https://phys.org/news/2018-05-theory-traffic-jet-stream-abnormal.html

The deadly 2003 European heat wave, California's 2014 drought and the swing of Superstorm Sandy in 2012 that surprised forecasters—all of these were caused by a weather phenomenon known as "blocking," in which the jet stream meanders, stopping weather systems from moving eastward. Scientists have known about it for decades, almost as long as they've known about the jet stream—first discovered by pioneering University of Chicago meteorologist Carl-Gustaf Rossby, in fact—but no one had a good explanation for why it happens.

"Blocking is notoriously difficult to forecast, in large part because there was no compelling theory about when it forms and why," said study coauthor Noboru Nakamura, a professor in the Department of the Geophysical Sciences.