Low temperatures dipped to the mid and upper 30s across parts of the Midwest Friday morning. Clear skies and light winds that promoted the chill were followed by an increase in cloudiness which inhibited daytime warming. The city’s official low of 50 and high of 64 yielded a daily temperature deficit of 7 degrees, only the 5th day this month to run below normal. As of Friday, the month’s average temp was running about 3 degrees above normal. That surplus is likely to erode further in coming days as a high amplitude wave develops in the jet stream flow across North America. This pattern will produce a sustained period of cool, northwesterly wind flow from the Great Lakes to New England through early next week with temps forecast to average nearly 6 degrees below normal during the period of September 24-29. The chilly pattern is expected to ease by early October when late-season warmth is forecast to spread across much of the nation.