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News media being evacuated from White House; president is not at the White House – ABC

This 2011 image provided by the National Oceanic and Atmospheric Administration shows a field survey site in California's Sierra Nevada mountains. A new study published online Thursday, Feb. 28, 2013 in the journal Science found snowfall in the Sierras was influenced by dust and microbes from as far away as Africa. (AP Photo/Jessie Creamean/NOAA)

Associated Press
This 2011 image provided by the National Oceanic and Atmospheric Administration shows a field survey site in California's Sierra Nevada mountains. A new study published online Thursday, Feb. 28, 2013 in the journal Science found snowfall in the Sierras was influenced by dust and microbes from as far away as Africa. (AP Photo/Jessie Creamean/NOAA)
This 2011 image provided by the National Oceanic and Atmospheric Administration shows a field survey site in California's Sierra Nevada mountains. A new study published online Thursday, Feb. 28, 2013 in the journal Science found snowfall in the Sierras was influenced by dust and microbes from as far away as Africa. (AP Photo/Jessie Creamean/NOAA)
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