Minecraft – How to generate an empty world in Minecraft

In this thread on Reddit, I found a command line script that will generate terrain by repeatedly restarting a server with varying spawn points until the area you specify is filled. This seems like just what you're looking for.

There's also a Bukkit plugin named WorldGenerationControl which can do it on a live server.

This is caused by starting the world without mods, and then adding mods that do something to the world gen (like for example adding new biomes or ores).

The part you explored before you added mods is still generated with the vanilla worldgen, the newly explored parts are generated based on the modded worldgen. Both are generated by using the same seed. But a slightly different worldgen.

If you are thinking that you never explored this far: that's normal, the game generates some area around wherever you are, so you came 5 chunks away from said area (or at least I think it was 5 anyway, might be more)

If you remove the chunks generated before you added mods with MCedit, they will be regenerated as new chunks with the new world generator.

I don't know which area of your world was made with the original worldgen and which part was not, but let's assume the bottom part is the old part. And the snowy area up top is generated once you had mods. Removing some of the birch wood forest area in MCedit should cause it to turn into snow forest biome (biome names are made up based on how it looks, they might actually have a different name). If my assumption is wrong, delete some snowy forest and it'll turn into birch forest.