US Temperatures Unusually Hot and Unusually Cold

The persistent ridge of high pressure that stagnated over the Eastern Pacific Ocean since last December has been responsible for this sharp contrast in temperature anomalies across the lower 48 states. The system caused the jet stream to flow over the top of the ridge and southward to the east of the high, thus funneling cold air into the central third of the nation. In itself this is not unusual, but what has been “unprecedented” (to quote NOAA) is how persistent this pattern has been over the past 7-8 months.
http://www.wunderground.com/blog/weatherhistorian/comment.html?entrynum=302