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

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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.

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

The views expressed are those of the author and are not necessarily those of Scientific American.

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