It would only make sense to require them every winter if the large majority of winters were quite cold. As such, nowhere in the US requires winter tires, but someone else noted that Quebec does. This makes sense, since even a mild Quebec winter will spend most of the time getting below freezing.

I would be very surprised if there were a place that required winter tires by law where the winter does not always get and stay at or below freezing for significant periods of time almost every winter.