While it’s still early in the season for many parts of tornado alley, these low numbers don’t really surprise me given what the El Nino phenomenon can do to the tornado frequency across the Plains. El Nino, though weakening, is still in place. And the jetstream structures that often characterize it can modify the shape and propagation mechanisms of the storm systems that generate severe weather over U.S. – often in a way that does not favor tornado production. That seems to be what we’re observing so far:

http://www.spc.noaa.gov/wcm/torngraph-big.png

Of course a six year database is not a large sample when considering a seasonal tally. And we need to be particularly wary of the meaning behind these stats in this case, given that a single El Nino cycle (including the opposite phase La Nina) can last several years before dissipating. Only 4 of the last six years were “null” seasons (neither El Nino nor La Nina). It is also interesting to note that the tornadically active 2008 season occurred during a strong La Nina.

The bottom line in all of this is that the 2010 tornado frequency so far is down. And if El Nino is indeed having a negative impact on the count, I wouldn’t be surprise to see it pick up later in the Spring given El Nino’s (expected) impending demise.