Recipe for a Blizzard

High above the U.S. East Coast, a clash of the titans is underway, with two winter storms merging into what meteorologists are calling a historic blizzard, dubbed Nemo by the Weather Channel.

Which makes us wonder: What's the difference between a bad winter storm and a full-fledged blizzard?

According to the National Weather Service, storms that involve large amounts of snow, winds in excess of 35 miles (56 kilometers) an hour, and visibilities of less than a quarter mile (half a kilometer) are classified as blizzards.

Visibility is the most important element in the mix, said meteorologist Matt Kelsch, with the University Corporation for Atmospheric Research. "Usually, it takes that strength of wind—[35 miles (56 kilometers) an hour]—to pick enough snow off the ground to reduce visibility that much," he said.

"[But] it doesn't always have to be snowing to be a blizzard," Kelsch added. In the U.S., areas of the Great Plains and western states can get something called ground blizzards: storms in which high winds pick up snow from the ground and whip it around, greatly reducing visibility.

Will blizzards become more frequent in the future as a response to climate change? That's difficult to know for sure, said Kelsch, but warming oceans may increase the contrast between warm, coastal air and cooler air over land, which could increase the frequency of storms.

"Weather and climate are directly connected. You can't change one and not expect an effect on the other," Kelsch said.

Forecasters expect this storm to dump 2 to 3 feet (0.5 to 1 meter) of snow on Boston in about 24 hours, while coastal areas around Cape Cod will get slammed with 20-foot (6-meter) waves and 70-mile-an-hour (112 kilometer-an-hour) winds. (See pictures of weird ice and snow formations.)

Louis Uccellini, a winter storm expert and newly appointed director of the National Weather Service, said February is notorious for producing major winter storms along the East Coast.

But in this blizzard's case, the rapid development of a system off the mid-Atlantic coast, together with a second system coming in from over Lake Erie—which will intensify conditions—has produced an especially severe storm. (Check out running updates on the storm from The New York Times.)

"For Boston, we're pretty confident this blizzard will be in the top ten storms of the past century," he said.

With reporting by Kate Andries.