Why the huge storm last week wasn't a bomb cyclone -- despite what you may have read

By Adriana Navarro, AccuWeather staff writer
April 15, 2019, 10:22:14 AM EDT

Colorful sat of massive storm 4/12/2019

A massive storm strikes the central U.S. on Friday morning, April 12, 2019. (Satellite/Northern Illinois University)

If you were reading the news online this week, you may be under the impression that a bomb cyclone pummeled the middle of the country again. Several prominent news outlets blared headlines that carried the term.

"'Bomb Cyclone' Shutters Schools, Makes Roads Impassable In Central US," a Wednesday NPR headline read. "A Bomb Cyclone Storm Is Hammering The Central US, Disrupting Travel," TIME screamed in a headline for a video posted on YouTube Wednesday. And numerous other newspapers and websites splashed the news largely because of an Associated Press headline that declared, "'Bomb cyclone’ snow, wind making travel dangerous in Midwest."

Here at AccuWeather, the term bomb cyclone never appeared in continuing coverage of the storm. AccuWeather journalists and meteorologists used language like "monster storm," "powerhouse blizzard," and "winterlike storm." That may leave readers wondering, "Was that huge storm a bomb cyclone or not?"

While the slow-moving weather system brought a host of impacts to a large swath of the country this week, including 30 inches of snow in some places, and its strength rivaled the power of last month's bomb cyclone, it did not technically qualify as a bomb cyclone, AccuWeather meteorologists point out.

"A storm is like a giant vacuum cleaner: The air is rising in the middle of that storm," AccuWeather Senior Meteorologist Alex Sosnowski said. "The lower the pressure, the more the air is rising in the middle. When that happens, the air has to rush in from the sides to take its place. It’s that rushing of air that creates the strong winds."

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But defining a storm as a bomb cyclone is a complicated matter.

As a storm strengthens, the pressure in the center of the storm drops. It is how quickly the pressure falls that determines if it qualifies as a bomb cyclone. The drop in pressure is measured in inches of mercury or millibars (mb).

The National Oceanic and Atmospheric Administration (NOAA) defines the criteria of a bomb cyclone as when "a mid-latitude cyclone rapidly intensifies, dropping at least 24 millibars over 24 hours." That 24 millibars equates to 0.72 of an inch.

AccuWeather uses NOAA Weather Prediction Center's (WPC) barometric pressure analysis to determine if a storm meets this official criteria from NOAA.

"The greatest pressure difference in 24 hours was 20 mb, so it didn't come close to NOAA's criteria," said AccuWeather Meteorologist and Social Media Manager Jesse Ferrell. This equates to a difference of 0.59 of an inch.

The bomb cyclone back in March had exceeded the criteria, reaching a difference in pressure drop of 0.97 of an inch (33 mb) in 24 hours.

When comparing this storm to the one in March, Sosnowski notes that it took longer for the pressure to fall and didn't meet the pressure difference of 0.72 of an inch.

David Roth, a prominent meteorologist on Twitter, fueled much of "bomb cyclone" coverage after he posted on Twitter suggesting the storm had undergone bombogenesis.

Other meteorologists weighed in on social media agreeing with Roth that the criteria was met when using a different definition of bomb cyclone.

Rather than follow the 24 mb in 24 hours model, Roth used the equation from Fred Sanders and John Gyakum's research findings released in 1980, which factors in latitude to the equation and explains the difference in conclusion.

The equation sets different thresholds for storms to meet in different latitudes with the goal of adjusting for the variance in deepening cyclones with latitude.

"The problem [with the 24 mp/24 hours criteria] is that such deepening rates for extra tropical cyclones are harder to achieve at more southern latitudes due to decreasing planetary vorticity as you lose latitude," Roth wrote in an email. He notes that for the same reason they are easier to achieve the further north you go.

The American Meteorology Society (AMS) also follows this equation to define a bomb cyclone, but Roth notes that the AMS definition committee will be looking into the topic. He guesses that NOAA's NWS would follow the AMS lead to have a consistent definition.

But for now, NOAA's official definition still uses the 24 mb in 24 hours model, disqualifying the April storm as a bomb cyclone.

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