What is a ‘Bomb Cyclone’?


I am sure that many of you have heard the headlines this past week “Bomb Cyclone to hit the Pacific Northwest,” well what is a Bomb Cyclone and how do they form?

We have to start from the beginning, a cyclone is just another term for a storm system or low pressure that develops. In our latitudes, they are typically called mid-latitude cyclones, and I am sure you know about the tropical cyclones, typically referred to as tropical storms or hurricanes.

In the early stages of storm development low pressure begins to organize, it is during this beginning stage that the storm system undergoes cyclogenesis. This is when we would typically see the formation of fronts in association with the system. To get a storm system or low-pressure system to develop you need to have temperature gradients present. Typically in the Northern Hemisphere you would have warmer air situated south and east of the storm system and would see colder air north and west of the system.

Sometimes while undergoing cyclogenesis, a storm can encounter extremely favorable conditions and rapidly intensify or undergo what we call Bombogenesis. Remember I mentioned pressure, atmospheric pressure is typically measured in millibars, and for a storm system to be classified as a “bomb cyclone,” it would have to have a 24 millibar drop within 24 hours, otherwise known as rapid intensification. This would typically occur when a very cold air mass collides with a very warm air mass.

But it isn’t all about what is going on at the surface. Systems like these need upper air support as well and in the case you see below, we start to see an upper level low develop north of the “surface” storm.

As time goes on, the upper-level low eventually “captures” the surface feature, and then we see the storm system rapidly strengthen, going from 998 millibars all the way to 952 millibars in a little more than 24 hours, thus classifying the example below as a “bomb” Cyclone as this is 46 millibar drop in about 30 hours.

Typically these systems can become quite strong, not only with precipitation, but also the winds and we will typically see wind advisories or high wind warnings issued with these. Precipitation, both rain or snow usually becomes quite intense.

The area off the northeast coast is well known for “Bomb Cyclones” typically referred to as Nor’easters. Cold air will slowly seep into the northeast and clash with the warmer sea surface temperatures off the coast, along that boundary is where we would see a rapidly deepening low-pressure system that would “bomb” out and produce precipitation and wind. While we typically associate nor’easters with snow, they can produce just rainfall, especially if it is early, or late in the season. These systems can develop any time of the year, however, they are most prevalent from October through March.

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