The frigid and stormy weather system delivering subzero wind chills and blizzard conditions to parts of the United States this week is forecast to strengthen so quickly that it will earn an ominous meteorological distinction: “Bomb cyclone.”
If that sounds menacing, it’s because it is supposed to: The term was designed to convey a degree of intensity and danger that is typically associated with hurricanes, but that even winter storms can carry.
Here is what it means, and what it could mean for the millions of people in the storm’s path.
The definition is clear-cut and technical: A bomb cyclone is a mid-latitude storm whose central air pressure falls at a rate of one millibar per hour for at least 24 hours.
Normal air pressure is about 1010 millibars, a measurement of the force exerted by the weight of the atmosphere. But in stormy weather, air pressure drops well below that — the lower the pressure, the stronger the storm.
The pressure of the storm system sweeping across the country this week is forecast to fall from 1003 millibars Thursday night to 968 millibars Friday night, a drop of 35 millibars. That is more than enough to qualify as what meteorologists call “explosive bombogenesis,” a rapid intensification that warrants the “bomb cyclone” label.
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As with any storm, they develop when drastically different air masses clash — typically, cold and dry air moving down from the north and warm, moist air coming up from the tropics. The warmer air rapidly rises, creating cloud systems, lowering air pressure and developing into a storm system that circulates counterclockwise around that center of low pressure.
Rapid storm strengthening is a signal that increasing amounts of warm air are being drawn into a storm’s circulation, spiraling toward its center and rising out its top. When more air escapes out the top of the storm than is being sucked inward, air pressure drops even further.
The differences in air temperature that feed this process can be especially pronounced when a polar air mass is as cold as the one surging into the United States. Air temperatures were dropping to more than 30 degrees below zero in Montana on Wednesday.
Bomb cyclones, often occurring in the fall or winter, typically produce heavy rain or snow, coastal flooding and hurricane-force wind gusts.
For example, one that hit New England this year dumped as much as 2 feet of snow; another that hit the Pacific Northwest in 2019 produced a 106-mph wind gust.
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The term was coined in a 1980 research paper by MIT meteorologists Frederick Sanders and John R. Gyakum. Gyakum told The Washington Post in 2018 that it was born out of a need to better communicate the intensity of storms outside summertime and hurricane season.
“Given their explosive development, it was an easy path to take to just call these systems ‘bombs,’” Gyakum said.
He said that while some winter storms had similar intensity to hurricanes, many people would assume severe storm risks passed with the end of hurricane season in the fall. “Our goal was to help raise awareness that damaging ocean storms don’t just happen during the summer,” said Gyakum, now a professor of atmospheric science at McGill University in Montreal.