What exactly is a 'bomb cyclone?'

Despite its appearance, experts say a 'bomb cyclone' is not a hurricane

(KTLA) – A powerful Pacific storm known as a bomb cyclone is set to slam California, prompting evacuations andcausing Governor Gavin Newsom to declare a state of emergency Wednesday.

But what exactly is a bomb cyclone?

A “bomb cyclone” goes by many names, among them, “explosive cyclogenesis,” a “weather bomb,” “explosive development,” or the less menacing, “bombogenesis.”

“Bomb cyclone” has become the most widely used term.

Experts describe bomb cyclones as storms that typically form in winter when a midlatitude cyclone undergoes “rapid intensification” at speeds of at least 24 millibars, the measure of atmospheric pressure, over a 24-hour period.

Conditions for this rapid intensification often result when a cold and warm air mass collide, which is precisely what is occurring over the Pacific with this latest storm, according to Daniel Swain, a climate scientist with UCLA.

“This is a textbook mid-latitude cyclone, with well-defined warm and cold fronts, and it’s even developing an eye-like feature near its center,” Swain says. “A ‘bomb cyclone’ refers directly to the rate of strengthening, not necessarily to its absolute strength. It has been found that the presence of nearby atmospheric rivers can actually enhance the rate of strengthening of low-pressure systems, so that is likely part of what’s going on here.”

While the eye of this storm is expected to remain well offshore in the Pacific, Swain says the impact on California will be widespread, nonetheless.

“A very strong cold front associated with the storm will indeed make landfall in California, and this is the event that will bring widespread strong/damaging winds as well as heavy precipitation that will likely lead to flooding and mudslides, especially in northern California, but also as far south as about Los Angeles.”

Although both are low-pressure systems that can bring high winds and heavy rainfall, Swain says a bomb cyclone is not a hurricane.

“Hurricanes derive their energy from warm tropical oceans whereas ‘mid-latitude cyclones’ (like the present one) derive their energy from horizontal temperature differences in the atmosphere.”

Late last month, a severe winter storm intensified into a bomb cyclone, bringing bitter cold and blizzard conditions to the Midwest and rainy conditions to the East Coast, as well as high winds for many states.

That bomb cyclone was rare, though. Alex Lamers, a warning coordination meteorologist with the National Weather Service, told The Hill that conditions for bombogenesis are typically better along the coast, not as far inland as the Midwest.

The Hill contributed to this story.