One of the most powerful bomb cyclones to form in decades hit the U.S. West Coast this week, bringing hurricane-force winds, heavy precipitation and power outages to portions of northern California, Washington and Oregon.
To many Louisianans, satellite images of the large, swirling storm barreling toward land looked eerily familiar.
But while they might look like hurricanes, spin like hurricanes and wreak havoc on land like hurricanes so often do, "bomb cyclones" like the one that hit the Pacific Northwest differ from tropical systems in several critical ways, according to Jill Trepanier, a hurricane researcher and professor at LSU.
"Cold air in a tropical system is a killer," Trepanier said. "This system is built off of cold meeting warm."
What is a bomb cyclone?
Trepanier said her phone was flooded with texts from friends and family asking her to "please tell me what a bomb cyclone is" after the storm's landfall Tuesday evening.
"It's very similar in idea to a rapidly intensifying hurricane," Trepanier said.
"Bomb cyclone" is a term used to describe a mid-latitude cyclone that has undergone bombogenesis, which occurs when it rapidly strengthens in a 24 hour period, according to the National Oceanic and Atmospheric Administration.
The NOAA says mid-latitude storms form where there's a strong temperature gradient between 30 and 50 degrees latitude.
How quickly a mid-latitude storm intensifies is measured by drops in atmospheric pressure, Trepanier said. Though the rapid intensification of hurricanes is generally measured by increasing wind speeds, Trepanier said the two different measurements amount to the same thing.
The lower the atmospheric pressure, the higher the wind speeds, she said.
"So the bomb cyclone and rapid intensification, they're effectively the same thing," Trepanier said.
Defining bombogenesis varies by latitude, according to NOAA: At 60 degrees latitude, a pressure drop of at least 24 millibars in 24 hours is considered bombogenesis. At the latitude of New York City, a drop of 17.8 millibars in 24 hours meets the criteria.
A hurricane is considered rapidly intensifying when its maximum sustained winds increase by at least 35 mph in 24 hours, according to NOAA.
How do bomb cyclones differ from hurricanes?
The main differences between bomb cyclones and hurricanes stem from how the systems initially form, Trepanier said.
While tropical systems feed on warm sea-surface temperatures and warm, moist air, bomb cyclones form when cold and warm air collide, often cool air over warmer ocean waters.
Cold, dry air won't wear away a bomb cyclone the way it will a tropical system, Trepanier said. Bomb cyclones, in part, feed on the cold.
But Trepanier said that relationship between warm and cold can also keep a bomb cyclone's atmospheric pressure higher than a hurricane, in turn keeping its wind speeds in check. So while bomb cyclones are often much larger in size than hurricanes, that means they're also usually significantly weaker, according to Trepanier.
A record-breaking storm
Bomb cyclones certainly aren't unheard of, but Trepanier said the one that hit the Pacific Northwest this week was historically powerful.
"We have had them before, she said. "This pressure drop that it is exhibiting -- it is a record."
As the storm grew south of the Aleutian Islands earlier this week, its pressure dropped by 27 millibars in six hours, eventually bottoming out at around 950 millibars, the Seattle Times reported.
That's low for the region, Trepanier said.
Hurricane Wilma, which peaked as a Category 5 storm in October 2005, holds the record for lowest recorded pressure ever recorded in a hurricane at 882 millibars, according to the NOAA.
Louisiana's normal atmospheric pressure is around 1,115 millibars. If you drop that number to 980 millibars, then you'd have "a crazy thunderstorm," maybe tornados, Trepanier said. At 920, you'd have the typical Category 4 hurricane.