Chicago is being treated to a few days of mild temperatures, and that will last through Friday – but big changes are in store beginning Friday night and for several days after. Daytime readings in the lower and middle 70s will be the rule through Friday. A powerful slow-moving storm system will be developing across the northern Great Plains on Thursday and Friday. Low pressure at the core of this system will lie over northern or central Minnesota by Friday and it will move only to just north of Lake Superior on Sunday. Chilly air of Canadian origin arrives late Friday, preceded by showery and thundery weather Friday. But with the storm system so far to the northwest of Chicago, conditions should improve markedly on Saturday, though temperatures will be much cooler. And it will become quite windy as well.