After selecting a service in the timetable, keep pressing UP on the controller until the cursor gets to the top of the right hand panel, where it says Dynamic Weather and then change it to Custom Weather. You won’t see the cursor going up through the weather settings while they are greyed out (due to very poor UI design) but it is happening behind the scenes. You’ll only see it when it gets above the weather settings section. Once Custom Weather is selected, you can change all the sliders.
Just to complete: You won't be able to set weather in journeys or scenarios, only if you run services from the time table.
