Go to Quick play mode from the main menu and choose a country (I had to boost some kilometers for the platinum in the end so I chose Finland). In the stage selection menu you have to push
to configure the weather. Make sure that "dynamic weather" is on and confirm your configuration in the menu. This is important, if you do not use the confirm button, dynamic weather will be turned off. Select a stage now (I chose Ouninpohja as it gives you 10 kilometers in about 5 minutes) and win. Do this three times to win the trophy.
I configured the weather before each race again to make sure dynamic weather was still active.