I don't know if this is only xbox but is you use any of the winter weather options with precipitation it is raining and get rid of the snow London underground only
I noticed on sand patch that if I use a winter timetable scenario, and add precipitation, it just snows- especially at nighttime. Not sure if this is what you mean, but I notice if you have a nighttime scenario, the temperature drops, and so precipitation (instead of rain) becomes snow. Which is slightly annoying (since we can’t change the temperature), but does make sense.