A combination of a series of three storm systems and a blast of Arctic air are prompting forecasts of an extended cold snap across the US, with temperatures in some parts of the north-central states expected to dip as low as 30ºF below normal. The regions expected to be affected include the Upper Midwest, Great lakes, Ohio Valley, Mid-Atlantic, and New England.

Mashable’s Andrew Freedman describes the February-like conditions that are visiting the continent as yet another consequence of the polar vortex pushing Arctic air in a southeasterly direction. 12 inches of snow is forecast for higher elevations in West Virginia, Virginia, and Maryland, with Philadelphia, New York City, and Boston possibly seeing lighter snowfall on Saturday.
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An mass of cold Arctic air has descended from the north over the continent, causing temperatures to plunge across Canada and the eastern half of the United States. Many regions between Nunavut and Ontario are seeing temperatures well below zero — Fahrenheit, that is (-32ºC), cold enough to crack windows in Alberta and Saskatchewan. Although low-temperature records in many regions aren’t being broken, these conditions are encroaching on century-old records for the duration of regional cold snaps.
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La Niña conditions are currently forming in the Pacific Ocean, however forecasters expect that if these conditions persist through the winter, this will only be a mild episode, compared to La Niña that have occurred in the past.

La Niña, Spanish for "little girl", occurs when sea surface temperatures drop to below normal along a band following the equator in the Eastern Pacific, the counterpart to the phenomenon of above-average sea surface temperatures known as El Niño. These two phenomena are part of the El Niño-Southern Oscillation (ENSO), one of the strongest drivers of climate variation in North America and around the world.
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