Originally posted by the Voice of America.
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                                   US WEATHER

   People in much of the United States are dealing with cold weather not
   seen in 20 years.
   The National Weather Service is calling the air mass sweeping across
   the country "dangerously cold," and has posted wind chill warnings for
   Monday that stretch from North Dakota to New York in the north and as
   far as Alabama in the deep south.
   Carl Erickson, a forecaster with AccuWeather, told VOA that wind chills
   in major East Coast cities could reach 20-to-30 degrees (Celsius) below
   zero, while in places like Montana and North Dakota those figures could
   hit minus 50 degrees.
   "The good news is, although this is a very intense cold air mass system
   that we haven`t seen in decades, it will not be long-lived. Even as we
   go into Wednesday the winds will begin to lessen, the cold air eases,
   and although no big warm-ups it will definitely feel a little bit
   better Wednesday compared to the next couple of days. Going into
   Thursday and Friday, looks like temperatures actually rebounding to
   near average levels, probably in that 5-to-10 degree above zero range
   in the big cities by Thursday and Friday."
   The cold and fresh snow measuring more than 30 centimeters in some
   places in the Midwest have created dangerous travel conditions, forcing
   schools to close and airlines to cancel thousands of flights.
   Forecasters say the widespread chill is the result of a relatively
   infrequent alignment of weather conditions, allowing a so-called polar
   vortex to travel unusually far to the south from its normal place in
   northern Canada.
   A polar vortex is a counterclockwise rotating pool of cold, dense air.
   It is expected to knock temperatures in half the nation down to minus
   17 degrees by Wednesday.
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   [1]http://www.voanews.com/content/us-weather/1824018.html

References

   1. http://www.voanews.com/content/us-weather/1824018.html