Upper-level "steering winds" (the winds, blowing 20,000 to 30,000 feet aloft that govern
the movement of weather systems at the surface) will shift from westerly to northerly
early this week.
That means chilly air centered over western Canada will be directed into the Great Lakes
region.
Only the southern (and mildest) portion of that Canadian air mass will affect Chicago
today and Monday, but it will be enough to lower readings 10 degrees from Saturday's
balmy 81-degree high. An additional 10-degree drop occurs Tuesday.
WARM-UP EXPECTED IN SEVEN DAYS
By next weekend, the jet stream is expected to shift to a southwesterly flow over the
central portion of the United States, bringing significant warming to Chicago and the
upper Midwest.
--By Richard Koeneman, WGN Weather Center Meteorologist
