What are the steps you go through for building out long term weather pattern predictions, if any?

I'm curious because geography and weather affect society down to the most baseline level. The Nordic countries would never have had a raiding culture if they had lush land for agriculture and a warmer climate for example. It'd be an entirely different people that would be foreign to us. If their lands were a bit more south and had warm water currents they may have a similar disposition to Britain, Scotland, and Ireland where they're unusually warm and rainy for the latitude they're at.

There's also a period in European history known as "the little ice age" where because of colder overall weather the period is marked by a huge uptick in wars, rebellions, and general violence from less bountiful harvests and harsher winters. Knowing when in the history of a world you're going to get prolonged periods of warm, cold, extreme weather, or other things, could help map out major international conflicts and could help to spur on interesting historical events.

For something as important as weather for making a reasonably realistic culture for a region I don't see it talked about as much as geography. I'm curious as to what steps you need to follow to be able to create at least a rough idea of how weather will change over time in the world in different regions