Millions of Americans west of the Mississippi are facing severe weather, from thundersnow in South Dakota to dust storms in Las Vegas.

A historic storm with blizzard conditions is underway, according to the National Weather Service, putting 15 million Americans at risk of intense winter weather.

The 1,600 miles worth of winter weather advisories span from from the Rocky Mountains to Michigan. In Denver, which had 80 degree weather on Tuesday, a blizzard warning goes into effect at noon Wednesday. Northern Nebraska, much of South Dakota, and parts of southwest Michigan can expect anywhere from 18 inches to two feet of snow.

That was quick... #Thundersnow already reported in South Dakota with quickly forming Plains #Blizzard pic.twitter.com/9F5DQjGGwQ — Bill Karins (@BillKarins) April 10, 2019

In South Dakota, thundersnow, when a thunderstorm produces rains snow instead of rain, is already being reported.

The National Weather Service says blizzard conditions are expected to be heavy, with wet snow and strong winds. As the storm travels north, snow will hit the Upper Great Lakes Wednesday night, and eventually turn into a rainstorm.

Even more, areas still recovering from the Midwest flooding that caused almost $1.4 billion of losses in Nebraska alone are expected to be hit by the storm, worsening conditions in the already-vulnerable region.

In South Dakota, the Pine Ridge Reservation is expected to be hit by this week's storm, when just last month tribal leaders of the Oglala Sioux Tribal reported over 75 damaged structures and 1,500 people displaced by the weather. Chase Iron Eyes, public relations director for the Oglala Sioux Tribe, said in a statement that as his community braces for Wednesday's storm, "we are once again largely without help" from the federal government.