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CHICAGO — An unusual late-season “polar vortex” dropping south from Canada in coming days will likely bring freezing temperatures to the Chicago area Friday night into early Saturday.

A “wavy” jet stream diving southward is pushing a late season “tropospheric polar vortex” over the Midwest and Northeast (see graphic below). Climate scientists refer to this cold in the east and warm in the west set-up as the “East-West divide.”

In the Chicago area, clearing skies and diminishing winds are forecast to fall out of the 40s through the 30s Friday, possibly dropping into the upper 20s during the pre-dawn and early morning hours Saturday morning.

Plants and other activities sensitive to freezing temperatures should be protected. As temperatures dip into the 20s away from the Lake Michigan, they could even put Saturday morning’s record low of 27 degrees set back in 1987 within reach.

As all that’s happening, record cold will likely break records on the East Coast at the same time as record heat breaks records in the west.

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A “wavy” jet stream (orange-shaded region) pushes a late season “tropospheric polar vortex” (blue/green-shaded pool) over the Midwest and Northeast