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    The Makings of a Mild December

    By By Dan DePodwin, meteorologist/online journalist
    November 30, 2012, 6:21:33 AM EST

    While December officially marks the beginning of meteorological winter, the weather will feel more like fall across parts of the nation.

    Meteorologist Matt Alto points out, "a gradual and dramatic warm-up will occur across the center of the country through the coming weekend." In fact, the turn toward milder weather may last a bit longer into December, especially across the central part of the nation.

    Is the Cold Coming?

    The current weather pattern only supports quick, short-lived shots of cold. One of these bursts may dive into the East during the middle part of next week, but temperatures are expected to rebound quickly.

    So, while December may be stormy in the West, the beginning of meteorological winter will be on the mild side in the Plains and East. Residents of these areas may remember a similar start to last winter. This time, however, AccuWeather.com meteorologists expect a turn toward colder weather as the season progresses.

    An Unusual Occurrence

    As the days grow shorter and we head toward the winter solstice on Dec. 21, it becomes harder and harder to have mild daytime temperatures. However, such warm stretches are not unheard of but do require certain ingredients.

    With a persistent flow from the south or southwest across much of the eastern half of the nation, warm air from the southern latitudes gets transported northward.

    This southerly to southwesterly wind brings air from Mexico and the eastern Pacific northward. If the flow was from the west, the air would have originated in the northern Pacific and would be much chillier.

    In combination with plenty of sunshine, high temperatures can climb 10 to 20 degrees above normal. In some cases, that will be enough to break records!

    Although residents of the eastern United States will bask in the relative warmth, the cold and stormy weather must reside somewhere and in this case it will be found in the West and north in Canada.

    Waves in the Atmosphere

    Similar to shaking a rope up and down, the atmosphere is made up of waves. Ridges bulge northward and troughs dip southward.

    A persistent trough of low pressure stationed off the West Coast is expected to pump plentiful moisture into that region (LINK). To the east of this trough lies the large ridge that will be responsible for the eastern U.S. warmth.

    In order to break down the current weather pattern, a ridge would have to build in the western U.S. This would then create a trough in the East, putting an end to the forthcoming mild weather.

    In addition to the placement of troughs and ridges, the location of the jet stream also influences the widespread warmth. When the jet stream is located toward the Canadian border, as it is now, the arctic air remains locked in Canada.

    It is important to note that while millions of folks in the East will be experiencing warm weather in the next week or two, cold air is lurking somewhere else in the world.

    As meteorologist Evan Duffey points out, "There is plenty of cold air in the world right now, just not in the eastern U.S. and until the pattern changes, not much chilly air is expected."

    Colder air is forecast to fight its way into portions of the Upper Midwest and East this winter, at least enough to distance the area well away from the positive temperature departures that occurred last winter. Part of the South, Appalachians and Atlantic Seaboard are expected to be stormy.

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