This is a lesson learned that I just remembered so I'm passing it on. Many years ago I was in the PA mountains on a bear hunt. November weather is tricky here. I didn't know the forecast and the day started out pleasantly warm so I dressed for that. As the day went on it started getting colder. The temps were really dropping.The sky opened up an it poured rain for only a min or two but enough to get me wet before I could get my poncho on. I found a great solution to get warm really fast. When I plan on hunting all day I carry a small can of Sterno with me to heat up an MRE or Mountain House meal. I got my poncho and Sterno out of my pack and put on the poncho and sat on a rock that was low enough that the poncho touched the ground. I opened and lit the Sterno and placed it under the poncho between my feet.
Within in minutes I could feel the sterno doing its magic and getting me warm. I had my arms inside the poncho (not in the poncho arms) so I could open my jacket and let the heat in.
There was a lot of steam coming off the poncho but at that time I didn't care. Now I make sure that at least my light weight jacket is waterproof because you never know for sure what the weather will bring.