Fast-moving undulations in the jet stream flow aloft will mark significant changes in Chicago’s weather pattern during the week ahead.
A northwest upper level flow will bring chilling Canadian air into the western Great Lakes today and Monday. A west to southwest flow steers low pressure over or just to the north allowing warm, moist air to briefly invade the area midweek. Northwest flow aloft returns Thursday followed by shift to the SW later Friday and Saturday.
Subtle variations in this high-level pattern can significantly impact the forecast for northeast Illinois. Computer models and forecasters are divided on the actual low pressure storm track which will be crucial to the Chicago projections. Should the low track follow a path through central Illinois, temperatures could be some 10 to 20° cooler than present forecasts. Either path will result in significant rainfall Wednesday.

So dramatic was the north to south temp spread across the Chicago area Friday that it was as if two seasons were in progress simultaneously. Summerlike 80s reached north to Lansing (82°), Gary (84°) and Valparaiso, Ind. (85°). But, it was a different story farther north—chilly, damp and raw. At midday, readings varied 38°! In Kankakee, SW winds gusted past 30 m.p.h. and tropical air was in firm control, the temp had soared to 86°. While Waukegan reported light SE winds with drizzle in progress and a temp of just 48°. It was at that time, t-storms exploded to life over Grundy and Kendall counties and raced east across Will and Cook counties. The towering 46,000 ft. tall clouds turned skies black and pelted areas with marble-size hail. It took only 15 minutes for the rain to total 0.42” at Midway Airport and suburban Maywood reported a barrage of 1” hailstones.
Other clusters which flared to the east, swamped Warsaw, Ind. with 2.45” of rain in an hour.
-Tom Skilling