There are many different factors which determine weather patterns and events. While we, as humans, do know some of them, we don't know all of them, and predicting what will happen with a weather event can be very difficult.
Note that while the experts can often get details of a weather event wrong, they are still the best we have at even coming close to getting it right.
As research continues and technology improves, the predictive power will improve.
I say, just be happy that we can predict these events at all. Imagine what it was like when knowledge of a hurricane event only occurred when the storm hit your city, and everything you owned was destroyed because you couldn't get out in time.
Check out this article by the NOAA for more information: http://celebrating200years.noaa.gov/magazine/devast_hurricane/welcome.html