This should not be a big problem. The weather simulation can take that into account, for example by assigning a climate to each map (predefined template of advanced settings). To account for edge cases, we could have advanced settings. The settings could include things like temperature, rain, humidity etc. By using a little bit of science, such as if temperature less than 0 -> we have snowfall. In addition, if the temperature is less than 0 after x days, water will be frozen and soldiers can walk over a lake.
I am almost certain that we could simulate weather and seasons very well using a simplified scientific weather model.