Storms, hurricanes and tornadoes are all heat engines. They are powered by heat differentials. If the atmosphere heated up evenly there wouldn't be any storms.

The warming of both the sea surface and air will not necessarily mean more storms. But when cold northern air travels south the interface between the cold and hot will be alive with storms.

As this interface shifts northward I would expect to see the band where storms are common to shift north also. A cyclonic storm, essentially a hurricane, was seen in the mid-west not long ago. This is highly unusual.

This may be the trend of the future as extreme weather, normally seasonal and mostly confined to coastal areas and well known belts of activity, becomes less seasonal and less confined to their normal areas.