AgWeb published an article in late March describing the changes that have occurred in growing season across the US since 1980, including graphs of both spring and fall frost changes and length of growing season. You can read it here. They attribute the changes to increasing temperatures over that time period.
The article also relates the increase in temperatures to an increase in severe weather; however, if you look carefully at the graph, there is an increase in F0 tornadoes over time but none in F1 and higher tornadoes. I believe that increase in F0 tornadoes is due to increased observations of small tornadoes by people with cameras and not to an increase in severe weather overall, since there is no trend in the larger tornadoes which are more likely to be observed through their damage paths.
