Soundings are atmospheric profiles. Soundings measure the temperature, dew point, winds and pressure. Weather balloons are launched with instruments on them that measure all those variables. The most common instrument used to measure soundings is a radiosonde. These variables are then plotted on a chart, usually a Skew-T log-P diagram (see the examples below). Temperatures are plotted in Celsius, so freezing is 0 degrees instead of 32.
Now how can we determine what type of precipitation will fall?
Actually it is quite easy by looking at the charts. The first image is an example of a freezing rain sounding. Notice the warm layer above the surface, located approx. 3,000 to 7,000 feet. This is where the green line crosses to the right of the pink freezing line. (Click on the picture to see a larger version.) This layer melts any precipitation to liquid, before refreezing when it reenters the cold air.
The next images is an example of a sleet sounding. The layer of warm air, located around 5,000 feet, is very shallow in this sounding. In other words, the vertical line barely touches/crosses the diagonal freezing line. After the precipitation melts and reenters the cold layer, it has enough time to refreeze into solid ice, also known as sleet.
The final sounding is of snow. The entire sounding above the surface shows that the atmosphere is below freezing, so snow is the only precipitation that will fall. The vertical temperature line get close to but never crosses the diagonal 0˚C line.
About 800 weather balloons are released twice a day around the world. The National Weather Service releases approximately 100 of those here in the US, and they share the data with meteorologists across the country. St. Louis is one area that weather balloons are not used. The closest cities with weather balloons are Springfield, MO; Lincoln, IL; and Topeka, KS.
Instead of using soundings from weather balloons, St. Louis meteorologists use computer models to produce forecast soundings. That is how we try to forecast what precipitation type falls in large winter storms.