Farnley

Are you sure this isn't caused by heating?

It's explained on the wiki.

/weather setprecip auto

I don't think that's possible, since climate is tied to world generation. /weather setprecip [-1..1|auto] lets you override the precipitation value with a constant value (values from -1 to 0 will disable precipitation with varying intensities of rain clouds) until you change it again. You could also modify the minRain, maxRain, rainRange, and possibly durationHours values of existing weather patterns in \Vintagestory\assets\game\config\weatherpatterns (ideally as a mod), or create your own weather pattern and load it with /weather setir. Modifying existing weather patterns would affect all regions and all worlds though. This wiki page and looking at existing weather patters should help you create your own patterns.

/weather lp gives you a list of weather patterns. Then use /weather setir [weatherpattern] ([weatherpattern] = one of the list entries) to change the weather in your current region, or use /weather set [weatherpattern] to change the weather in all regions.

In case you haven't tried it yet, I assume /wgen regen [chunk radius] (at the bottom of the page) does that. You can use .debug wireframe chunk to show/hide the chunk borders. Make sure to backup your world before you start using the command, and be careful around traders, as it might just delete them - at least it did when I played around with it.