Not a whole lot. We got pounded last night and I had to drain some water - about an 1 inch of draining. My levels didn't change a bit. If you had heavy rain for a couple days straight or more and had to drain a lot of water, then you might see some reductions in FC, CYA or CH. pH might get driven up a little with heavy aeration. You might even see a little increase in CC from atmospheric particles that were taken to the water from the rain. But in general, one good rain storm isn't going to throw you off too badly.