There is a bug in the weather code for SH3, concerning the handling of stormy weather.
This has been fixed in two different ways, once by H.sie (as part of his code patch for SH3), and once by me.
Both solutions replace the damaged envsim.act in stock SH3 with repaired code.
H.sie's solution stops constant storms with random changes to the weather; my solution corrects the original error, to restore the intended weather pattern, but also forcibly interrupts continuous storms.
My solution (which, naturally, I prefer) can be downloaded here.
http://www.subsim.com/mods1/nygm/EnvSimAct_Stiebler.7z
Just unzip the 7z file, and install with JSGME. It will replace (reversibly, if you use JSGME) the stock envsim.act file.
Don't fiddle around with claimed solutions that alter parameters. They can help a little - I wrote the RealWeatherFix myself some time ago - but the two different code patches fix the original error in stock SH3 completely.
Stiebler.