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

Beginning Wednesday and on into Thursday, areas of Northern and Southern California have been undergoing excessive rainfall, widespread flooding, landslides, heavy snowfall and extremely high winds. And there are two culprits behind it all, according to the National Weather Service: a major storm known as a “bomb cyclone” and an airborne phenomena called an “atmospheric river.” 

The hazardous weather has already claimed two lives — a young child died after a redwood tree fell on a home and a 19-year-old woman died after crashing her car into a utility pole on a flooded road. 

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 regions of the sunshine 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 presently touching down in California.

Specifically, those powerful gusts are likely to rip across the coast at speeds reaching between 60 and 65 miles per hour (97 and 105 kilometers per hour). On higher terrain, according to the NWS, their speeds may even reach a staggering 80 miles per hour (129 kilometers per hour).

To make matters worse, this bomb cyclone is accompanied by what’s known as an atmospheric river.

What’s an atmospheric river?

Atmospheric rivers are essentially narrow currents, or rivers, 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 this week’s is even stronger than the last atmospheric river that hit California, over New Year’s weekend, resulting 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 could continue to cause quite a bit of damage across the Californian coast, from places near Sacramento all the way down to Los Angeles. The bomb cyclone will more or less be “dragging” the atmospheric river in.

It won’t be a ‘one and done’ storm

As for a timeline, the NWS believes the storm’s rain showers and thunderstorms will continue through Thursday. When all is said and done, the agency explains, there will likely be about 1-3 inches (2.5-7.6 centimeters) of flooding in urban areas, 3-4 inches (7.6-10.2 centimeters) in North Bay valleys. By Friday morning, 3-6 inches (7.6-15.2 centimeters) of flooding is 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. 

The risk of shallow landslides, rockslides and mudslides in hilly or mountainous regions will also persist. On Friday, there may be a break in rainfall, but an extension of the storm is expected to continue intermittently through Tuesday — and could possibly stretch until about Jan. 16. 

“The message to convey is resiliency as this is not a ‘one and done’ storm,” the NWS said. “Of course, timing and details of subsequent systems will be subject to change. Be sure to stay tuned to the latest information in the coming days,” the discussion concluded.

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