Just a simple additional weather preset, that will create the lowest possible ceiling and visibility conditions achievable in MSFS (that I've been able to figure out, anyway).  Perfect for practicing those CATIII approaches!

Simply unzip, and place the folder "CatIIILowestPossible" in your Community folder.  Then, choose the preset "CATIII Lowest Possible" from your weather presets drop-down menu in the sim.