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What in the world is a ‘bomb cyclone’?

This map shows maximum expected wind gusts for Saturday.National Weather Service

The powerful storm that’s expected to wallop Massachusetts Saturday with heavy snow, high winds, blizzard conditions, and coastal flooding will be a “bomb cyclone,” forecasters are saying.

What’s that? It’s a colorful - and ominous-sounding - meteorological term for describing a rapidly intensifying winter storm.

A storm becomes a bomb cyclone — or undergoes bombogenesis — when its pressure drops steeply within a 24-hour period and rapidly strengthens. Other ways of saying it: A storm “bombs out” or “undergoes explosive cyclogenesis.”

The technical definition of bombogenesis is when the atmospheric pressure drops at least 24 millibars over 24 hours.

Saturday’s storm is expected to be a big “bomb,” in terms of dropping pressure.

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National Weather Service forecasters said in a Web posting Friday that explosive cyclogenesis will occur off the mid-Atlantic coast Friday, with an “impressive” pressure drop of about 40 millibars in 24 hours.

The pressure drop leads to intensifying winds - and Saturday’s storm is expected to feature wind gusts of up to 75 miles per hour at the tip of Cape Cod and 55 miles per hour in Boston.

MIT meteorology professor Frederick Sanders. Sanders, who died in 2006, is credited with first using the word “bomb” to describe such storms, MIT says.

“He made important contributions to the analysis, understanding, and prediction of fronts, low pressure systems, hurricanes, squall lines, and flood-producing storms, and he coined the term ‘bomb’ to describe explosively intensifying winter storms,” MIT said in an obituary on its website.

Sanders and colleague John R. Gyakum wrote about “bombs” in a 1980 paper titled, “Synoptic-Dynamic Climatology of the ‘Bomb’.”

The term has become familiar to many in Massachusetts during winter storms in recent years.

“During winter, such cyclone bombs can cause intense cyclones just off the east coast of the USA with storm-force winds, high waves, and blizzards or freezing rain,” according to the textbook “Practical Meteorology” by Roland Stull.

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Material from prior Globe stories was used in this report.


Martin Finucane can be reached at martin.finucane@globe.com.