Weather Talk: Wet snow is hard to forecast
Weather systems that produce a mixture of rain and snow can be devilish to forecast.
If the precipitation is heavy enough, a couple degrees of temperature can make a huge difference in the amount of snow that accumulates. There is a lot more to this than predicting the surface air temperature.
Precipitation in these cold weather systems almost always forms as snow in clouds well below freezing, so it is very important to be able to forecast the temperature of the air all the way down to the ground.
Further complicating this is a process known as evaporative cooling. Some of the precipitation will evaporate as it falls and this process cools the air, so heavier precipitation is more likely to fall as snow and precipitation falling through dry air is more likely to fall as snow.
Warm ground temperatures further complicate all of this because melting snow draws heat out of the ground, causing it to cool. This means a heavier snow is more likely to accumulate than a lighter snow.