One of the most popular weather terms as we entered 2018 was bombogenesis or the coined phrase “bomb cyclone” as most media outlets had labelled very powerful Winter Storm Grayson on Jan. 4-5, 2018. Although the term bombogenesis sounds very ominous and technical, it actually is fairly simple to understand. Bombogenesis comes from merging the words “bomb” and “cyclogenesis” and is defined as a rapidly intensifying low pressure system containing a pressure fall of at least 24 millibars within a 24 hour period. What made Grayson so unique was that it more than doubled the bombogenesis requirements with an actual pressure drop of 59 mb in 24 hours. The process is commonly associated with larger winter storms like nor’easters but can even occur over land across the mainland of the United States as well.

Before we get into the finer details of bombogenesis, let’s start off by first defining what a low pressure system is. Low pressure is the area in which the pressure of the atmosphere at sea level is lower than the surrounding area. One of the key weather fundamentals and the development of low pressure is related to the rising or falling of cold and hot air masses. Wind will flow from high pressure to low pressure, which causes winds to converge at the surface and thus rising air in a low pressure system. Rising air cools the water vapor in the atmosphere, forms clouds and results in eventual precipitation.

Although there are many different ways to represent or display air pressure, the most common way is in millibars (or mb). Average sea level pressure is 1013.25 mb, and a typical strong low pressure system contains a reading in the upper 900 mb. Winter Storm Grayson became one of the most explosive East Coast storms to ever form as NOAA’s Weather Prediction Center estimated the lowest reading during the storm to be 950 mb. To put this number in perspective, only some of the more devastating tropical storms and hurricanes that affected the Northeast had lower readings than Grayson (Hurricane Sandy – 946 mb, Hurricane Irma – 914 mb).


March 1993 SUPERSTORM “STORM OF THE CENTURY” – 33 mb in 24 hours

February 2013 WINTER STORM NEMO – 29 mb in 24 hours

February 2016 WINTER STORM MARS – 25 mb in only 18 hours

March 2017 WINTER STORM STELLA – 31 mb in only 19.5 hours

January 2018 WINTER STORM GRAYSON – 59 mb in 24 hours

The term bombogenesis was first introduced back in the 1940s but became more widely known in the meteorological vocabulary during the 1980s and even more so today. The factors at work for a low pressure system to “bomb out” or undergo bombogenesis usually depends on several factors all taking place at the same time. To achieve the intense drop in pressure associated with bombogenesis, you need a strong gradient in ocean temperatures, an overlapping jet stream (dividing line between warm and cold air) and a relatively warm core.

During the life of a nor’easter, you have a low pressure system over the warmer ocean water of the Gulf Stream combined with frigid arctic air advancing in from the north. The clash of these air masses will lead to an increase of rising air at the center of low pressure and resultant rapid strengthening. With such rapid intensification, wind speeds increase and the development of heavy precipitation occurs. During this intensification process, you typically see heavy bands of 2 to 4 inches per hour snowfall rates, strong winds with blizzard conditions, coastal flooding, and in some instances even thunder snow.


  • Strongest bombogenesis event along East Coast in FOUR DECADES
  • BLIZZARD / WHITEOUT conditions from Virginia to New England
  • Top snowfall accumulation of 22 INCHES IN ETNA, MAINE
  • Snow rates of 3 INCHES PER HOUR observed in Providence, Rhode Island
  • RECORD COASTAL FLOODING along the east and north facing shores of
    Massachusetts, including Boston Harbor
  • Several reports of 70+ MPH WINDS AND POWER OUTAGES from Virginia to

While buzz words like “bomb cyclone” will continue to dominate news and social media outlets, it always remains important to understand the true meaning of the term as it relates to the weather conditions affecting your area. Bombogenesis is an impactful weather phenomena and can result in dangerous or even life-threatening conditions during winter storms.

Winter Storm Grayson was a direct reflection of how impactful a storm can be when Tallahassee, Florida, saw its first snow in 28 years and Disney World in Orlando, Florida, closed portions of their attractions because of the resultant cold temperatures. As always, check with your most trusted weather forecast provider to make sure you and your operations are prepared to handle an upcoming storm.