A tarp creates an dry island of refuge if it is properly rigged; if not it becomes a flapping, dripping, water-catching pain.
Here’s an idea from Cliff Jacobsen’s book Camping’s Top Secrets on rigging two tarps to create a chimney to keep a fire going in the rain.
Here’s a view showing how the tarps overlap for a foot or two by tying them together at the grommets. The back edge of the top is rigged up to the tree to create the opening that forms a chimney.
A view from above showing how the tarp is laid out.
A close up of the chimney.
Needless to say the fire must not be so large that it threatens to burn the tarp: a small, well maintained fire will provide plenty of heat and cheer in a rainstorm.