Friday, May 10, 2013

A Drought Worth Having

Droughts can be quite nasty with it comes to rainfall.  However, droughts can be a good thing when it comes to severe weather.  The U.S. is currently experiencing such a drought for tornadoes.

The monthly number of tornadoes for 2013 compared to the 1981-2010 average. Click on the image for a larger version.  Image Credit: Climate Central.

The year started off looking above normal, but that quickly turned in February.  Significantly fewer tornadoes were observed in March and April.  May is normally the peak of tornado season for much of the country.  The drought has continued and will likely continue into the middle of May.  It is hard to say if it will continue beyond that, but it is likely.

The second busiest year on record for tornadoes was 2011.  The weather pattern that year was coming off of a strong La Nina.  The map below shows the path for many of the tornadoes nationwide as well as the intensity.  Compare that with 2012 which like this year showed a low number of tornadoes.

The top chart shows all of the observed tornadoes for 2011.  The bottom chart is for 2012.  Click on either map for a larger version.  Image Credit: NOAA/SPC.

A major rainfall drought hit much of the tornado belt in 2012.  The lack of moisture yielded fewer thunderstorms which are required for tornado development.  While the rainfall drought has continued this year it has eased for much of the tornado belt.  This year it has been the unseasonably cool spring that has reduced tornado development.  The thunderstorms needed to produce tornadoes need heat and moisture.

South Carolina is not in the tornado belt of the central U.S., but there has been a tornado drought here as well.  The chart below shows that only one tornado has been observed so far and that was near Ridgeway in Fairfield County.  The peak month for tornadoes in this state is May.  There is a secondary peak in August associated with tropical activity.

The tornado drought in South Carolina for January-April, 2013.  Click on the image for a larger version.  Image Credit: Climate Central.

While there is a tornado drought in progress, it is always best to be prepared for any severe weather.  The National Weather Service has published an excellent guide explaining much of the severe weather for South Carolina.  There is a great deal of useful information in this short guide.  What’s more you can get it online here.

In 2011 there was talk of climate change affecting tornadoes because of the large number.  How can we reconcile the current drought of tornadoes which is the lowest in about 60 years?

Tornadoes are weather events.  Global warming is changing the climate and hence weather patterns.  This may at times impact the production of tornadoes.  However, the scientific literature does not answer how climate change will alter the number or placement of tornadoes.  It is reasonable to expect that as the earth warms tornadoes may expand their range northward in Canada.  Last year was a very busy year for tornadoes in central Canada.

It is less clear how this will play out in the central U.S.  Tornadoes are complicated beasts, affected not only by moisture and temperature but also by wind shear and other factors. So far, there’s simply not enough information to say anything definitive about the future of tornadoes under global warming.