A secondary area of low pressure will slide through Northern Illinois Friday morning. This will not only bring light rainfall to Northern Illinois and Southern Wisconsin, but also pull down a colder air mass for the afternoon.
Scattered showers will last into the afternoon with a cold front, but just enough instability may be present in the atmosphere to produce a few isolated storms. Severe weather isn't expected, but some storms could contain small hail or graupel (soft hail).
Most times during thunderstorms the freezing line, or point in the atmosphere where the temperature drops below 32 degrees, is several thousand feet above. With the cold air arriving Friday afternoon, the freezing line will be lowered as colder air tries to move down to the surface. Water droplets within the atmosphere and the cloud won't have to travel very far up before freezing. The rain that occurs within the downdraft of the storm will help bring those hail stones down to the surface. So don't be surprised if there is very small hail with any rain showers Friday afternoon.