Sorry I missed your post yesterday. Your game set-up appears to be fine. Are you saying that it is the re-worked campaign files that caused the issue? Do those files have changes to the weather in any of the lines numbered 10 to 20 in each of the MIS files? This is the "default" lines:
Fog=0
FogRand=0
Clouds=0
CloudsRand=0
Precip=0
PrecipRand=0
WindHeading=0
WindSpeed=5.000000
WindRand=0
WeatherRndInterval=5
SeaType=0
... the game also has a tendency to get "stuck" in some weather patterns under certain conditions, and one way to get out of that would be to Save the game, exit the game to Windows, re-start the game and re-load the Save. However, that does prolong the agony sometimes, since it re-sets the "counter" (that WeatherRndInterval=5 line). There are members here more fully qualitfied to be "weathermen" than myself, and may have more insight into what you're experiencing...
Another help might be the use of the LAA (Large Address Aware) utility. CapnScurvy has a thread on its use in Windows 10 here:
Windows10; Large Address Aware; SH4 Install. While it is for Windows 10, most of that does apply to Windows 7 also, ~if~ you have a 64-bit version. LAA will help the game run better, by allowing the game more system RAM use...
