Under the Rotunda of the U.S. Capitol, Donald Trump was again inaugurated, this time as the 47th president of the United States, capping a historic return to power that serves as resounding rebuke to the established political order.

A polar vortex dipping down from Siberia will bring a cold front with frigid temperatures to nearly 300 million Americans. See maps of the arctic blast.

According to the National Weather Service, a polar vortex will impact most of the country this weekend, here is what you need to know.

A rare winter storm is getting set to slam the Gulf Coast and South, with cities from Texas to Florida under Winter Storm Warnings.

A polar vortex is slated to sweep most of the continental US bringing winter storm warnings and a hazardous freeze to millions.

A polar vortex is currently hitting the United States, with schools closed in many areas and at least four deaths caused by treacherous conditions.

People living "basically anywhere from the Rockies eastward" will see extremely cold temperatures over the next several days, a meteorologist says.

The polar vortex is a ring of cold air that typically circles the Arctic. A strong jet stream essentially acts like a fence, keeping it locked there. If there's any weakening in the jet stream (the fence), the frigid air can become unstable and spill out. This sends the normally trapped cold air much farther south.

The most unusually cold air in the Northern Hemisphere will be over the United States early next week, bringing dangerously frigid conditions.

A blast of Arctic air is set to cover much of the United States with temperatures below freezing starting on Friday and into next week, impacting millions of Americans in nearly all of the contiguous states.

Despite a cold start to the day, most of the U.S. will experience seasonably mild temperatures on Thursday. However, dangerous changes are on the way as a polar vortex tightens its grip on the nation as sends temperatures tumbling below freezing starting this weekend.

An intense blast of arctic air blanketed the Mid-Atlantic and the Northeast with frigid temperatures, and a rare winter storm was expected along the Gulf Coast.