A major storm in the North Atlantic caused extreme winds and monster waves in the United Kingdom on Wednesday. Scotland's mountainous St. Kilda Island even recorded a 144 MPH wind gust during the height of the storm. People are breathlessly calling this a "weather bomb." What exactly is a weather bomb, anyway?
The two most common types of low pressure systems we have to deal with are extratropical cyclones and tropical cyclones. The latter needs no real explanation—tropical cyclones are warm-core low pressure systems that derive their energy from intense thunderstorms that make up the storm's eyewall. The former—extratropical cyclones—are the most common type of low pressure system to affect the United States and Europe.
An extratropical cyclone—a cyclone that contains cold and warm fronts—forms as a result of lift caused by an upper-level trough or strong winds in the jet stream. Both a trough and certain parts of a jet stream foster a process known as divergence, or winds spreading out at the upper-levels. When winds in the upper-levels diverge, air has to rush in to fill the void, and that air comes from the surface. Air rushes up to fill the space left behind by the diverging winds aloft, and this process creates low pressure at the surface. Stronger winds in the jet stream can lead to stronger divergence which, in turn, can lead to stronger low pressure systems.
Sometimes two jet streaks—areas of enhanced winds within the jet stream—can align in such a way that they create major large-scale lift in the atmosphere. This kind of setup is often responsible for some of the stronger extratropical cyclones we see, and they can deepen very rapidly. If the minimum pressure in an extratropical cyclone deepens strong enough and fast enough, we say it underwent "bombogenesis."
Bombogenesis, or that "weather bomb" everyone is talking about, occurs when an extratropical cyclone strengthens by at least 24 millibars in 24 hours. If a relatively weak 1004-millibar cyclone at noon on Thursday deepens to 980 millibars by noon on Friday, it underwent bombogenesis, and we sometimes call it a "bomb."
In the case of the storm in the North Atlantic, it rapidly deepened beyond the limits of bombogenesis, so it's accurate to call it a bomb in the context of the term's definition. The storm was immense—on Wednesday morning, the GFS model analyzed the low's minimum central pressure down around 950 millibars with surface winds of ~70 MPH across a huge swath of the North Atlantic. Due to the strong winds and incredible 1,200+ mile fetch from Greenland to the U.K., the storm created incredible waves off the western coast of the United Kingdom and Ireland. The video at the top of this post shows the high waves and stiff winds as they battered Scotland's Orkney Islands on Wednesday.
While there is an argument to be made that many news organizations are just calling it a weather bomb to get you to tune in or click their story, there is a solid meteorological basis behind the term. The reason so many people think it's pure hype is because the media has resorted to scouring their thesauruses searching for the next colossal, frightening adjective to describe a storm. The public's thirst for weather drama is like a hard drug. Once you get high on the storm of the century, it takes a bigger, badder storm to give yourself your next nerd high. Superstorm. Polar vortex. "Weather bomb" achieves something we'll call the CNN Equilibrium—it's the perfect term to give people the hype they want without being called out for misrepresenting the facts for ratings.
We often see cyclones undergo bombogenesis in the winter and spring months, so we'll hear the term more often as the seasons change and strong storms become more commonplace.
[Image: WeatherBELL | Video: Wind/rain/waves in Scotland's Orkney Islands, via BBC]