Meteorologists have many sophisticated tools to them forecast the weather. however, weather forecasts are still sometimes incorrect. why do you think this is the case?

I wouldn't say that weather forecasts are wrong. I would say that they are sometimes wrong. ;-) 

A forecaster uses very complex computer-generated models when forecasting the weather. These models describe what will happen in the atmosphere (for instance, concerning winds, temperature, and humidity) over the coming hours or days. 

The models are not perfect, partly because the research about the weather is far from finished, and partly because the physics in the atmosphere is so complicated that some approximations have to be made. 

But the biggest problem is the fact that the computer models require observational in-data. Information from weather satellites, weather radar, soundings, and weather observations are put into the computer models, and then the computer starts "calculating the weather into the future". But there are not enough weather obserations and weather radars, especially not over the oceans. If this would change, weather forecasts would improve considerably