Severe weather season is usually running full-bore in late May, but not this year.
A large dome of cool, dry air moved southward from Canada May 15-16 and that is one reason. The other perhaps more significant continuing cause for a recent lack of tornadoes is the position of the jet stream.
It is located along the U.S. Canadian border, and looks as though it will stay there through the end of May.
The jet stream is the primary steering belt of wind for storms; it also provides a “supercharge” effect over thunderstorms as it provides more lift. So when the jet is not around the overall severe potential will generally be lower.
Tornado Season 2009 to this point has been in line with the 1954-2007 average; if this lull continues for very long, we’ll find ourselves below average for U.S. tornadoes, which is not a bad place to be.