For those who won’t believe the link between polar warming and mid-latitude weather until they see the physics, Jeff Masters has a quick summary of it in the post “Our extreme weather: Arctic changes to blame?”
To see the connection, you have to look at the way temperature and pressure give rise to wind speed and direction. The less consistently cold Arctic decreases the speed of the jet stream, and this is making a jumble out of the upper-level winds. The jet stream is the high-speed, high-energy upper-level wind pattern that keeps the other upper-level winds in line. When it has less energy, upper-level winds can blow in all directions.
In particular, more upper-level loops are forming. An upper-level loop keeps the same air mass over the same area for a period of time. This can create persistent weather patterns that give rise to extreme storms, floods, droughts, and high winds.