I roll a d8 for the weather, higher = better. What that actually means depends on climate, time of year, and any previous weather roll.
Since it's Free Kriegsspiel the result is always plausible/reasonable to me at least; and my players seem happy.
I think there should be a lot more effort put into teaching GMs FK techniques like this, certainly for simulating all the parts of the world they don't want to use a mechanistic system for.