The National Weather Service said the tornado that struck near Oklahoma City Friday night was an EF5, the most powerful there is. There hasn't been a storm system so intense since 1999. Anna Werner ... more 
The National Weather Service said the tornado that struck near Oklahoma City Friday night was an EF5, the most powerful there is. There hasn't been a storm system so intense since 1999. Anna Werner explains why the atmosphere over the Southern Plains was so volatile. less 
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Tue, Jun 4, 2013 10:54 PM EDT