What Is a ‘Bomb Cyclone’? Here’s Why It’s So Brutal

Since Jan. 4, areas of Northern and Southern California have been hit by sweeping amounts of rainfall, widespread flooding, landslides, heavy snowfall and extremely high winds. This hazardous weather has already claimed at least 12 livesincluding a toddler who died after a redwood tree fell on a home and a 19-year-old woman who died after crashing her car into a utility pole on a flooded road. 

There are two culprits behind it all, according to the National Weather Service. The first is a major storm known as a “bomb cyclone,” and the second, an airborne phenomenon called an “atmospheric river.” 

So, what do bomb cyclone and atmospheric river mean? And why are they menacing enough to result in fatalities and prompt the governor of California to declare a state of emergency, with high-risk regions of the state ordering mandatory evacuations?

What’s a bomb cyclone?

Simply put, a bomb cyclone is a large, intense storm that’s associated with a sudden and significant drop in atmospheric pressure. 

In general, cyclones, which are basically giant, rising columns of air, form when a mass of low-pressure air meets a mass of high-pressure air. But bomb cyclones happen when the pressure suddenly and starkly drops in the low-pressure-mass section. That makes the pressure difference between both masses much more pronounced, which intensifies winds correlated with the storm. 

You can think of the bomb cyclone’s column as rising super fast all of a sudden, lowering air pressure at the center far too quickly and creating a sort of vacuum effect, producing ultrastrong winds in the process. 

Meanwhile, Earth’s rotation pushes these high-intensity winds across the globe — like those touching down in California.

Specifically, those powerful gusts were initially expected to rip across the coast at speeds reaching between 60 and 65 mph (97 and 105 kilometers per hour). On higher terrain, according to the NWS, their speeds were expected to exceed a staggering 80 mph (129 km/hr). However, this bomb cyclone has already topped speeds over 100 miles per hour (161 km/hr) in some areas.

And to make matters worse, all of the chaos is accompanied by what’s known as an atmospheric river.

What’s an atmospheric river?

Atmospheric rivers are essentially narrow currents in the air that carry lots of water vapor across the world. They transport most of that water vapor outside of the tropics, then release it in the form of either rain or snow. 

Smaller, weaker atmospheric rivers usually don’t pose a major threat — most of those mini ones are actually considered good for replenishing our water supply — but more extreme versions of these events have the potential to create floods and cause mudslides. 

And the one at hand is even stronger than the atmospheric river that hit California over New Year’s weekend, which resulted in major floods, dozens of cars stranded on highways and thousands of homes without power.

So, coupled with the bomb cyclone bearing down on the West Coast, this particular atmospheric river has been causing quite a bit of damage across the California coast, from places near Sacramento all the way down to Los Angeles. The bomb cyclone more or less is “dragging” the atmospheric river in.

“Nearly all of California has seen much above average rainfall totals over the past several weeks, with totals 400-600% above average values. This has resulted in nearly saturated soils and increasingly high river levels,” the NWS said during a short range forecast discussion on Tuesday, according to an online transcript of the conference.

It won’t be a “one and done” storm

As for a timeline, the storm’s rain showers and thunderstorms poured from Jan. 4 into Jan. 5, with Jan. 5 night and early Jan. 6 offering a bit of a respite. But then it continued through the weekend, fell into Monday and has persisted on Tuesday. 

“Just as the last episode of heavy precipitation across California is beginning to wind down early this morning, another energetic low pressure system is quickly gathering strength off the West Coast and heading once again toward California,” the NWS said on Tuesday. “In addition to being highly moisture-laden, this rapidly intensifying system is also packing some thunderstorms.”

The bomb cyclone on Wednesday, Jan. 4, moving toward the coast of California.

NOAA Satellites

The core of the system, the organization explained, will slam onshore with moderate to heavy rain resuming across much of California through Tuesday night, while several more feet of snow is possible along the Sierra Nevada.

When all is said and done, the agency projected at the start of the storm, there will likely be at least 1 to 3 inches (2.5 to 7.6 centimeters) of flooding in urban areas, and 3 to 4 inches (7.6 to 10.2 centimeters) in North Bay valleys. About 3 to 6 inches (7.6-15.2 centimeters) of flooding was expected to pile up in coastal ranges and over 8 inches (20.3 centimeters) in the wettest areas like the Santa Cruz mountains, just over 70 miles (112 kilometers) south of San Francisco. 

As for the end of this menacing cyclone, the NWS said it could possibly have to wait until about Jan. 16. 

“The message to convey is resiliency as this is not a ‘one and done’ storm,” the NWS said during a Jan. 4 forecast discussion — a sentiment that has proven to be absolutely correct.