Historically, velo has moved with the weather. Starting low in April and peaking in August. Around 1.5-2 MPH on average. Some of this is the weather itself (i.e. guys can throw harder when it's warmer and they're looser), some of it the nature of guys ramping up from the offseason.
I haven't seen any studies focusing on the last few years, but anecdotally it seems to vary much less over the course of each year. My read on why is that guys have now trained enough to throw at essentially 100% right away on February 15th. So a few years ago I'd look at all those guys who were flat and be all "if they're flat now then by June they'll be up quite a bit!" but I'm less confident than ever of that being inevitable. It still happens, Drew Smyly is a great case last year (91.9 in April, 93.3 in August), but it's no longer the universal truth it used to be.