"It's called 'black ice' because it tends to look like the rest of the pavement on the road, although in reality, it's actually clear," the US Forest Service notes. "Black ice forms without creating bubbles, which allows it to blend in with any surface it forms over. Black ice is dangerous precisely because it's hard to detect."

Drivers, keep an eye out for ice on Houston-area roads. On Tuesday morning, Houston TranStar was reporting over 50 closures across the area due to dangerous driving conditions.

A snowstorm warning was in effect for 31 million people - from southern Texas east through Georgia and north to the Carolinas and into Virginia - until Wednesday morning, the National Weather Service said.