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These powerful storms can pack a hurricane punch. How do they form?

Science Behind the Weather: What is a nor'easter?

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    Scott Sutherland
    Meteorologist/Science Writer

    Saturday, February 14, 2015, 9:21 AM - Anyone living in Atlantic Canada or the U.S. Northeast certainly knows (and perhaps cringes at hearing) the term nor'easter, as these notorious storms have a history of inflicting powerful winds, heavy rains and driving, burying snow on those regions. What exactly is a nor'easter, how does it differ from a tropical storm or hurricane, and what gives these storms their strength?

    A nor'easter is, as the name implies, a storm that packs strong northeasterly winds along the coast. They're typically seen along the northeast coast of the United States and through Atlantic Canada, between the months of September and April. These storms can rival the expanse of the largest hurricanes, and can pack comparable winds as well. However, rather than having the swirling vortex shape of a tropical cyclone, they retain that distinctive 'comma' look of an extratropical storm.

    March 2014 Nor'easter. Credit: NOAA

    The biggest differences between a nor'easter and a tropical storm or hurricane, though, are in how they form and the source of their energy.

    Tropical systems form due to warm, dry waves of air from western Africa meeting warm, humid air rising quickly over warm equatorial waters. They thrive on all that heat, drawing it up into the vortex surrounding the core (eye) and using it to fuel their powerful winds, and then re-heating it by sucking it down through the eye so that it can gather more humidity and cycle back through the stormclouds. This is why these storms are considered 'warm-core' systems.

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    By contrast, nor'easters are born out of the coupling of two different air masses - cold dry air pulled down across the eastern half of Canada and the United States by a strong dip in the jet stream meets warm, humid air from over water (such as from the Gulf Stream or even the Gulf of Mexico). The mixing of these two very different types of air is volatile enough, as this is how most weather systems over land get their start, but a nor'easter gets a boost from two different sources. The first is that the warm air being drawn up the eastern side of the dip in the jet stream flows right over the warm waters of the Gulf Stream, adding more warmth and humidity to that flow. The second (and perhaps most important) is a strong vortex at the top of the storm, at the same level as the jet stream, that effectively sucks air upward from the surface very rapidly. This direct, rapid flow of air upward through the low-pressure core of the storm gives this kind of system the name 'cold-core' low, and it's this cold core that drives the storm, giving it its strength.

    Regardless of their differences, the effects of these two types of storms can be indistinguishable at this time of year, as both can bring lashing winds, heavy rains, storm surges and flooding. In colder weather - late fall through early spring - these can turn into nasty snowstorms and blizzards, dumping feet of snow on some regions of northeastern North America.

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