Although this is my 4th year of (unsuccessfully) hunting turkey, I still have a LOT to learn.

How will a late afternoon thru overnightthunderstorm effect turkeys on roost and then the following morning?

It can be fatal for them if I am lucky enough to be nearby when t-storm approachs and their gobbling at the sound of thunder gives away their location to me(and I'm there in the AM)

Also if its still raining at roost time they may prefer to head for the conifers as opposed to hardwoods for the night

That's kind of what I was thinking also!

How much rain and/or lightning n thunder will keep them roosted or hiding?

If it is a light rain, they will go ahead and pitch down. Generally if it is hard they may stay on the roost till the rain stops. What I noticed in Nebraska this season is that after a night of rain, the birds pitched to the west side of the draws or where they would get 1st sun (to dry them out). On clear, warmmornings generally pitching to east side and heading to the bottoms(where the timber is)during the day to escape the hot sun.