At least 18 states have declared a state of emergency as a significant winter storm hits much of the United States.

U.S. crude and natural gas production fell and spot ‌power ​prices spiked as a winter storm with frigid temperatures started ‌to sweep across the country on Friday.

Thousands of power line workers were on standby, flights were canceled and bottled water flew off the shelves Thursday as a huge winter storm that could bring catastrophic damage, widespread power out...