Preliminary reports indicate that 23 days passed between November 7th and 29th, 2009 without a single tornado report in the continental United States. That period turns out to be about the average duration of an annual tornado drought based on recent records.
The U.S. should experience tornado droughts spanning a little over 20 days about once per year. The longest tornado-free span in the last two decades occurred over a 45 day period from December 31, 2002 through mid-February, 2003.
Other tornado droughts in the longer-term record include the following:
57 days, ending on Nov. 14, 1952.
54 days, ending on Jan. 28, 1956.
52 days, ending on Feb. 2, 1986.
45 days, ending on Jan. 19, 1961.
45 days, ending on Jan. 22, 1981.
In an attempt to better visualize tornado “drought” durations, and where those episodes have occurred on the calendar, I put together a chart depicting the longest annual tornado droughts since 1990.
This chart seems to indicate that most November tornado droughts are relatively short in duration. There have been six November droughts in the last 20 years. The November 2002 drought was the longest, at 30 days, and continued into December before ending. Over half (11) of the tornado droughts began or ended in December but it is not uncommon for tornado droughts to span January/February too.
There are other times in the year when there may not be a tornado for a few days, or up to a week. Those spans are not shown here. Analysis of the records suggests that our most recent tornado-free November ended just about on schedule after 23 days.
Incidentally, the Storm Prediction Center (SPC) issued no Tornado or Severe Thunderstorm Watches during November 2009. This is the only November in the watch database (since 1970) when no watches have been issued during the month. But, that’s a subject for another post, perhaps!
Harold Brooks has been conducting a return period analysis on the tornado record for different tornado (E)F scales. He is likely to provide additional insight in an upcoming blog post on this subject so stay tuned!