Bomb Cyclone Powered by Atmospheric River Heads for US. What Are They?

A “bomb cyclone” and an “atmospheric river” are poised to deliver intense weather to the West Coast of the U.S. this week, bringing heavy rain, snow and strong winds, according to forecasters.
These dramatic terms describe powerful meteorological phenomena with the potential for widespread impacts. But what do they actually mean?
A bomb cyclone refers to a storm system that undergoes bombogenesis, a rapid intensification process where the atmospheric pressure in the storm drops by at least 24 millibars (mb) in 24 hours.
This steep pressure drop causes the storm to strengthen explosively, resulting in violent weather.
The term combines “bomb,” indicating explosive development, and “cyclogenesis,” which describes the formation of cyclones—low-pressure systems that rotate counterclockwise in the Northern Hemisphere.
While many bomb cyclones form over the ocean, their effects can extend far inland, impacting broad areas with high winds, heavy precipitation and severe disruptions.
“The term can be used for any type of storm, but is often used for non-tropical storms like nor’easters or the large system that is off of the West Coast this week,” Heather Zehr, senior meteorologist at AccuWeather, told Newsweek.
“That storm currently has a central pressure around 1,015 mb, but is expected to strengthen rapidly beginning tonight [Monday]. The fastest deepening of the storm will be from late tonight to early tomorrow night with the pressure expected to drop an impressive 50 to even 60 mb.”
At the heart of a cyclone is a rising column of air that creates a vacuum effect, lowering atmospheric pressure.
The faster the air rises, the more intense the storm becomes. Winds rush toward the low-pressure center, generating fierce gusts, while the rapidly ascending air cools and condenses, producing intense rain or snow.
An atmospheric river is a narrow, but long, corridor of concentrated moisture in the atmosphere, often described as a “river in the sky.”
These phenomena transport vast amounts of water vapor from tropical regions to other parts of the globe, sometimes stretching thousands of miles across the ocean.
When an atmospheric river reaches land, it can release heavy rainfall or snowfall, depending on the temperature.
Atmospheric rivers form when warm, moist air rises from tropical oceans, where evaporation rates are high. This moisture is then carried by strong winds in the upper atmosphere, typically along jet streams, to mid-latitude regions.
When the moist air encounters landmasses or mountain ranges, it is forced upward, cooling and condensing to form clouds and precipitation.
The intensity of an atmospheric river depends on how much moisture it contains and how long it remains stationary over a region. Some of the strongest atmospheric rivers can transport up to 15 times the water flow of the Mississippi River, according to the National Oceanic and Atmospheric Administration (NOAA).
“With this storm system, the best moisture heads toward the coasts of Oregon and Northern California tomorrow afternoon and then is mostly funneled toward the northern half of California for the rest of the week,” Zehr said.
AccuWeather is forecasting heavy rain, snow and wintry storm conditions through to Friday.
“Strong winds will create large waves offshore and along the Washington and Oregon coasts and can lead to sporadic power outages in coastal and mountainous areas,” the latest AccuWeather forecast update said.
Residents in low elevation areas along the Interstate 5 corridor should prepare for flooding, while exceptionally heavy snowfall is expected in the Olympics and Cascades.
Atmospheric rivers often play a role in fueling bomb cyclones. As a bomb cyclone intensifies, it can tap into an atmospheric river, drawing in immense amounts of moisture.
In a kind of feedback loop, bomb cyclones can then strengthen atmospheric rivers, working together to increase their impacts.
Zehr added, “The storm is already tapping into moisture from west of Hawaii now. As the storm deepens, it pulls that moisture toward the U.S. West Coast, while maintaining that connection with the central Pacific.”
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