"Bombogenesis": Cool Word, But What Does It Mean?

If you live in the  North East, you’ve no doubt heard about the “bomb” cyclone that rode up the coast from January third through the fifth. You also may have heard about the storm undergoing “bombogenesis.” So what exactly was this storm?

The short answer is that it was a nor’easter. But the long answer is so much cooler.

The origin of the storm can be traced back to the polar jet stream, the river of fast-flowing air high above the icy tundras of the arctic circle. This jet normally keeps frigid arctic air contained to the north, but occasionally, a section of the jet will significantly weaken, allowing the arctic air mass to spill south. This process of the polar jet weakening and allowing frigid air masses to migrate south is often dubbed the “polar vortex” by media outlets and weather presenters. As the arctic air mass spills farther and farther south, it induces a sharp “U” shape in the mid-latitude jet stream (this jet is exactly the same as the polar jet, but it resides much farther south over the mid-latitudes of the Northern Hemisphere, and it is the driving force behind much of the weather in the U.S.). This “U” shape is called a trough, and it will be where the trigger for the nor’easter comes from. Off the east coast of Florida, the boundary between the warm, moist subtropical air southeast of the trough and the cold, dry air in the trough provides the potential energy needed to fuel a powerful storm. What sets the energy off is a small disturbance in the jet stream that rides up the trough, inducing an area of low pressure along the warm/cold boundary. The immense potential energy built up through the temperature difference can finally be put to work, as the area of low pressure begins to deepen. Normally, the low would rapidly intensify as it rode up the East Coast, wrapping moisture around the circulation to the western side, where it would fall as snow, accompanied by gusty winds.

But the nor’easter 5 that began on January third had a little extra “kick” than a typical storm.

As the fledgling storm moving up the coast, it encountered not one, but two separate upper air disturbances (waves of energy in the upper atmosphere). They positively interacted with the low, allowing it to undergo that buzzword you probably heard, “bombogenesis.” Bombogenesis is when an area of low pressure drops 24 millibars (a unit of air pressure) in 24 hours. It is relatively rare, usually only happening one or two times a year, and is the upper echelon of non-tropical storms. What makes the recent storm special is that it dropped 59 millibars in 24 hours, more than double the “bombogenesis” criteria. This incredible drop in pressure makes it one of the most rapidly intensifying non-tropical storms on record.

The storm’s structure was right out of a textbook.

The central circulation was so intense that it bent the warm front on its eastern side back on itself, trapping warm air in the center of the storm. This warm air seclusion showed up on satellite as a well-defined “eye,” something usually only seen in strong tropical storms. The northwest sector of the storm featured an extensive precipitation shield, the result of moisture being wrapped around the warm southeast sector of the storm to the cold northwest sector of the storm, where it condenses and precipitates as snow. Accompanied by strong winds, blizzard conditions prevailed in the precipitation shield. A cold front sprawled from the center of the storm in the Mid-Atlantic to Central America, spawning thunderstorms along its 2,300 mile extent. A dry slot showed clearly on satellite images, the result of dry continental air flowing around the center from the south.

The storm’s effects were significant, but fortunately mitigated some by its distance from the coast.

Snow totals exceeded a foot over a wide swath of New England accompanied by very strong winds, especially near the coast. Those strong winds combined with a drop of pressure caused a storm surge along parts of the New England coast, inundating towns with ice-covered water. But the storm’s most far-reaching impact was after it moved out of the area. Record cold spilled in behind the storm’s departure, and affected the entire Northeast.

The nor’easter from January third through January fifth was extraordinary, but not unprecedented. The National Weather Service (NWS) estimated the return interval on a storm of this magnitude to be around 30 years.

If you see anyone trying to link this event to climate change, do not believe them. While climate change may be affecting nor’easters, meteorology’s understanding of that potential link is so limited that it might as well be zero.  So wait until cutting edge research catches up to our questions.

I am obsessed with the weather. I am slightly less obsessed with photography.

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