I'd go ahead and plant provided the 1/2 inch is expected over several hours rather than in a 20 minute burst.

Thanks for the reply. As it turned out, I didn't get that far -- the wind was too high to work with the dry mix.

I've heard of Mel's Mix handling 7 inches over a 24 hour period. The seedlings needed a bath but it drained well enough. I know water and low spots get puddles and I turn around for ten seconds and it's drained.

Here is a link that might be useful: Sinfonian's garden adventure!

what happened? if those seeds are supposed to constantly be kept moist, heavy rain won't hurt them or will it?