What are bomb cyclones and how do they form?

A powerful storm system known as a "bomb cyclone" was unleashing its fury across Washington state on Wednesday, causing widespread power outages, claiming at least one life, and creating treacherous travel conditions across the region. The storm knocked out power to more than 600,000 homes and businesses across Washington, Southwest Oregon, and Northern California.

  

Reuters

Ricco Anthony, San Francisco Public Works laborer, lifts a sandbag to drop it into a city resident's car trunk, as a "bomb cyclone" storm heads for Northern California, in San Francisco, California, U.S., November 20, 2024.

In Lynnwood, just north of Seattle, the severe weather turned deadly when a falling tree struck a homeless encampment, killing one woman. Two additional people were injured when another tree fell on their trailer in Maple Valley, southeast of Seattle. The storm's impact was so severe that schools across western Washington were forced to either cancel classes or delay their start times.

With tropical storm-force winds reaching 50 miles per hour and gusts around 70 mph, the weather system has already dropped 2-3 inches of rain over Southwest Oregon and Northern California. However, meteorologists warn that the worst may be yet to come. "The storm is just beginning," said Rich Otto, a meteorologist with the NWS Weather Prediction Center in College Park, Maryland. The system is expected to stall over Northern California in the coming days, potentially dumping 10-15 inches of rain by Friday, with some areas receiving up to 20 inches.

What is a 'bomb cyclone'?

A dangerous weather phenomenon called a bomb cyclone that occurs in mid-latitudes - between Earth's tropics and the polar regions - can bring strong and damaging winds, torrential rains, heavy snowfall, flooding and frigid temperatures.

It displays some traits of a hurricane, but it is not one.

A bomb cyclone, also referred to as explosive cyclogenesis or bombogenesis, is a mid-latitude cyclone that has rapidly intensified. A cyclone is a low-pressure weather system - one where the atmospheric pressure is lower at its center than in surrounding areas - with winds rotating inward. It circulates in a counterclockwise direction in the northern hemisphere and a clockwise direction in the southern hemisphere.

A low pressure storm system known as a "bomb cyclone" forms off the coast of the U.S. Pacific Northwest and western Canada in a composite satellite image November 19, 2024.Reuters

A bomb cyclone's winds can reach hurricane force - 74 miles (119 km) per hour - and stronger. These storms tend to form during winter and can spawn copious amounts of precipitation. They have life spans of about a week during which they grow to peak intensity over roughly four to five days and then dissipate over the last two, according to Jon Martin, a professor of meteorology at the University of Wisconsin at Madison.

How does it form?

Bomb cyclones form when the conditions at the surface and at the jet stream level are ideal for the storm to intensify. The jet stream is a narrow band of strong winds in the upper atmosphere. A variety of atmospheric processes combine to produce these storms. Almost all bomb cyclones have a precursor disturbance in the winds in the middle part of the troposphere - the lowest region of Earth's atmosphere - about 3-5 miles (5-8 km) above the planet's surface, Martin said.

Another important feature common to many, but not all, explosive cyclogenesis events is a warm ocean surface. Many of the most intense bomb cyclones form over oceans. Precipitation can be prodigious. When water vapor changes into liquid and ice, as it does in these storms, enormous amounts of energy - called latent heat energy - are released. Some of that energy further intensifies the storm.

By virtue of the atmospheric pressure getting so low, differences in pressure across the storm can become very large, powering strong winds that can have devastating effects.

When and where are they likely to occur?

Explosive cyclogenesis occurs mostly over oceans and most commonly during the cold season in both hemispheres - roughly November to March for the northern hemisphere and roughly May through August in the southern hemisphere, though these storms can be earlier or later than that. Areas particularly prone are situated in so-called storm tracks along the east coast of continents because this is where the warmest ocean currents exist, such as the Kuroshio off Japan and the Gulf Stream off North America, Martin said.

Bomb cyclones can be very destructive and pose particular danger to shipping interests, since many of them occur over the oceans, according to John Knox, an atmospheric scientist and professor of geography at the University of Georgia. Some bomb cyclones have occurred on the Great Lakes of North America and caused shipwrecks there, too, Knox said.

Isn't it the same as a hurricane?

While bomb cyclones can unleash hurricane-force winds and sometimes display characteristics of a hurricane, they are not hurricanes. They form from different physical processes and do not have the symmetry of hurricanes, which also are low-pressure systems. Bomb cyclones occasionally develop "eyes" resembling those at the center of a hurricane, Knox said.

But a bomb cyclone has its origins in the mid-latitudes and is associated with weather fronts - a boundary between two air masses with different characteristics such as temperature - and a strong jet stream, Knox said. A hurricane originates in the tropics and is not associated with either weather fronts or a strong jet stream, Knox said.

Are they becoming more common?

Global climate change, according to experts, is causing more frequent and more extreme weather events around the world. But are bomb cyclones becoming more common or more intense? Martin said it is not clear whether that is the case. The fact that Earth is warming has ramifications for cyclone dynamics that scientists are currently trying to figure out, Martin said.

A warmer planet means more water vapor in the air and that would tend to make at least the latent heat portion of the empowerment of these storms stronger, Martin said. However, the warming is not uniform, Martin said. Since observations suggest more warming at high latitudes, Martin said, this could render the bomb cyclones weaker in general.