Here are the dates of the #polarvortex formation in the NH for ERA-interim since 1979. This year looks to be one of the later dates in this record (possibly *the* latest) but that's not saying much since there's so little variability. Note last year was one of the earliest dates. pic.twitter.com/A7eymkstyT

— Dr. Amy H Butler (@DrAHButler) August 22, 2019