Tried using the command /weather rain, and the sky changes to overcast, but it never actually rains. I changed to thunder and received the message "Set weather to rain & thunder" but still no rain. However the thunder did appear.
/weather clear brought the weather back to clear. Setting the weather directly back to thunder made the thunder return, but no rain.
seed -8300275442758179407
xyz: 1286 69 975
Edit: It is raining the biomes next to the savanna I'm in, but it won't rain in the savanna. Adjoining biomes are beach and lukewarm ocean.
Works as intended. Mesa, desert, and savanna biomes all have a temperature above 1.2, (Savanna=1.6, Desert=1.8, Mesa=2.0) it is too "hot" for rain to occur.