Cold

I am new to Plumes and am not sure how to handle them in zone 9a. They have been down to 41 degrees. They are losing their leaves. Is this too cold for them and will they come back out?

They usually will lose their leaves at this time of year (they go dormant). 41 is fine, just keep them DRY when it's that cool. Dormant plants don't need watering.
In March you will see the new leaves appearing.

oh--no watering at all?

A little drop now and then if it makes you feel better ;-)

With temps ranging from 30 to 80 I just figured I should water.

Really, just a tiny bit. They are dormant and not actively growing so they are not really taking up any water.

I water sparingly in the winter, but mine aren't dormant and still have their leaves. I just water when the soil looks dry, but I have extremely good drainage, and we still have warm days. Plumerias are hardy to Zone 10 and cannot tolerate any temps at freezing and below so it is recommended that you protect your plumies if temps are going to drop below about 35. Damage can start at 32, even for brief periods. Death happens at 27 or 28 or long periods of 32 and below.