When on a vanilla snapshot server, both /toggledownfall and /weather clear 1000 do not stop the rain. Even sleeping no longer stops the rain. After trying one of these methods, restarting the server stops the rain.

I expected any of the three methods above to stop the rain with minimal delay, but instead the rain kept going as if nothing had happened. Even after waiting for (Minecraft) days the rain seemed frozen on. After a reboot the rain stopped instantly every time I tried.