I played a scenario created with the scenario planner. When I started it, I set the weather at snow. I saved the game and resumed it a day later. within seconds all snow disappeared. Not sure if this has to do with the scenario planner or with the game save feature. Edit. It is a game save issue. I tried again and I explicitly set the weather before resuming the game.
Thats on every map when loading the savegame. I use unreal unlocker to modify my weather back with the console.