Despite global warming, the United States still experiences freezing winters, and a new study explains why. Prof. Haim Garfinkel of the Fredy and Nadine Herrmann Department of Earth Sciences, leading an international team of researchers, discovered how two changing patterns in the stratospheric polar vortex direct extreme cold waves to different regions of the United States. The findings, published in Science Advances, could significantly improve winter forecasts in an era of climate change.