays across the US and Canada.