I had a mfx surgery done back in 03, and another one about 3 months ago (different areas). And yes, there are times (not always) that when we have a sudden weather change (barometric pressure) I'll be a bit of a hurting puppy. It's usually a moist heat that helps ease the achiness....as well as a couple of ibuprofens or tylenols. Also noticed that it helps if I drink lots of water.... I really don't know what that's about, but I'm a better predictor of pressure changes than the weatherman!
Hope it's not affecting you too much - what part of your knee did you have your mfx done?