USA Today reports:
The USA in the last 12 months has seen the fewest number of tornadoes since at least 1954, and the death tolls from the dangerous storms have dropped dramatically since 2011.
... The reason: An unusually cool weather pattern from the Rockies to the East Coast. "Generally, the lower the temperature and/or the drier the air, the lower the number of thunderstorms," reports AccuWeather meteorologist Alex Sosnowski.
... So far in May — usually the USA's most active month — only three tornadoes have formed. All have been rated EF-0 on the Fujita scale of tornado intensity. EF-0 is the weakest rating for tornadoes, with wind speeds of about 65-85 mph.