I don't recommend a jacket, I recommend layers.

I wear my lightweight hi-vis windbreaker over layers. I can stop and peel off a layer if I get too warm, and I carry an extra layer in my backpack in case I get too cold, or if it is much colder on the ride home. I also carry three pairs of gloves for the same reason. Whatever small weight penalty this incurs is offset by the comfort of being able to "fine-tune" my outfit to match the conditions.