Choose two random Pokemon that don't have auto-weather Abilities.
Set the weather manually to one of the weather conditions (not sure if Primal weather is also affected by this).
Switch to another Pokemon. Change the weather to "none".
Select another Pokemon. The weather is automatically defaulted to whatever whether you chose (like Rain). Even if you choose, say, MegaZard Y and it switches to Sun, switching from MegaZard Y to another Pokemon will immediately change the weather back to Rain.
http://nuggetbridge.com/forums/topic/13623-nugget-bridge-damage-calculator-bugs-feature-requests/?p=182609