What is happening is that the very necessary approximations are continually being refined and improved. The aim has to be to produce the best forecast possible although this may be far from the best possible forecast. In a sense, there is some experience built into the models. This comes through verification of the results of trying to describe all the various mathematical processes. Representation of the effects of convection or radiation can only be very approximate.
Much effort goes into finding the optimum representations.
Thirdly, there are small, local variations in the weather that are currently too small for the computers to replicate. The mathematical models cannot represent every minor headland, cliff, area of warmer or colder sea that lead to small variations in weather that may be important to the yachtsman. Improvements have occurred in forecasting. These are probably most evident in the ability to provide useful, although by no means always accurate, information out to five or six days ahead. There have been improvements in the shorter period forecasts although these are often difficult to perceive because of the difficulties in verification and the natural variability inherent in the weather.