Midnight Update
The only change I have to the forecast this evening is to extend the threat for freezing rain ever-so-slightly farther South. Our new guidance has come in a little colder and brings a freeing rain threat as far South as Shelbyville. I still believe the heaviest and most significant ice accumulations will occur just North of Interstate 70 from Noblesville to Muncie over to Greenville, Ohio.
So many of you have asked how forecasters tell what type of winter precipitation will fall? Well we use skew-t's and thickness soundings.
Above is the latest skew-t for Muncie. The bottom white line (1000) is the surface. Notice the temperature (white line) is slightly below zero degrees celsius, about 29 degrees. But as you go higher in the atmosphere the white line (temperature in the upper layers of the atmosphere) gets warmer than 0 degrees celcius/32 degrees F. Thus, the precipitation that starts to fall as snow, melts, and becomes rain. When it reaches the surface, temperatures are below freezing and thus you get freezing rain.