What Is This “Bomb Cyclone” Threatening the U.S.?
New England is nervously awaiting heavy snow and strong winds as “Winter Storm Grayson” barrels up the U.S. Atlantic coast. Already, the storm has hit regions not accustomed to severe winter weather—Florida, Georgia, Virginia and the Carolinas—with a mixture of snow and rain, according to news reports. This is no typical winter storm—meteorologists have been predicting Grayson will soon turn into a particularly intense system called a “bomb cyclone.” Scientific American spoke with Jeff Masters, director of meteorology and co-founder of Weather Underground, about the science behind the powerful winter storm hitting the eastern U.S.
[An edited transcript of the interview follows.]
What exactly is a “bomb cyclone”?
A bomb cyclone is a low-pressure system that intensifies very rapidly—you have to have a fall in pressure of at least 24 millibars in 24 hours to qualify as a “bomb cyclone,” or “bombogenesis,” event. When a storm has its pressure rapidly fall like that, it’s going to drive stronger winds, because winds try and blow to equalize differences in pressure. The atmosphere doesn’t like to have different pressures, so what will happen is the wind will flow from high pressure to low pressure to try and balance out the difference.
With this storm, which [has been] named Grayson, the biggest concern is very strong winds gusting as much as 60 miles per hour along much of the east coast. Those strong winds are probably going to cause a lot of power outages.
Does this “bomb” effect just create strong winds or also colder weather and more precipitation?
When you get a rapidly intensifying storm, all of the impacts increase. As the winds grow stronger, you’re also going to be pulling in more water vapor from the periphery of the storm into the center where it then gets forced upwards and condenses—and you get increased precipitation. At the same time, because the storm is getting deeper, it’s able to pull in Arctic air from northern Canada, much more so than if it weren’t so intense. So its reach increases. That means that it’s going to have much colder air on its northern side and, conversely, much warmer air down on its southeastern side. The center of the storm is going to be over the very warm Gulf Stream, and that’s going to provide a lot of evaporation of moisture into the storm, driving heavy snowfalls when it wraps around where the cold air is on the northern side.
What kind of physical conditions create a bomb cyclone?
What has happened is the jet stream has gotten into a big kink. The jet stream is the band of high-altitude winds that goes from west to east over the mid-latitudes. But the jet stream can take a big dive and get a kink in it, so that it has a big loop that goes far to the south and then comes back far to the north. This means that you’re now bringing in very warm air on the east side of this kink flowing northwards—in this case, out over the Gulf Stream where you have got a lot of warm water, too. And just a few hundred miles west of there, now you have got cold Arctic air adjacent to this warm moisture that’s being pulled northward. Those two air masses of very contrasting temperatures are interacting, and the storm forms right along that high-energy boundary there.
What [the storm is] doing is, it’s drawing energy from the difference in temperatures—more technically, the difference in densities—between the two air masses. So the bigger the contrast between that cold Arctic air on the one side and the warm moist air from the ocean on the other side, the stronger your storm is going to be. In this case, that kink is very sharp, so there’s a very intense difference in air masses on either side of this jet stream boundary—and that’s driving the storm. In addition, you’re getting a little bit of energy coming from the ocean itself, like a hurricane does. Hurricanes derive their energy from warm ocean waters—they pull the energy right out of the ocean. This storm, Grayson, is also going to get some of its energy from the very warm Gulf Stream waters.
How unusual is this type of storm?
You usually get one [bomb cyclone] per year, as far as along the northeastern coast of the U.S. goes. We had one in March 2017 that helped drive very heavy snows in the Northeast and brought a bigger Arctic outbreak to the southeastern U.S. that caused over a billion dollars in freeze damage. There was also one the previous year, in January 2016, a big nor'easter that caused over $2 billion in damage. We’re getting our one for this year, hopefully. Although all bets are off—when you set up a pattern like this, it’s not uncommon to get two major nor'easters in a month. I’m not seeing anything in the models right now suggesting that’s going to happen this year though.
But if you look at the central pressure they’re expecting for this storm, it’s going to go below 960 millibars, which is unusually low. I haven’t found any storms in the past 10 years that have done that. So this is going to be an unusually intense one, and it’s going to bring a lot of impacts along the coast. Particularly, I’m a little concerned about coastal flooding south of Boston, along some of the shores there. We’re expecting major coastal flooding with storm surge of two and a half to three feet [there].
Is “bomb cyclone” a new term?
The bomb terminology has been around since at least 1980. That’s when it was first defined. I've been using that term for 35 years, but it became more fashionable just in the last few years. The media probably helped drive some of the fascination with this term.
Some people have been comparing this storm with a hurricane—is that fair?
In some ways. The structure is different, and the energy source is different, so you’re going to have considerable differences in what the impacts are. But it’s the same size as a hurricane, and it’s got similar sorts of winds. The pressure is actually far lower than what you would see in a hurricane with equivalent winds. But a winter storm like this doesn’t focus its strongest winds in an eye wall like a hurricane does, so it takes a much lower pressure to get hurricane-force winds with a winter storm than it does with a hurricane.
Temperatures are predicted to drop very low after this storm—why?
The storm is a counterclockwise rotating area of low pressure, which means that on its northwestern side those counterclockwise blowing winds are going to be pulling a lot of Arctic air from Canada down over New England. It’s going to be frightfully cold Saturday morning—I think in Maine you’re going to see some 30-below readings, and it’s going to be tough to get above zero in New Hampshire and northern Maine. Probably you’ll get above zero in Boston; New York, maybe single digits for a high.
Could climate change be playing a role in this bomb cyclone?
We would expect it to be playing some role, since climate change is fundamentally affecting the atmosphere and changing the base state in which storms arise. So potentially you would have more moisture available to this storm, just because the oceans are hotter because of global warming—and that could potentially increase the impacts of a storm like this. There’s also some question about whether global warming might be affecting the jet stream, which is an important trigger for this storm. [Climate change] may be making the jet stream have more meanders, more of these large loops—these kinks—that can drive these sorts of storms. It’s an active area of research, and there hasn’t been unanimous scientific agreement about how climate change is affecting these storms.