I want to manually start a disaster in my city to see what happens and test my escape routes, how do I start a disaster on demand rather than just waiting for one to randomly happen?
In the user interface is a new section with all the disasters. It's called "Landscaping and Disasters", see screenshot bellow:
Now you just have to select a disaster and place it on the city. There also is a bar to set the severity of the disaster (1 = weak, 10 = strong). The disasters have a cool-down time as you can see in the following screenshot:
After placing your favorite disaster on your poor citizen you can lean back and wait for it to happen. Just fast forward time, it automatically slows down when the disaster occurs. You also could place more than one or different types of disasters at the same time.
Have fun playing god! :)