On Thursday, March 19, 2015 at 9:21:49 AM UTC, wrote:
If you, like me, are interested in tornado activity in America this is the site to bookmark:
http://www.spc.noaa.gov/wcm/#data
This year, so far, looks unusually quiet for both thunderstorm and tornado activity.
The tornado count is less than 10% of average for this time in March, and the last three years have also well below storm activity.
Being NOAA they provide archives back to 1950, which as you can probably guess I have already downloaded and written a viewer for, but I have my work cut out for me with this kind of data, because this site is chock-a-block with all kinds of statistics that I can't begin to match. Because a lot of people die each year as the result of these events (47 last year with 632 injured) NOAA take the whole matter of forecasting and issuing 'watches' for them very seriously.
Why thunderstorm and tornado activity is so low at the moment is debatable, and it will be interesting to see how the rest of 2015 pans out.
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There are, as you might guess, several reasons posited. The tornado count has been lower than average during the past three years.
In 2012 a rather persistent high pressure ridge developed across the U.S. interior. In both 2013 and 2014 the early spring was unusually cool across the eastern 2/3 of the U.S., with a western or west-central ridge / eastern trough pattern in place. This was rather persistent during 2014, and can be partly attributed to the anomalous warmth in the eastern Pacific and emerging positive PDO pattern.
With a strongly positive PDO at the moment this sort of pattern is still apparent, and east of the Rockies temperatures have been and continue to be below average. I don't see a significant change in pattern through the rest of March or start of April, so most likely it will stay relatively quiet. Severe storms might be more likely in the S and SE rather than the Plains and Midwest during that period.
Additionally, El Niņo - although currently weak - has been shown to have an inversely proportional effect on tornado and severe storm numbers. This has been thought to be the case for a while and now, in timely fashion, there is a paper on the subject:
http://www.earth.columbia.edu/articles/view/3239
http://www.nature.com/ngeo/journal/v.../ngeo2385.html
Stephen.