If there aren’t any clouds, then your sunset will be pretty boring. You will only have a nice gradient of color going from blue to orange.
I usually don’t bother going out when there is an overcast day to shoot sunsets, because the results are quite disappointing. The clouds cover the sky and you can’t see it, so you will end up having no colors at all.
If there is some light rain during the evening, then you can be pretty sure you will get an amazing sunset. I really can’t explain it, but trust me, it works all the time.