[math-fun] "great walls" stop tornados?

http://www.bbc.com/news/science-environment-26492720 http://www.usatoday.com/story/weather/2014/02/25/giant-walls-tornado-alley/5... Physics Prof. Rongjia Tao claims if we build 3 giant walls 1000 feet high across a large part of the USA midwest, then tornados will diminish a lot in frequency. While it seems somewhat plausible to me, I doubt this question can be answered either by armchair theory or perhaps not even by computer modeling. More of an experimental question. And there might be many other side effects (good or bad?) of such a construct and its cost would be enormous.