Wunderground.com - Dr. Jeff Masters, 1/14/13
Los Angeles, California recorded its coldest temperature in 22 years on Sunday, and record daily lows fell across large portions of the Western U.S. over the weekend. But meanwhile, much of the Eastern U.S. basked in record-breaking warmth, with temperatures reaching the upper 60s in New York. What's going on? Well, the jet stream--the upper level river of strong winds that marks the boundary between cold, Arctic air to the north and warm, subtropical air to the south--has worked itself into a very extreme configuration.
The jet is diving far to the south over the Western U.S., creating a U-shaped trough of low pressure that allows cold air to spill southwards out of Canada. The 34°F reading on Sunday morning in Los Angeles was the coldest temperature there since December 23, 1990. (The lowest temperature ever recorded in downtown Los Angeles was 28°F, most recently on January 4, 1949.) The -20°F observed at Grand Canyon, Arizona on Sunday morning tied for the coldest temperature measured there in 37 years of record keeping.