The northerly jet stream flow that has brought repeated outbreaks of arctic air to the region is in the process of breaking down. By Sunday, westerly upper-level winds will become established, allowing air of Pacific origin to surge across the continental U.S.

The area will remain in polar air Saturday. Subzero morning temperatures are expected to rise to the mid-teens. It will mark the city’s 10th day with a low temperature of zero or below. The opening five days of January have been the coldest here since 1979.

Milder air is expected to reach the Chicago area Saturday night. Minimum temperatures will occur during the early evening, with readings rising through the overnight hours. Warming will begin in earnest Sunday, as strong southwest winds send late-day readings climbing to near freezing.