It's rain...it's sleet...no, it's graupel

What?!?  Graupel may be a meteorological term you haven't heard too often, but that's exactly what may be mixed in with those showers later this afternoon. 

The strong low pressure system moving through Minnesota and Wisconsin this morning will become a little more centered over southern Wisconsin and northern Illinois this afternoon and evening.  The temperatures above associated with the low are very cold compared to mid-October normals.  When very cold temperatures are present several thousand feet above the surface it can sometimes increase the instability in the atmosphere.  We tend to see this a little more on days after a cold front has passed and the day starts out with sunshine but then ends cloudy with instability showers forming.  The showers that develop are caused by the rapid upward movement of the air.  A similar scenario may play out this afternoon (minus the sunshine) with what's known as graupel.  Snowflakes form high in the clouds due to well below freezing temperatures.  When those snowflakes encounter supercooled water droplets (yes, liquid can have a temperatures below freezing and still remain liquid instead of freezing) it causes the water droplet to quickly freeze into small ice pellets.  This process is different than what's needed to cause sleet because sleet forms when there is a thin, warm layer above the surface of the earth causing the frozen precipitation to partially melt before falling into a below freezing layer at the surface.  There will not be a warm layer present above the surface today and tomorrow.  So, if there are tiny frozen things falling from the sky later this afternoon you now know what it is.