Seems to me that rural feudal societies were slaves to the weather.
i.e. typical harvest, good harvest, bad harvest. Most years would be average, but on an exceptional year does the crown collect the extra grain in the granaries or does the province keep it (better fed peasants raise the population etc), do you ignore a famine or spend a fortune on bread to stop the peasants from rioting?
Maybe there needs to be an annual test for the weathers impact on agricultural provinces. Usually the test would be okay with nothing but a note to the player, a bumper harvest might improve province income and fertility and a bad harvest is a calamity with the option of the player intervening to prevent famine (ie. buys food). A bad run of luck and 3 years of famine might dent a countries ability to make war, conversely, 3 years of bumper harvests would strengthen the nation.
Title - The weather report for <year>
Trigger Date = Summer Solstice (21st December)
Description a) "My Lord, we have had a bumper harvest. There is plenty of food for the peasants and we will make a tidy profit on our estates".
Text on Button 1 (on event A) "Pass another quail" farming income +20%, Population +10%, stability +1, all agricultural provinces (e.g. grain/wool) gain 50% income
Text on Button 2 (on event A) "We must invest this windfall in farm walls and better breeding stock" income for agricultural provinces raised permanently by 5 %
Description b) "Not much to report, a good year for picnics"
Text on Button (on event B) "Good"
Description c) "My Lord, we are in trouble! Unseasonal weather has ruined the harvest!"
Text on Button 1 (on event C) "Let them eat cake" income (hungry people are too weak to maintain the farms)" stability -1, farm income -20%, Population -10% Revolt risk +20%, farming provinces income lowered permanently by 5%
Text on Button 2 (on event C) "We will make a token effort to appease the masses" farming income -20%, Population -5%, all agricultural provinces (e.g. grain/wool) lose 20% income
Text on Button 3 (on event C) "Fill the granaries chancellor, you must go abroad and buy food for our people" stability +1, income from grain provinces -20%, revolt risk -10%, all agricultural provinces (e.g. grain/wool) lose 50% income
Maybe the percentage effects are enhanced (both good and bad) for marginal areas - eg arctic.
You could imagine your joy if you were portugal and castille had suffered 3 consecutive years of famine? Reduced income would make them impoverished and open up a temporary window of opportunity. Of course, the same holds true in reverse.