Snow that has partially melted, but hasn't completely turned into water yet, is called "slush".
"Slush" is different from "sleet", which is a mixture of snow and rain that falls from the sky. "Slush" is only what results after the snow has fallen and started to melt.
Use the phrase "turn to slush" to describe snow becoming slush:
It's supposed to get up to 50 degrees today, so most of the snow on the roads will just turn to slush.