With a hat tip to Jonathan A, here is the scoop from John Timmer at Ars Technica.
Charles Greene of Cornell has concluded that:
Greene’s paper describes a key determinant of the Northern Hemisphere’s winter weather: the Arctic Oscillation. When that is in its positive phase, a strong set of winds called the Polar Vortex forms. These winds help trap Arctic air masses at the pole, keeping the cold out of the mid-latitudes. This also allows the jet stream to take a more direct route around the globe, moderating the weather.
But over the last few years, the Oscillation has been strongly negative; in fact, in 2010, we saw a record for the most strongly negative period we’d ever recorded. During this phase, the winds of the Polar Vortex weaken, allowing the cold Arctic air to intrude or mix into the air at lower latitudes. As a result of this, Greene told Ars two things happen to the jet stream: it gets substantially weaker, and it tends to meander widely from north to south as it traverses the globe. This can lead to the severe chills the US and Europe have experienced over the past several winters, but the meandering jet stream can also draw warmer southern air north, as happened in the US this spring.
I would go further. These cold outbreaks “use up” the cold pool, allowing the atmosphere to recover from the winter more rapidly. So an extraordinarily warm spring somewhere in the middle latitudes is doubly likely.