This animated JET STREAM FORECAST shows how the northern and southern streams are to combine or "phase" to generate the vertical motions in the atmosphere which will drop Central U.S. barometric pressures precipitously---generating wind and significant precip in the process. The color scheme highlight wind velocities. Note how a powerful phased "wind max" develops aloft over the Central U.S. in this animated forecast. It's that wind max that is to drive rapid intensification of the winter storm

It's very mild in the UK at the moment, but a change in the jet stream means a change in the weather here later this week... | Met Office