What does it take to get an extreme cold outbreak in California? A complete breakdown of westerly flow--& even a *reversal* to weak easterly flow (caused by upstream blocking high pressure) that allows cold continental air to spill westward. This occurs very rarely. #CAwx (13/17) pic.twitter.com/HRNTt3oADu

— Daniel Swain (@Weather_West) February 18, 2021