The reason behind the different types of precipitation
This weekend, a potential wintry mix is in the forecast for parts of central Missouri. This system is set to enter the area Saturday night, exiting throughout the early morning hours of Sunday. Most will see rain, but small differences in the vertical temperature profile will lead to others seeing this wintry mix and snow.
One of the more basic vertical temperature profiles coincides with the observation of rain. In this temperature profile, the precipitation type typically starts as snow or ice crystals as it leaves the cloud it was produced from. As this precipitation falls, it quickly enters a deep layer of warm air that extends all the way to the ground. This allows for the snow or ice crystals to melt and stay in liquid form throughout the rest of their trip from the cloud to the ground.
Freezing rain introduces a slightly more complex vertical temperature profile. It is similar to rain in that precipitation starts as snow or ice crystals as it falls from the cloud. A warm, above-freezing layer is still in place as precipitation falls, allowing for the transition to liquid raindrops. The difference is that this warm layer does not extend all the way to the ground. Instead, a thin, below-freezing layer sits at the surface. The depth of this layer is not large, so the raindrops falling through it do not have enough time to refreeze. So, instead of freezing before impact, the freeze comes after impact with the ground. This can lead to conditions that deteriorate rather quickly, as ice accumulates on any surface it is exposed to.
Sleet has almost the same vertical temperature profile as freezing rain, but the difference is that the depth of the cold layer at the surface extends higher into the atmosphere. This means that the rain falling through it has enough time to partially re-freeze before hitting the ground. The inside of the sleet pellet usually still consists of liquid water, as the drop did not have enough time to fully transition back into a solid ice pellet.
Snow is the simplest of these vertical temperature profiles. The precipitation starts as snow and ice crystals as it falls from the cloud, and it stays that way throughout the entire trip to the ground. This is because the vertical temperature profile does not consist of any areas that are above the freezing mark or leave enough time for melting to occur.
The system this weekend does not look to be impactful in terms of producing noticeable traffic, especially since freezing rain will not be much of a problem. Precipitation chances will pick up after sundown Saturday for most, exiting through the early morning hours on Sunday. Snow will be possible in areas close to Macon and Paris, with more of a mix of rain and snow possible for areas near Moberly, Brunswick, and Mexico. Accumulations in areas with snowfall in the forecast will be trace amounts at most. Rain becomes the primary precipitation type along and south of I-70.
