I don't think it's more and more developments, at least at first glance. For example, Falls Village which is in the far NW corner of CT by the NY/MA line where it's about as middle of nowhere as you can get in CT, used to have an average first frost of 9/16, and now a decade later it's up to 9/23. I don't think there's too much developing going on in a place with a population just over 1,000 people. Some of it is likely methodology in calculating the average, so it's important to note that the way normals are calculated does change over time, but that likely does not explain all of the moves. We've also seen reduction in snow cover days at some of these middle of nowhere in the woods stations as well like Norfolk at 1400 ft elevation. The Falls Village average last frost moved up from 5/30 to 5/24, so the growing season there has increased by two weeks in the last ten years (officially 50% probability of frost free period was 108 days and now it's 120).