I tried to let it snow in all biomes with this script:

allBiomes = forAllBiomes()
allBiomes.set("temperature", -0.2)
allBiomes.set("enableRain", true)
allBiomes.set("enableSnow", true)

It sets the values, e.g., output/Jungle.json

{
  "ID": 21,
  "Name": "Jungle",
  "Class": "net.minecraft.world.biome.BiomeJungle",
...
  "Temperature": -0.2,
...
  "Enable Rain": true,
  "Enable Snow": true,
...
}

But still it's normal rain, even in previously snowy biomes.

Not really sure what to tell you. I tried your script and it's snowing everywhere.

Do you have any screenshots, logs, etc.?

@superckl What he said is true because I'm facing the same problem.
Maybe he forgot to say this problem happens on multiplayer game. I tried the same seed and script in singleplayer and multiplayer game. And I noticed where it should snow in local game became rain in the server.