A northwest flow aloft will bring a brief period of cooler, less humid air over northern
Illinois and Wisconsin through Monday. However, the primary jet stream then moves back
north, establishing a strong west-east flow along the U.S.- Canadian border. This setup
will allow warmer and more humid air to surge back into the Midwest on the nose of
strengthening southwest winds. For three straight days, Tuesday through Thursday,
Chicago's highs will most likely exceed 90 degrees, while readings at night will hold just
above 70 degrees. Should these conditions occur, daily heat indexes will peak in the
mid-to-upper 90s, with little respite at night.
