The winter outlook forecasts contrasting temperatures across the United States. December is expected to be significantly colder than January and February for many regions. An unusually cold spell is anticipated to stretch from the Northwest to the Ohio Valley this December. La Niña and a weakening polar vortex are the driving forces behind this chill, both contributing to deeper dips in the jet stream. These dips will usher in colder air masses across the northern and central parts of the country.