haven't measured one yet, but i can. there must be hundreds of thousands of them out there now. i have never seen such a profusion of insects of this size in the woods here!

I'm getting the same here. I've seen hundreds upon hundreds, but there must be ten times that. The number of specimens is immense here, and in the woods next to my house the chorus is almost deafening. I'm curious what decibel level it's at. But no doubt based on specimens I've seen and the chorus, it's a bunch M.tredecim in my neck of the woods.

My emerging nymphs from last night at home now have the orange abdomen and mark behind the eye supposedly characteristic of M. tredecim. I've got a couple measured, about 28 mm body length. They seem noticeably larger than what I believe to be tredecassini I got the other day--yes, a couple of measured captives are about 20 mm body length.

It doesn't get much better than this, insect-watching-wise! I got a few slides in '98, but did not know all the ins-and-outs.

They are crawling up the sides of my house even as we speak--they are coming for me! If they get me tonight, have everyone know that I went down with strobe flash firing, and a memory card full of images!

measured two cicadas body length: 32mm and 35mm.

... that confirms they are M. tredecim. M. tredecassini is smaller, and with dark abdomen and behind the eye (from what I've read).
M. tredecula is supposed to have more striped abdomen underneath. Certainly don't see any sign of that in your specimens, so I think that you are safe to move these to the new guide page for M. tredecim.
Again, I'm always willing to stand corrected, but I think I understand at least those two species now. (We know this is a 13-year brood, so that rules out all of the sept-... species.)
Had a chorus of M. tredecassini at 8:30 this morning, temp. at 55 F!

have fun - can't wait to see what you get!

I hope you are getting some audio of the chorus? That would help identify the species. I'm pretty darn sure you are seeing M. tredecim. I've seen both M. tredecassini and M. tredecim now, and they are pretty easy to tell apart by abdominal color. M. tredecassini has started singing already, but I have not heard M. tredecim.
Cool wet weather for a couple of days here, so that will probably put a damper on things.

I have recorded some video w/ audio of the cicadas several times on my 7D but the sound seems rather muted when played back... I'm going out this afternoon and will take a measurement. May try another video too - they're going at it (singing) as I type.