When storms 'bomb out:' Explaining how a bomb cyclone forms

The name bomb cyclone comes from the term 'bombogenesis'

IN OCTOBER, the U.S. experienced three severe weather systems known as a "bomb cyclone," spanning each coast.

The name comes from the meteorological term "bombogenesis," or "explosive cyclogenesis," when a storm strengthens at least 24 millibars within 24 hours. A storm that achieves this mark becomes known as a "bomb cyclone." Basically, it's a low-pressure storm that intensifies very quickly, bringing intense impacts.

Bomb cyclones are more common in the Pacific Ocean but do happen in the Atlantic Ocean.

"Bombogenesis is fairly common in the Pacific Ocean region because there is enough water surface area for strengthening. It has happened a few times on the Atlantic coastline; however, it is not as common there," said FOX Weather Meteorologist Stephen McCloud. "Forecasting this phenomenon is rather difficult to do especially when there are changes in forecast model runs. Most of the time, this happens in real-time situations. It is easy to pick out, especially on the satellite when you see bombogenesis occur."

The first of two bomb cyclones in the Pacific Ocean formed on Oct. 21, when its central pressure dropped 50 millibars in 24 hours. A second bomb cyclone three days later set records on the Pacific coast when it strengthened to 942.5 millibars for its central pressure, marking the lowest measures for a storm in the region of the ocean near the Pacific Northwest, according to the National Weather Service. Its central pressure dropped 46 millibars in 24 hours.

The combination of an atmospheric river of rain and the bomb cyclone brought a foot of rain, 3 feet of snow and the highest mountain wind gust of 159 mph.

Early this week, a nor'easter brought flooding rain and damaging winds to the northeast. After moving off the Atlantic coast and dropping 28 millibars in 24 hours the system met the criteria for a bomb cyclone combining two severe weather events into one. 

The Boston area saw some of the most severe winds at 80 mph.

According to the NWS, the last bomb cyclone to occur off Boston's coast was in October 2019.

However, not all bomb cyclones happen in the fall and winter months.

A study published in the Journal of Applied Meteorology and Climatology looked at 783 bomb cyclones over 15 years in the Pacific Ocean.

The study found in 69% of cases bomb cyclones frequently happened from December to February and early March. The frequency depends on the region of the Pacific with a peak in March with a second peak in October, November and December, according to the researchers. 

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