Forecast and prediction of agriculture and related issues depends on the real time observation in well connected and properly installed observatory. The area for which you want to predict needs number of observatory depending on the size. More is the number of observatory more accurately it can predicted or vice versa. Now question is the availability of data... If you don't have the observatory you can use the gridded data recorded by the satellite sensors and can be downloaded from USGS website for any point on the earth surface. It uses the google Earth . Visit USGS and find the data.... Another link is Marksim GCM which provides the data simulated by 17 GCM models...
However it is not the substitute for real time obsreved data...