WSET.com - ABC13Why So Many Twisters? by Matt Ferguson

Why So Many Twisters? by Matt Ferguson

We all know how devastating this year's tornadoes have been, not only in our region, but across the entire Southeast.  

Here are some interesting statistics for you from the Storm Prediction Center in Oklahoma. So far in 2011 there have been 1,036 tornadoes (thru April) in the United States. The average number is around 460 and the yearly average is 1,469 twisters. For the month of April 2011 severe thunderstorms brought the country a record 600 (possibly or more) tornadoes. The previous record was 267 tornadoes set in April of 1974. The previous record for any month was May 2003, when the US saw 542 twisters. Therefore, to say the least 2011 has been an unprecedented year for severe thunderstorms and tornado outbreaks.

So many of you may be wondering, why so many tornadoes this year??? Well unfortunately, there is no easy answer and scientists do not know exactly the reason for the record breaking number of deadly tornadoes. I'm going to try to give you some insight on what may be the cause of the numerous and widespread severe weather events this season.

We all know thunderstorms form ahead of cold front in most cases. Each year we have cold fronts, but this year these have been more intense containing more wind energy aloft in the atmosphere. Right now we are in a La Nina phase of the Pacific Ocean (cooler than normal sea surface temperatures). This pattern often causes more extreme clashes of cold and warm air masses across our country. In addition this year some data suggests the Gulf of Mexico is warmer. So not only is the air feeding into the South warmer, but it more humid due to ample evaporation. This all leads to severe thunderstorm development along the cold fronts that form as cool Canadian air moves south. Now you have the storms, but to get tornadoes you must have rotating thunderstorms called super cells. To cause a storm to rotate you must have wind shear in the atmosphere above. This is the change of wind speed and/or direction vertically or horizontally above ground usually starting beyond 2,000 feet high. Wind shear is caused by jet streams, which are strong rivers of wind high in the atmosphere. There is usually a low level jet stream around 5,000 ahead of storms (winds of 50mph plus) and the normal jet stream driving the cold front roughly 30,000 to 40,000 feet high (winds of 120mph plus). This year's jet streams have been much stronger than usual. The jet stream's upper level strong winds (up to 180mph) are a product of temperature differences across a large area. Since this year's clashing of warm and cold air masses are extreme the jet streams are stronger, causing more wind shear in the atmosphere. This leads to thunderstorm rotation and eventually the development of tornadoes. All this wind energy in the atmosphere has lead to powerful tornadoes this year, with winds between 150 and 200mph.

Hopefully this gives you a better understanding of tornado development and some reasons as to why tornado records have been broken!