Growing Grapes in a Northern Climate

We've had some weird weather and it has been touch and go as to whether my grapes would plump up and ripen before the snow. We did have snow in summer while the clusters were still green. Then we had unseasonably hot weather, followed by cool rainy weather. Here they are today, looking promising. There is a frost warning for Thursday (2 days from now). Should I pick the grapes now or leave them a bit longer? That is the question...