Winter Storm Diego is expected to dump a mix of heavy rain, snow, ice — and possibly tornadoes — onto large parts of the country as it moves from coast to coast in the next few days.

The monster storm was already dumping buckets of rain in Southern California on Thursday and is expected to intensify as it begins its 2,500-mile crawl across the southern US, meteorologists said.

By midday Thursday, about 20 million people were under some sort of weather alert and tens of millions more should be added over the next few days, according to USA Today.

About 4 to 8 inches of snow and some ice are expected over eastern New Mexico, western Oklahoma and the Texas Panhandle on Friday, meteorologists said.

“The greatest risk of severe weather that includes the possibility of a few isolated tornadoes is over portions of central and South Texas on Friday afternoon and evening,” AccuWeather meteorologist Alex Sosnowski told the outlet.

By Saturday, rain and threats of flooding will shift largely to the Deep South and Southeast. And more freezing rain and ice is likely from Missouri to the Carolinas.

Diego will save the worst for last — hitting the central and southern Appalachians with heavy snow late Saturday and Sunday. Parts of North Carolina and Virginia could see a foot or more of snow.

But the Big Apple will be spared Diego’s wrath.

“The storm is going to stay south of the Mason-Dixon Line … so you’re not going to get anything from the storm,” Accuweather senior meteorologist Paul Walker told The Post.

Instead, it’ll just be cold and dry in the city, with highs in the mid-30s throughout the weekend, Walker said.