A “bomb cyclone” hit the Pacific Northwest this week, killing at least two and knocking out power to over a half a million people. Now, just ahead of the busy Thanksgiving travel season, the storm will continue its path across the U.S., set to wreak havoc across the nation as it makes its way east.
In the wake of news reports warning of a “bomb cyclone” and even a first time ever “triple-bomb cyclone” event, a lot of weather nerds went scrambling to enter “what causes a Bomb Cyclone” into their search engines. This weather nerd is one of them.
What is a Bomb Cyclone?
To put it simply, a bomb cyclone, which occurs through the process known as bombogenesis, is basically a winter hurricane. It “occurs when a mid-latitude cyclone rapidly intensifies,” or quickly drops in atmospheric pressure, marking the strengthening of the storm, according to the National Oceanic and Atmospheric Administration (NOAA).
According to The Weather Channel, bombogenesis itself is an ominous-sounding term frequently used in the winter to describe powerful low-pressure systems that intensify rapidly. The process of bombogenesis begins as cyclogenesis, meaning the development or strengthening of an area of low pressure. To be classified as a weather bomb, or having undergone bombogenesis or “bombing out,” the central pressure of a low-pressure system must drop at least 24 millibars within 24 hours.
A 2021 study lead by Robert Fritzen from Northern Illinois University found that only about 7 percent of all non-tropical low pressure systems near North America from 1979-2019 were bomb cyclones. The large majority of those occurred off the East Coast of the U.S., which averaged one such bomb cyclone per year.
What causes Bombogenesis?
In the case of the recent storm surge that targeted the West Coast from Washington down to Northern California, bombogenesis results when there is a large temperature gradient, usually between a cold continental air mass and warm sea-surface temperatures, whether it be Pacific or Atlantic.
However, it can also be the product of a cold polar air mass and much warmer air from the south, say, over the Plains state. Over that temperature contrast, a powerful, intensifying jet-stream disturbance triggers air to rise, kicking off the bombogenesis process. This happens most often from October through March, but it can happen any time of year. Frequently, nor’easters are weather bombs due to cold air surging southward from Canada combined with the warm ocean waters of the Gulf Stream.
So, what happens when a weather bomb strengthens? Winds increase dramatically, and precipitation, including snowfall, can become intense. Blizzard conditions can occur, sometimes accompanied by lightning as the system is “bombing out.”
Our East Coast is Notorious for Bombogenesis Events
The corridor off the East Coast of the U.S. is notorious for bombogenesis events, particularly in recent years. According to weather.com, in March of 2018, the first of the so-called four-easters, Winter Storm Riley, bombed out off the East Coast and drove destructive coastal flooding into parts of the Eastern Seaboard. Two months prior to that in early-January 2018, Winter Storm Grayson intensified at the most rapid rate on record for the western Atlantic Ocean, plunging roughly 59 millibars in 24 hours to a low of 950 millibars.
Winter Storm Mars underwent bombogenesis in February 2016. Blizzard conditions were verified in multiple locations on Cape Cod, and a wind gust of 65 mph was measured on Nantucket Island. In 2015, Winter Storm Iola brought winds strong enough to cause coastal flooding at some locations in Massachusetts.
But even though weather bombs typically are found during the winter with a marine influence, this is not always the case. An example of bombogenesis over land was the “Octobomb” that impacted portions of the Plains and Midwest Oct. 25-27, 2010. All-time record-low barometric pressure readings were set by this system in Minnesota and Wisconsin.
Pacific Coast Atmospheric River + Bomb Cyclone = Disaster
What made this week’s bomb cyclone event so much more devastating was that it hit at the same time as an atmospheric river event, and that was a recipe for disaster.
An atmospheric river is exactly what it sounds like – it’s a long, narrow river of water vapor in the lower atmosphere. These rivers in the sky transport moisture from the subtropics to the mid-latitudes. When an atmospheric river runs up against North America’s West Coast, the mountains and complex topography force the air to rise, cool, and the moisture to condense and precipitate. That can mean feet of snow at high elevations and rainfall elsewhere.
Atmospheric rivers tend to form out ahead of cold fronts that are associated with low-pressure systems known as extratropical cyclones. These are air mass boundaries circulating around the area of low pressure. A bomb cyclone is a rapidly intensifying extratropical cyclone. These two weather phenomena go hand in hand. A strong atmospheric river will feed moisture into the low-pressure system, providing fuel for the cyclone. The stronger the low-pressure system becomes, the stronger the atmospheric river becomes.
While this current atmospheric river is hitting the coast, the bomb cyclone will sit over the ocean off the Pacific Northwest and spin. As it spins, it sends small frontal waves through the atmosphere that push the atmospheric river inland. That creates a lot of uncertainty for forecasts.
If you picture the atmospheric river as a fire hose pointed at the coast, these frontal waves are essentially the fireman taking his hands off the fire hose and letting it go all wavy. It can move northward, and then back southward. The question is how far it will go before pivoting, then how quickly it will pivot back. The harder and longer it rains, the more impacts you’re likely to see.
The saving grace with this storm is that it’s early in the season, so the soils are relatively dry. That means they’ll be able to soak up more of the moisture. If a storm like this hit in winter, after the soils were already saturated, more water would run off, causing more widespread flooding than an early-season storm.
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