Outside the volcanic ash theory, I would agree at the UHI sites like the hothouse around and near Harrisburg. Harrisburg is on a run of only one 100+ day in 11 years , if you count this year which is obviously not set yet. That single 100 temp was right on 100 as well so as unimpressive 100 reading as you can get. The last time this happened at MDT is 1967-1979. So, we are setting a lot of yearly average records or near records without actual extreme heat. That suggests to me that whatever the reason is, it is going to take something extraordinary to not hit 90 at least once at the UHI sites. I do not think MDT has ever had a year without hitting 90 once. 1907 it was right on 90.