Nope, the weather checks are done according to the game time from what I understand. And weather check doesn't mean weather change, there is a probability that after check weather stays the same.
Anyway I am a SH3 Weather user and I like it. Haven't tried the last version yet, just installed it. (Available in the mods workshop forum)

Do we know about how long the interval is that the game checks to see if it will do a weather change?

So we are talking realtime checks here right? TC will not have an effect? I have to actually sit at the computer X amount of time for a change?

I read somewhere that there is a random chance of the weather changing every six hours ingame time. Not sure what TC does to that though.

say for example you were doing a mission in 1x tc, would you see the weather change gradually or would it just instantly change or because your in 1x tc would the weather even change at all??

In my situation I sometimes only get an hour or so a day to play SHIII using the save game feature at the end of each session. If the weather counter gets reset at every save and exit point, and the weather changes every six hours of game time, I should still have the calm whether that I had when I left port. However, since leaving port, the weather has changed to rough seas and rain and now just rough seas and very overcast. I run at 128 TC between waypoints.

Nemo

What I hate the most is the feeling that the weather may get stuck in it's current behavior. Am currentöy doing a patrol outside Cuba, and it's like a hurricane and has been like that for at least 7 days now!

There's always hurricanes around there.

I'm currently running a 1x mission on the western approaches. Yesterday the weather was partial clouds with wind at 4 m/s. Now it's overcast, medium fog and wind speed at 7 m/s. So it does change, even at 1x.