Sunday is forecast to mark the 10th consecutive day of below-normal temperatures in Chicago. Daytime highs since Oct. 25 have averaged more than 7 degrees below normal. Meanwhile, overnight lows have averaged closer to normal, mainly due to cloudiness and northeast winds off Lake Michigan. This trend is forecast to continue through midweek as a deep storm system develops over the Eastern U.S. As a result, northwest to north upper level winds will continue to supply the Great Lakes region with cool air. Jet stream orientation is then forecast to undergo a radical transition as the East Coast low weakens and heads out to sea. By late week, storminess and cold air are forecast to become established over the Pacific Northwest. They will turn our upper level winds southwesterly, sending a surge of much milder air into the Midwest.