After weeks of cloudy and wet weather, Chicagoans are in for an extended autumnal treat with several days of sunny and seasonably mild weather expected this week. With a sprawling high-pressure system dominating the weather pattern, area residents will be treated to a week of nearly perfect fall weather characterized by mild, sunny days and clear, chilly nights. However, big changes are expected by the end of the week as a strong, autumn storm develops. Gusty thunderstorms will herald the arrival of sharply colder air by late Friday as temperatures fall more than 20 degrees from midweek highs in the lower 70s to readings struggling to reach 50 by next weekend. A sure sign of the changing seasons, this week’s storm will likely bring the season’s first accumulating snowfall to portions of the northern Plains and the upper Midwest.