Well I will say if you happen to get distant thunder it can be a great locator! But I do not advocate hunting through lightning storm!
At any rate I have harvested birds in heavy and light rain, it just depends on how full of gusto I am that particular day. I do love to get out after a good shower and a front passes. That can be a great time to field hunt and find the birds out "bugging". General rule of thumb turkeys are going to do what they do any normal day, whether rain, sleet or snow. It may be harder to hear them or elicit a gobble if they are vocal, but you can harvest them none the less. Just wear good rain gear and stay dry if you get after them in inclimate weather.