As fall gives way to winter, meteorologists from the U.S. East Coast to the shores of Western Europe will be watching for signs that the next storm heading their way will turn into a bomb.

That’s right — a bomb.

It’s when a storm — for the U.S., often a nor’easter coming up the East Coast — gets significantly stronger over a 24-hour period, said Paul Kocin, a meteorologist with the U.S. Weather Prediction Center in College Park, Maryland.

The result is usually more snow, rain, wind and waves than the average nor’easter would produce for places like Washington, New York and New England. Bombs are most common from late fall through the winter in the Northern Hemisphere.

“It’s mostly winter time that’s really the key season,” Kocin said by telephone. “When something bombs out, meteorologists get excited.”

An example of the power of a bomb occurred in February 2013 when a storm dropped 2 to 3 feet of snow across New England. Massachusetts Governor Deval Patrick banned travel in the eastern part of the state.

Bigger Storms
As impressive as that storm was, Kocin said it was really a marginal example. Even bigger ones occur along Canada’s eastern coast and in northern Europe.

In January, a massive storm hit Wales with high winds and crashing surf, Kocin said. It was one of a series of winter storms that raked the British Isles, knocking out power to thousands, disrupting rail traffic and causing coastal flooding, according to the U.K. Met Office.

“Farther to the north and out in the Atlantic, they are fairly common,” Kocin said. “Newfoundland gets a lot of them; that’s why there are not a lot of people who live there. They can get hit with some incredible storms.”

Kocin said he was recently in Ireland and was struck at the sight of a coastline that had been carved by centuries of these powerful storms.

Coastal areas enhance sharp gradients in temperature that really can get the storms going. Cold air congregates over the land, while the ocean tends to be a little warmer. That leads to trouble.

A hurricane draws its power from warm water and keeps its strongest winds tightly wound around its core. A big winter storm builds on that difference between warm and cold air and can have winds rivaling those of a hurricane spreading throughout its mass.

Formal Definition
When working together at the Massachusetts Institute of Technology in 1980, Fred Sanders and John Gyakum codified the definition of a bomb as a storm in which the central barometric pressure drops by 24 millibars or more in 24 hours, Kocin said.

The threat of a bomb is enough to cause a commotion among meteorologists. Just before the U.S. Thanksgiving holiday, a storm moving up the East Coast had the potential to bomb out, and snow warnings and travel advisories covered much of the Northeast.

In the end, the large cities of the East Coast got mainly rain from a storm that will be forgotten before long.

Bombs are the storms people remember. Airports close, Amtrak shuts down train service, roads become impassable, beaches get washed away and power can be knocked out for days at a time.

As December turns into January, it’s just a matter of time before another one comes along.