Most of the impact models that predict weather effects on crop growth and productivity run on a daily basis. This is the resolution that you would need to capture effects of drought and extreme heat stress. Those models are based on biophysical processes in the crop-soil-atmosphere system, fairly well calibrated and most of them ready to use after a further local calibration. However, there are also models around that use monthly or even yearly data. You may even be able to build a statistical model to use monthly or yearly data, especially when working at the regional scale. However, to do this you will need to have quite a good data base on yield and weather data to construct a sensitive model.
Climate change impact on crops is a really interesting topic, but as Claas Nendel said, models run on a daily basis and future climate data on a daily basys is really complex process. In this way, I like developing monthly-yearly data-model better than weekly-daily data-model.
In this way, first try developing an actual climate database, using statistical procedures if you want or if you don't have data enough. Later build the a new future climate database and compare your new reults. You can compare using easy climate indices (summarize, average, median...).