Harvest is August-October, depending on varietal and ripeness. The vines are dormant, and as long as the water is not swift and ripping them or the trellis system out of the ground, they'll do ok. Obviously, it is difficult to prune vines if they're under water.
We just had an article posted here the other day blaming the Bears for eatiing all the grapes.
I think one way or another they are going to hike up the prices. JMO
Napa Valley is a wonderful place.
Haven't been there in 20yrs but it was great back then.