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A powerful storm is moving through the West Coast, bringing with it a terrifying weather term: a bomb cyclone.

Bomb cyclone is a term used by weather enthusiasts to describe a process meteorologists commonly call bombogenesis. That’s the rapid intensification of a cyclone in a short period of time, and it can happen during powerful storms like the one tearing through Washington into Northern California this week.

But what is a bomb cyclone, why does it happen and why are these large storms so feared?

What is a bomb cyclone?

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A bomb cyclone occurs during the rapid intensification of a cyclone located between the tropics and polar regions, according to the National Oceanic and Atmospheric Administration. It can happen when a cold air mass collides with a warm air mass, which is something that can occur over ocean water, says the authority.

The measurement needed to determine whether a cyclone qualifies as a bomb cyclone can be tricky, but it is largely a matter of rapid pressure drop. Atmospheric pressure is measured in millibars by the National Weather Service.

If a storm drops by 24 millibars or more in 24 hours or less, it can be considered a bomb cyclone, said Stephen Baron, a forecaster with the weather service in Gray, Maine.

“I would say rapid intensification of hurricanes is one of the most common times we see it,” Baron said. “We see that with Nor’easters sometimes.”

Why is it happening on the west coast?

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At least two people have died and hundreds of thousands have been left without power after the bomb cyclone slammed into Seattle, Washington, on Tuesday night as it charted a destructive path to northern California and Oregon.

The storm came as the region experiences a “significant” atmospheric flood, which is a long plume of moisture, over the Pacific Ocean.

Heavy rain and strengthening winds battered the Bay Area on Tuesday night, with the winter storm reaching maximum intensity on Wednesday, the Weather Prediction Center announced. Gusty winds gusting over 77 mph wreaked havoc in Western Washington on Wednesday with high surf along the Pacific Northwest coast throughout the day.

The WPC has also warned of life-threatening flooding and rockfall through the Golden State, with rainfall expected to peak Thursday, reaching up to 16 inches.

The “abnormally strong” storm intensified quickly enough to be considered a bomb cyclone, the agency said.

When else has that happened?

Bomb cyclones can happen in many places and are not unique to the West Coast. They can be found in several parts of the world’s oceans, including the northwest Pacific and the North Atlantic.

They have been associated with major weather events across the country in recent years. Hurricane Milton, which made landfall in Florida last month as a Category 3 hurricane, was a recent example of a bomb cyclone, Baron said.

A 2018 bomb cyclone, which helped popularize the term on social media, brought snow to the southeast and near-hurricane-force winds.

Another in 2022 brought extreme weather and bitter cold to large parts of the country.