I think I've found my correct weather seed, so I've been going through my calendar on MeteoNook for some time now trying to find a day with rain + heavy fog.
The closest I've found was rain with normal/light fog, and honestly, you can hardly tell that it's foggy at all. :[ Cloudy days still look very sunny now, casting a lot of light, which kind of ruins the vibe I want. Now I'm just wondering if it's even possible!
( Also, does anyone know how to predict a thunderstorm / which rain type will bring it? ; v; I've never experienced one and would love to see it lol. )
I've had a two thunderstorms this year but they're unfortunately really short, like only an hour long. From what I remember, it starts off looking like it'll be a normal rainy day but then it'll get dark and moody real fast. I usually get slight angled rain.
I wrote down the dates for them, so I could probably TT back and observe if someone doesn't give a better, clear answer for you.
I'm unsure for the other weather states as I don't wake up early enough for the fog, nor do I tt into the early morning hours.
I'm pretty sure this is possible as I have a vague memory of coming across this weather combination around 2 times before while TT'ing. I've never figured out my own weather seed or anything though, just going based off of memory.
As for thunderstorms, I generally would come across them more often during summer months. No clue how to predict them, but timing traveling during the summer is how I've experienced most of the ones I've seen.