I love Silent Hill so I suppose I can take a crack at it.

A Silent Hill in Springfield, Missouri would mean a much more abandoned Springfield. So of course there would be almost no people. In the Silent Hill Franchise they do a good job of using the ashfall as an excuse for most people having left and those still around being crazy, in Springfield's case I feel like we would want to play more into the weather, perhaps its is always foggy as hell at best, and rainy to downpouring at worst. Never a day of good weather and most have left as a result.

The natural indicator of normal reality to horror reality would remain sirens, and the extensive tornado siren system in Springfield would fit that bill pretty easily. The game would probably take place mostly downtown, and opt away from the neighborhoods. Although I also think that utilizing the various Caves and Springfield Underground throughout the city would be almost a requirement as well. City could lead to caves, which could lead to parks, and offer a bit of variety in setting.

In normal reality you are just left with a couple crazies somehow making by downtown, and there is weird undertones of some kind of cult living in the caves beneath Springfield, then when you transition to horror reality the weather gets substantially worse and it forces you indoors or to minimize your time outside as much as possible. Monsters may become a serious issue outdoors and anywhere connected to outdoors could have monsters spilling into them. The storm can also impact the water levels and shut off certain access points in the underground and tunnel systems, meaning you can only get certain places at certain times.

Lots of gun stores around where you could restock on supplies, and interesting areas you could explore downtown from the various train tracks, to under bridges to across city streets, parks, etc.

The monsters would probably include a mix of the normal horrors you might expect in silent hill, but could also include some local varieties of weird creatures, like Ozark Howlers as some kind of demon dog that becomes active during the storms in the parks.