No observable trends have been detected in the United States with either tornado or hail climatology, according to the scientific literature at large and the IPCC. The tornado events of the very active 2011 season are not unprecedented in history (Doswell et al., 2012). Such outbreaks have occurred before - in 1974, for example, and such outbreaks will occur again.
The incidence of severe convective storms is not expected to change under global warming. However, a recent study by Trapp et al. (2007) concluded that the number of days in which severe weather could occur may increase under global warming. More research is required in this regard. Flooding under intense rainfall events has been shown to increase and will increase further under global warming, as noted above.
In the absence of a detectable trend in severe convective events of tornadoes or hail, significant headway can still be made through adaptation of tornado and hail resilience measures of existing building codes and standards andrecommendations of the Institute for Business and Home Safety (IBHS). Tornado resilience measures in particular can save thousands of dollars under weaker tornadoes, and save lives under stronger ones. Such measures attain greater importance given the expanding population footprints, increasing population densities and increasing property values in the United States.