What is a bomb cyclone? Pacific Northwest warned over approaching storm

Alarmingly-named weather phenomenon common in North America in late autumn and early spring

Joe Sommerlad
Thursday 21 October 2021 13:09
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Related video: A ‘bomb cyclone’ forms off the west coast of the US

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The US Pacific Northwestern states of Washington, Oregon and California have been warned that a strong jet stream over the Pacific Ocean could be about to toss a “bomb cyclone” their way.

While the name sounds alarming, the phenomenon is relatively common in North America and the term, which was coined in 1980, has been criticised by some meteorologists for being unhelpfully sensationalist and inspiring needless panic.

“Bombogenesis is the technical term. ‘Bomb cyclone’ is a shortened version of it, better for social media,” weather expert Ryan Maue has said.

“The actual impacts aren’t going to be a bomb at all. There’s nothing exploding or detonating.”

The occurrence, also known as explosive cyclogenesis, essentially amounts to a rapidly developing storm system, distinct from a tropical hurricane because it occurs over midlatitudes where fronts of warm and cold air meet and collide, rather than relying on the balmy ocean waters of late summer for fuel.

The US National Oceanic and Atmospheric Administration says the process takes place when “a midlatitude cyclone rapidly intensifies, dropping at least 24 millibars over 24 hours”, citing the measurement used to record atmospheric pressure (the force being exerted by the weight of the air).

Like a conventional cyclone, a bomb cyclone is the product of a low pressure system, where the atmospheric pressure is lower at sea level than in the surrounding area. Air drawn into the system from the Earth’s surface rotates in the same direction as the Earth when it rises, drawing in whirling winds at its base and creating the cyclone.

So long as the air continues to climb to the top faster than it can be replaced at the bottom, the pressure will continue to drop.

As with a hurricane, lower air pressure yields a stronger storm, resulting in high winds, heavy rains or blizzard conditions depending on the surrounding air temperature, with bomb cyclones most common in the US in late autumn and early spring when colder conditions are prevalent but not dominant.

Currently, 60 to 70 miles per hour gales are being forecast for America’s west coast, according to Washington’s KXLY.

“All bomb cyclones are not hurricanes,” climate scientist Daniel Swain of the University of California, Los Angeles, has told NBC. “But sometimes, they can take on characteristics that make them look an awful lot like hurricanes, with very strong winds, heavy precipitation and well-defined eye-like features in the middle.

Thermal imagery captures a bomb cyclone developing

“Fundamentally, the impacts of a bomb cyclone are not necessarily different from other strong storm systems, except that the fast strengthening is usually a signature of a very powerful storm system.”

Those concerned about staying safe are advised, as with any other strong storm forecast, to stock up on canned food, water, first aid supplies, a torch and spare batteries in case power supplies are knocked out.

“Pay attention to your local weather forecasters, but understand that the occurrence of a ‘bomb cyclone’ does not mean a storm will be particularly apocalyptic,” writes Rachel Feltman of Popular Science.

“You should still be safe and prepared, but that’s true of pretty much any winter storm.”

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