This deserves a lot more explanation than anyone wants to read but... This graphic compiles the ratio of folks switching from Democrat to Republican and shows it as a ratio to the number switching from Republican to Democrat. Normalized to 100, so a value of 100 means the number switching from D to R and number switching from R to D are equal. The higher the number, the more folks are switching from R to D compared to the other way around. Numbers under 100 mean more folks are switching from Democrat to Republican. Put simply: high = good for Democrats low = good for Republicans.
I summarized the weekly data into 4 week segments to get rid of some weekly volatility. I did the same for both the state as a whole, and then for Allegheny County by itself. What isn't obvious is that the raw number of folks switching their registration is pretty volatile over the course of the year. So some of the data points are calculated on just a handful of voters (such as in the period just after an election), while at other times there are a lot of voters (such as just before registration deadlines going into an election) being captured in this. Nonetheless you can see some of the reason D's might have cause for concern looking at fall elections since this is what I come up with.