What is a ‘bomb cyclone’?

A powerful storm hits the West Coast and brings with it a terrifying-sounding weather term: bomb cyclone.

Bomb cyclone is a term used by weather enthusiasts to describe a process meteorologists commonly call bombogenesis. It is the rapid intensification of a cyclone over a short period of time, and this can happen during powerful storms like that Northern California and the Pacific Northwest are preparing for this week.

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

According to the National Oceanic and Atmospheric Administration, a bomb cyclone occurs during the rapid intensification of a cyclone located between the tropics and the polar regions. It can happen when a cold air mass collides with a warm air mass, something that can happen over ocean waters, the agency says.

The measurement required to determine whether a cyclone may exist classified as a bomb cyclone can be tricky, but it mainly involves a rapid pressure drop. Atmospheric pressure is measured in millibars by the National Weather Service. If a storm drops 24 millibars or more in 24 hours or less, it can be considered a bomb cyclone, said Stephen Baron, a weather service forecaster in Gray, Maine.

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

The National Weather Service Weather Prediction Center has issued excessive rainfall risks from Tuesday through Friday due to the powerful storm expected in Northern California and the Pacific Northwest. The storm is coming as the region experiences a storm atmospheric rivera long plume of moisture, over the Pacific Ocean.

The Weather Prediction Center said the storm intensified so quickly that it could be considered a bomb cyclone.

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

This storm is expected to bring heavy rainfall. That could lead to flash flooding and winter storms in different parts of the West Coast, depending on the elevation.

Strong wind storms are also expected in some parts of the west coast.

Travel is expected to be dangerous and power outages are expected. Significant damage to trees and infrastructure can also occur.

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

A bomb cyclone in 2018, which popularized the term on social media, brought snow to the southeast and winds close to hurricane force. Another in 2022 brought extreme weather and bitter cold to much of the country.