The crazy fluctuations in movement of the jet stream these days caused by the warming of the Arctic are leading to extreme variations in winter temps, which wreaks havoc on even the hardiest of grape varieties.
A lot of farmers in the US are denying climate change instead of anticipating and preparing for it. If they were smart, like European farmers, they would be doing something about it, or at least not denying its existence.
http://msue.anr.msu.edu/news/pruning_severely_winter_damaged_grapevines_in_michigan