Signs of oncoming rain from plants: increased smell from plants; pinecones expanding. And up to two days before poor weather, herding animals may move to lower ground and gather.
Try and collect and store as much rain water as you can improvising with the materials you have, not forgetting bark or plant leaves. One less obvious way of collecting rain is to wrap cloth around and around sloping branches or tree trunks, so that the end of the cloth dangles into a container. Something else worth knowing is that well-trodden clay is waterproof, so you could make a pond without a plastic liner.
Once collected, keep water covered to prevent algal growth or loss through evaporation. Once it's more than two days old, you'll need to start filtering and purifying it.