Bitter polar winds will plough into warm air left from summer to unleash a deluge of thunderstorms. A sharp change in the weather awaits the US as the jet stream surges southwards across western states. On the other side of the country, a northwards loop of the jet will keep temperatures above 60F. Where the two air masses collide, fireworks will erupt triggering torrential rain and the threat of ‘severe’ storms. Weather Channel meteorologist Johnathan Erdman said: “There are some changes coming later this week. “Models are forecasting a southwards plunge of the jet stream into the west, and t…