Nor'easter…. It's a term that by just saying it creates very distinct images of snow, wind, and terrible and unsafe travel conditions among other things. But what exactly is a nor'easter?
A nor'easter is an area of low pressure that typically begins in the Gulf of Mexico and then moves north up the coast of the eastern United States. They get their name from the wind directions people see during the worst of the storm – northeast. Although they can happen anytime of the year, they are of most interest and concern during the fall and winter months.
Like any recipe for your favorite dish or dessert, nor'easters have a recipe consisting of three key ingredients. The first is an area of high pressure centered over southeastern Canada or New England and cold air in the same area. Often times, areas of high pressure don't get as much attention as areas of low pressure, but in the case of a nor'easter they are an important part of the storm. The placement of this high is key, because the clockwise winds around a high will blow cold air to the south. This leads to cold air gets wedged up against the mountains and will continue to move south. This scenario is called cold air damming, and this process gets the cold air in place for a snow storm.
The second ingredient is an upper level trough that reaches into the Deep South. This is the most difficult of the ingredients to explain. This trough can help for an area of low pressure that will become the center of the storm to form. The alignment of this trough is also key. It needs to be "negatively tilted", which means the trough axis is aligned from northwest to southeast. In this alignment, upper level processes that can help create strong areas of low pressure at the surface are most efficient.
The third and final ingredient is moisture, because you can't have rain or snow with any moisture in the system. This moisture will come from the Gulf of Mexico or the Atlantic Ocean (or both) as the nor'easter moves over those bodies of water.
Nor'easters will typically strengthen as they move north. This is because as they do so, the upper level trough typically becomes more negatively tilted. These storms can even resemble a hurricane, but that is an incorrect description because the dynamics of the two storms are different. Nor'easters can have hurricane force winds, however.
Nor'easters come in different sizes and shapes, and no two storms are the same. But if these ingredients combine in the right way, these storms are capable of causing major accumulations of snow, high winds, power outages, unsafe travel conditions, and extreme cold.
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