The volcano lava watch is back at sea level, where it is pretty warm. If you get cold, just get closer to the lava, but do not step in it. I don't think you need a jacket there. Bring a flashlite, as that place is dark, and the surface is black.

The town of Volcano is up a few thousand feet. Get a B&B with a fireplace. the fire is a real delight. Not only is the weather cool, it is damp. If you wander around at night, you will probably need a jacket.