I feel like it’s more common in the inter mountain west, and areas in the Great Basin.
the most common time I would see graupel in Reno, it would be in late winter/early spring during weak disturbances, where the sun was stronger, and you’d have partly sunny days with scattered quasi-convective “snow showers”. Sun would heat up ground in shallow ways, then a snow shower comes in, temp at surface drops from 50 degrees to 30-35 quickly with lots of graupel.