Interesting enough I have two servers both are set to no weather (see settings below) However my one server still has weather while my other does not.
The Settings_RegionalWeather setting only determines if weather should depend on the current region, e.g. if it's raining in temperate biomes, it's snowing in arctic regions etc. If this setting is set to false, the current weather is a global effect, i.e. if you set it to rain, it will rain everywhere (even in deserts or arctic regions).
To turn off automatic weather changes, you can set Settings_WeatherPreset to 0 - this keeps the currently set weather effect active ![]()