For a winter storm to produce snow, the atmosphere must be below freezing from top to bottom.
While snow can reach the ground when temperatures are above freezing, the warm layer near the surface must be very shallow so that the snowflake does not melt before reaching the ground.
Sleet forms when there is a layer of warm air between two cold layers near the cloud and near the ground. The precipitation begins as snow in the clouds, but then partially melts when it falls through the warm air. It refreezes before reaching the ground and will fall as tiny ice pellets. You can often hear sleet hitting the ground and see it bouncing upon impact.
Freezing rain forms in a similar way as sleet but melts completely before reaching the ground. However, what makes this winter precipitation so dangerous is that once that liquid water drop reaches a surface that is below 32 degrees Fahrenheit, it freezes on contact. This includes on tree branches, power lines and roads. The ice can accumulate and is extremely heavy, bringing down trees and power lines.
Determining how deep those layers in the atmosphere will be is critical to forecasting which type of winter precipitation will fall.