Temperature, sky, wind and precipitation forecasts for three or four days away are as accurate as the forecasts for the next day were three or four decades ago. Most weather is a result of fairly basic concepts in fluid analysis. Computers are able to model the atmosphere and, based on measured initial conditions, forecast the future with reasonable accuracy.
Meteorologists use their experience with the models and with atmospheric dynamics to offer reasonably accurate forecasts most of the time. However, severe thunderstorms, and tornadoes in particular, are of a much smaller scale, with motions and forces often of a secondary nature. Meteorologists are still not able to say for certain which thunderstorm cells will produce a tornado, or which atmospheric setups will produce those dreaded tornado outbreaks.