It may be in service, you can see bus updates from PRT @PGHTransitAlerts on Twitter, and you can follow bus movements in real time via the Transit App. 5pm should be after the brunt of the storm, and I imagine roadways will be clear, but it’s hard to say for sure what conditions will be like.

The longer you wait at the airport— which I would recommend doing so in the airside terminal which has more to do/more seating area, once you leave via the tram, you will be post-security and stuck in the landslide terminal, which is much less full featured, has less seating, and will (presumably) be much more crowded— the better the roads will be. You can always check the status of bus schedules or Ubers remotely in the airside terminal before making the decision to leave.