For all of the natural disasters, there are two ways for them to occur. The first is through natural occurrence in the game, the other is to activate it yourself.
The first option can be manipulated in the “Options” section when you pause the game. On the “Gameplay” tab, under “In-Game Settings”, there is a “Use random disasters” choice that you can toggle on or off. Below that is “Random disaster frequency” that has a slider bar. Set it as you wish.
Personal note: I had it set right in the middle, playing for 3 hours on 3X speed in 150,000 person city, I experienced 0 disasters For the second option, you can activate the forest fire yourself, but you need to do have a few things. First, you need a forest area, not really dense but a lot of trees together in an area. Second you need to build a “Firewatch Tower”. This is under the “Emergency Services” button on the toolbar. It used to say “Fire Department”, but it still has the same icon. This area now has two sections, “Fire Department” and “Disaster Services”. The firewatch tower is last item under “Fire Department”. Place it in your forest.
Now you need to start a forest fire. The last button on the new toolbar is “Disasters”. Click that and there are two tabs, “Evacuation” and “Disasters”. Push
to get on “Disasters”. Now, there are two fire choices, “Fire” and “Forest Fire”. Select “Forest Fire” and choose your forested area. A fire will break out and the emergency will start, popping achievement.