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Nor'easter Bomb Hits East Coast Thanks to a Rex Block. Huh?

Okay, it's not the infamous polar vortex, but people living on the New England coast are about to get popped by a "Nor'easter bomb." And it was set up five days ago by a "Rex block." I'm not making that up.

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American


Okay, it’s not the infamous polar vortex, but people living on the New England coast are about to get popped by a “Nor’easter bomb.” And it was set up five days ago by a “Rex block.” I’m not making that up.

Last weekend weather forecasters were marveling wide-eyed about a potentially unheard-of storm caused by two colliding weather systems that could wallop the entire Northeast with extremely low temperatures and feet of snow. The collision is occurring further out into the Atlantic Ocean as of today, sparing much of the nation, but it will still slap Cape Cod, Boston and eastern Maine, as well as eastern Canada.

So were the forecasters using made-up language—an issue of late, with weather services pasting names on every snow flurry and windy day? Nope, Rex and the bomb are legit meteorological terms.


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A “bomb” is when the atmospheric pressure in a weather system drops by more than 24 millibars in 24 hours. For perspective, a typical pressure over the Northeast coast might be around 1,012 millibars, but it doesn’t take much of a drop to intensify a storm. The lowest pressures in the biggest hurricanes on Earth, such as Sandy, reach down to the 940s. So a 24 millibar drop is a lot, and when it happens in less than 24 hours, meteorologists get excited, because that’s rare.

In this case, the storm was fixing to be a Nor'easter—a low-pressure center off the Northeastern coast that circulates counterclockwise, sending winds inland from the Atlantic. So the dynamic duo was termed a Nor’easter bomb. The pressure was forecast to drop to the 980s or even 960s. Whether or not it gets that low, the drop in pressure is steep and confined, which causes strong winds, perhaps greater than 60 mph for the Cape and Maine.

Why did all this happen? The moisture came north in a system from the Gulf of Mexico, but the cold was created by—yep, the Rex block. That term describes an enormous pressure center that can get stuck in place. In this case, that place was over Alaska and the upper West Coast, which forced the jet stream to take a deep dive from the Arctic over the east-central U.S. When that system collided with the moist one from the Gulf, the two combined and began spinning, dropping the pressure inside and pulling up the moisture, turning it into snow. If you want more details about Rex, see a nice blog from last week by Eric Holthaus, a meteorologist who writes for Slate.

This latest bomb is just one more example of the jet stream acting weird, as our climate changes. And as Holthaus notes, although the Northeast is cold, the rest of the planet is not. Average global temperatures are expected to be higher than normal in the next few weeks, he says.

Photo courtesy of Serge on Flickr

Mark Fischetti has been a senior editor at Scientific American for 17 years and has covered sustainability issues, including climate, weather, environment, energy, food, water, biodiversity, population, and more. He assigns and edits feature articles, commentaries and news by journalists and scientists and also writes in those formats. He edits History, the magazine's department looking at science advances throughout time. He was founding managing editor of two spinoff magazines: Scientific American Mind and Scientific American Earth 3.0. His 2001 freelance article for the magazine, "Drowning New Orleans," predicted the widespread disaster that a storm like Hurricane Katrina would impose on the city. His video What Happens to Your Body after You Die?, has more than 12 million views on YouTube. Fischetti has written freelance articles for the New York Times, Sports Illustrated, Smithsonian, Technology Review, Fast Company, and many others. He co-authored the book Weaving the Web with Tim Berners-Lee, inventor of the World Wide Web, which tells the real story of how the Web was created. He also co-authored The New Killer Diseases with microbiologist Elinor Levy. Fischetti is a former managing editor of IEEE Spectrum Magazine and of Family Business Magazine. He has a physics degree and has twice served as the Attaway Fellow in Civic Culture at Centenary College of Louisiana, which awarded him an honorary doctorate. In 2021 he received the American Geophysical Union's Robert C. Cowen Award for Sustained Achievement in Science Journalism, which celebrates a career of outstanding reporting on the Earth and space sciences. He has appeared on NBC's Meet the Press, CNN, the History Channel, NPR News and many news radio stations. Follow Fischetti on X (formerly Twitter) @markfischetti

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