The National Weather Service has upgraded its freeze watch to a freeze warning for the suburbs overnight into Tuesday morning.
The service calls for freezing temperatures from 1 to 8 a.m. throughout the Chicago area, and worse over the Wisconsin border.
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"These conditions will kill crops and other sensitive vegetation," the service's warning says.
On Tuesday, there's a chance of rain mixing with some snowflakes in the afternoon with high temps around 44 degrees.