What you are describing is what often happens here in South Coastal Massachusetts. Starts out as snow, changes to sleet, maybe some freezing rain mixed in, and then finally rain. Left to itself, it makes a big, heavy, sloppy mess to clean up.
Like so many have already advised, I too would recommend the removal of the snow just prior
to the changeover. You don't want the snow to become saturated with rainwater because of the decreased throwing distance, not to mention more wear and tear on the machine. (And who the heck wants to work out in the cold rain?
). Not only that, but often there is a wind direction shift right after the precipitation stops, and cold air sweeps back into the area. If that slushy mess freezes up, you've got a real mess on your hands.
The trick however is knowing when
the optimum time is to get out there.....
For that, I have used a local TV station's website that uses Doppler radar. Not only can you see the speed of progress of the approaching rain/snow line, so you can estimate when it will get to you, but you can see the actual changeover pretty precisely from the comfort of your computer. The one I use has a delay of only about 5 minutes. That's certainly close enough.
This strategy is what works best around here, since it's not only easier to remove the snow before it gets wet, but since almost every snowblower leaves a trace coating of residual snow, the timing insures that the rain that follows will wash that away. In other words, the storm continues but the job is already done!!!