At one point, I did write a page that included food data that attempted to take seasons into account. It kept having all kinds of problems. However, this was back when there were variable harvests, which confused the calculations significantly.
I could probably write one now that would properly take seasons into account, with daily harvests.
There's never been a conscious effort to keep accurate medium-to-long-term food production data (ignoring weather, which is unpredictable) out of Lords' hands; it's just not been a trivial proposition.
If I'm given enough raw data to work with, I could make a prediction model, which I could then share publicly, along with the formulas to make it work.
Weather is unpredictable... and yet not. They have a set % chance of happening, and have defined values for their impact. On a small scale, it's best not to include it. But continent-wide and over many years, it could be factored in as an average.
I suspect, though, that it's info that most players don't care about. Seasons only affect two continents, and on these two continents, very few realms (especially if you exclude ones being conquered) have a meaningful food deficit. Most realms know they'll make it through winter, so what do they care for knowing by exactly how much? At most, they can note how much food they have in the warehouses at the start of a year, and a year later, as most of them probably don't trade much with other years. Because the food deficit isn't equally spread out.
All I want, really, is the means to predict how much food will be required to make it through to summer. Because that's how much food we need to try to plunder or buy. (mostly the latter, obviously, Falkirk won't be around for much longer)
That, and that the continent actually produce a surplus of food that is maintainable on the long term...
