agree, totally weird to ask a hunting question on a mountain bike site. But since we’re all here…
I’ve endured some significant storms with just a tarp for shelter. If you avoid laying in a low spot you’ll be fine. Don’t pick a spot that looks like a stream used to run through it.
If bad weather looks possible pitch the tarp pretty low in an A frame style. If you run the ridgeline cord under the trap you need to position 2 small drip lines at the edge of the tarp to keep water from running in. At least one of the trees should be right up against the open end of the tarp to sort of “close the door”. Put a scrap of Tyvek or HD painters plastic under your sleeping bag. methods that wrap the tarp around to serve as a floor are terrible. That just creates a bowl for water to collect in.
This method is about as good as I’ve seen https://youtu.be/UlJE_SkN_ng but again, picking the right spot is a big deal.