A wave of energy is beginning to move out of the Rocky Mountains. This wave will provide the energy needed to produce precipitation.
Temperatures are expected to drop overnight and it appears there will be a 5-6 hour window where temperatures at the surface will be below freezing to allow the snow to reach the ground.
Forecast models are indicating light amounts of moisture with this system. Both the morning runs of NOAA's global forecast model and the European forecast agency model are keeping less than 0.03" of liquid for St. Louis and Indianapolis. Generally speaking, I think we will see a dusting to less than a half inch of snow. (In many cases, it could be less than a quarter of an inch.)
- St. Louis metro - Starting around 10:30pm-12:00am. Ending around 6:00am.
- Indianapolis - Starting around 3:30am-5:00am. Ending around 10:00am.
Bottom line... Nothing to get concerned about. On my "Freak-Out-Meter" I'd put it at 0.5 out of 10.