A ‘Bomb Cyclone’ Is Threatening California. Here’s What That Means

The bomb cyclone as it moves toward the coast of California.
NOAA Satellites

On Wednesday and Thursday, areas of Northern and Southern California are poised to experience excessive rainfall, widespread flooding, landslides, heavy snowfall and extremely high winds, according to the National Weather Service. And there are two culprits behind the forecast devastation: a major storm known as a “bomb cyclone” and an airborne phenomena called an “atmospheric river.” 

“Now is the time to finish any storm preparations,” a NWS representative said on Wednesday, per an online transcript of the organization’s San Francisco Bay Area forecast discussion. “But this window is closing, as the winds will only continue to increase through the day.”

So, what do “bomb cyclone” and “atmospheric river” mean? And why are they worrisome enough to prompt organizations like the NWS to urge residents of the West Coast to prepare for a “brutal system” that could lead to loss of human life?

This storm would be fairly notable in its own right, as it will be associated with an unusually well-defined warm *and* cold frontal passages and an exceptionally moist and relatively warm #AtmosphericRiver. #CAwater #CAwx [3/n] pic.twitter.com/owveYNNQRz

— Daniel Swain (@Weather_West) January 2, 2023

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 ready to touch down in California on Wednesday and Thursday. 

Our #GOESWest🛰️ full-disc satellite shows a powerful storm undergoing bombogenesis off the northern CA Coast.

What is a “Bomb Cyclone”? By definition, it is a low pressure system that experiences a fall in pressure of 24 millibars in 24 hours. #CAwx https://t.co/zmYAPqXknp pic.twitter.com/IDnazirGPx

— NWS Sacramento (@NWSSacramento) January 4, 2023

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.

“These strong and gusty winds today will topple trees, and potentially create power outages,” the NWS representative said during Wednesday’s discussion, highlighting that the storm has already begun to close in but will soon intensify.

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 the one meteorologists are presently cautioning about is considered a big one. It’s even expected to be 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 headed to the West Coast, this particular atmospheric river could 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.

An atmospheric river approaches the western U.S. Observed by the ALPW product, derived from multiple polar-orbiting satellites. pic.twitter.com/oyE6KsZ07r

— CIRA (@CIRA_CSU) January 4, 2023

“Rain is likely to persist through much of the day across the North Bay where the atmospheric river will be places ahead of the main cold front,” the NWS said. “Don’t let your guard down if things appear quieter than you expected this morning.”

It won’t be a ‘one and done’ storm

The NWS also issued a warning that rainfall and thunderstorms will increase in both coverage and intensity between the afternoon and evening, as well as to expect rises in creeks, streams, rivers and flooding in urban areas. 

As for a timeline, the organization believes the storm’s high winds will diminish early Thursday morning, but rain showers and thunderstorms will continue through much of the day. 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, and by Friday morning, 3-6 inches (7.6-15.2 centimeters) 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 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.