First thought: funnel cloud! Second thought: stormchasers. I think you need to play up the strengths in the photo for they are getting lost it too much 'empty' space. A tighter crop would help keep focus and attention on that tornado. Cropping 1/3 off the top and a little off on each side that it comes close to each side of the cloud the 'mushrooms' would keep the focus on the storm and tornado. Second it that I would also crop a little bit at the bottom such that the road 'emerges' from the bottom left corner to act as a leading line in the photo. Lastly, I think that there is a bit of loss of detail in the tones and sharpness. Try playing with shadow/hightlight/midtones and USM (unsharp mask) to see if more detail can be garnered from the shot thus improving it's visual impact.
Very interesting image... I like how the horizon line is level and not in the middle of the picture. Some ideas... the cars tell a story however I think that it distracts from the subject (the tornado). I would have rather seen this without the cars so we could see more of the tornado. Must say that took some courage to capture!