I think your basic ideas about wind patterns and such seem reasonable.
I created an Excel workbook that will generate a year's worth of random weather when I was starting Kingmaker. I posted it to the boards so anyone can use it.
You can adjust the parameters to your heart's content; I used historic weather data from Warsaw, Poland as the basis for my version of the Stolen Lands.
I would suggest running the sheet once, then copying the data out of the calendar sheet and pasting the values only into another sheet - the sheet tends to run all the random generation whenever you open it. It can be locked, but I still found it would bypass that on a regular basis.
Let me know if you have any questions about the sheet, and I'll do what I can to help troubleshoot.