I can sympathize with you. We just had a pig roast last weekend. Once you start a pig cooking 20 hours in advance, with 100 people invited, there is no turning back-- rain or shine! And as luck would have it, we got a major downpour just as everyone was sitting down to eat. Here's how we coped before and during...
We got a couple of 12 ft tents from a discount shop (Ocean State around here) for around $100 each for folks to sit under to protect from sun or rain. We got a smaller 10 ft tent to cover the pig while I carved it. We made as much room inside as we could for folks if needed. The rain dropped our attendance to about half (yum... a freezer full of smoked pig!!). We managed to serve everyone before the rain came. People gathered in small groups wherever there was shelter and ate and visited.
In spite of the heavy rain, everyone was in great spirits and enjoyed themselves.
So this info may not be of use to you but if not too late, you can either buy, borrow, or rent tents. Do the best you can to make all guests feel ok about the change of weather. And you may find, like we did, that people make best of the situation and most will actually feel it is a very memorable event. Don't let a few hundred gallons of water dumped on you ruin a perfectly good party!
You probably don't need this tip but for anyone in a similar situation, we have some old scaffolding in our basement. One section of it with an old scrap piece of plywood over the top, made a great shield for our fire pit where we cooked our hardwood down to coals. It is much better than a flammable tent as the rain on top of the plywood kept it wet enough to not worry about it burning.
Best of luck!