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.
